foren material selector for ammonia and caustic soda- mti ms-6

Materials Selector

for Hazardous Chemicals

Materials Selectorfor Hazardous Chemicals

Michael Davies

MS-6: Ammonia and Caustic Soda

Publication No. MS-6

Materials Technology Institute

Copyright � 2004

Materials Technology Instituteof the Chemical Process Industries, Inc.

Printed and bound in the United States of AmericaAll rights reserved, including translations

ISBN 1-57698-031-6

No part of this publication may be reproduced, stored in a retrieval system, ortransmitted, in any form or by any means, electronic, mechanical, photocopying,recording or otherwise, without prior written permission of the publisher.

This document was prepared under the sponsorship of the Materials Technol-ogy Institute of the Chemical Process Industries, Inc. (MTI) and is approved forrelease. All data and information contained in this document are believed to bereliable; however, no warranty of any kind, express or implied, is made with respectto the data, analyses, or author of this document; and the use of any part of thisdocument is at the user’s sole risk. MTI, the author, or any person acting on itsbehalf, assume no liability and expressly disclaim liability, including without limi-tation liability for negligence, resulting from the use or publication of the informa-tion contained in this document or warrant that such use or publication will be freefrom privately owned rights.

Published byMaterials Technology Institute

www.mti-global.org

v

Contents

List of Figures .....................................................................................................xiiList of Tables .......................................................................................................xvForeword............................................................................................................ xix

Section I: Ammonia

Chapter 1: Introduction .....................................................................3

Chapter 2: Properties of Ammonia..................................................7

Physical Properties ..........................................................................................8

Chemical Properties ........................................................................................9

Safety and Health Considerations....................................................................9First Aid Procedures................................................................................ 10Recommended Protective Equipment ...................................................... 10Disposal, Spill, or Leak Procedures.......................................................... 11Fire.......................................................................................................... 11

Chapter 3: Production of Ammonia ..............................................13

Chapter 4: Corrosion by Ammonia................................................19

vi Materials Selector for Hazardous Chemicals

Passivity........................................................................................................ 19

Forms of Corrosion........................................................................................ 20General Corrosion ................................................................................... 20Pitting Corrosion ..................................................................................... 20Crevice Corrosion.................................................................................... 21Intergranular Attack ................................................................................ 21Erosion-Corrosion.................................................................................... 21Stress Corrosion Cracking........................................................................ 21Vapor-Phase Attack ................................................................................. 21High-Temperature Corrosion................................................................... 22Dealloying............................................................................................... 22

Chapter 5: Corrosion of Metals and Alloys.................................23

Aluminum and Its Alloys .............................................................................. 23

Iron and Steel ................................................................................................ 24Cast Irons ................................................................................................ 24Carbon Steels........................................................................................... 25

Stress Corrosion Cracking of Steels.................................................... 26Nitriding of Steels ............................................................................. 27Hydrogen Attack............................................................................... 27Temper Embrittlement....................................................................... 29Hydrogen Sulfide Attack ................................................................... 29

Alloy Steels ............................................................................................. 30Chromium-Molybdenum Steels ......................................................... 30Nickel Alloy Steels ............................................................................ 30

Stainless Steels............................................................................................... 30Ferritic Grades......................................................................................... 31Precipitation-Hardening Grades .............................................................. 31Duplex Stainless Steels ............................................................................ 31Austenitic Stainless Steels........................................................................ 32Cast Stainless Steels................................................................................. 33

Alloys for Use at Elevated Temperatures ....................................................... 34Creep Resistance...................................................................................... 35Metal Dusting.......................................................................................... 35

Nickel and Its Alloys ..................................................................................... 37

Copper and Its Alloys ................................................................................... 40

MS-6: Ammonia and Caustic Soda vii

Titanium and Its Alloys ................................................................................. 42

Zirconium and Its Alloys............................................................................... 43

Niobium........................................................................................................ 43

Tantalum ....................................................................................................... 43

Other Metals and Alloys................................................................................ 43

Chapter 6: Resistance of Nonmetallic Materials ........................47

Elastomers..................................................................................................... 47

Plastics .......................................................................................................... 47Thermoplastics ........................................................................................ 48Thermoset Resins .................................................................................... 49

Carbon and Graphite..................................................................................... 52

Ceramic Materials ......................................................................................... 52

Chapter 7: Corrosion in Contaminated Ammonia......................55

Ammonium Chloride .................................................................................... 55

Carbon Dioxide—Carbamates ....................................................................... 55

Chlorides....................................................................................................... 57

Chapter 8: Specific Production Equipment...................................59

Production Stages.......................................................................................... 59Desulfurization Section............................................................................ 59Primary Reformer.................................................................................... 60Secondary Reformer ................................................................................ 63High-Temperature Converter................................................................... 64Carbon Dioxide Removal System ............................................................ 64Waste Heat Recovery System................................................................... 64Ammonia Synthesis................................................................................. 64Distillation Columns................................................................................ 65Heat Exchangers...................................................................................... 65

Heaters.............................................................................................. 65Coolers and Condensers .................................................................... 66

viii Materials Selector for Hazardous Chemicals

Storage Tanks .......................................................................................... 66Refrigerated Storage Vessels .............................................................. 66Pressurized Storage Vessels ............................................................... 67

Piping...................................................................................................... 68Pumps..................................................................................................... 68Compressors............................................................................................ 68Valves...................................................................................................... 69Gaskets, Seals, O-Rings, and Hoses ......................................................... 69Bolting..................................................................................................... 70Transportation Equipment ....................................................................... 70

Rail Car Transport (Tank Cars) .......................................................... 71Tank Trucks ....................................................................................... 72Marine Transport............................................................................... 72Pipelines............................................................................................ 72

Section II: Caustic Soda

Chapter 9: Introduction ...................................................................77

Chapter 10: Properties of Caustic Soda........................................81

Physical Properties ........................................................................................ 81

Chemical Properties ...................................................................................... 82

Safety and Health Considerations.................................................................. 84Recommended Protective Equipment ...................................................... 84Fire and Explosion................................................................................... 84First Aid.................................................................................................. 85Disposal, Spill, or Leak Procedures.......................................................... 85Caustic Dilution ...................................................................................... 85

Chapter 11: Production of Caustic Soda ......................................87

Production Processes ..................................................................................... 88Diaphragm Cell Process .......................................................................... 88Mercury Cell Process ............................................................................... 89Membrane Cell Process ........................................................................... 89Concentrated Solutions and Solid Caustic Soda....................................... 90

Impurities...................................................................................................... 90

MS-6: Ammonia and Caustic Soda ix

Chapter 12: Corrosion by Caustic Soda........................................93

Passivity........................................................................................................ 93

Forms of Corrosion........................................................................................ 94General Corrosion ................................................................................... 94Localized Corrosion................................................................................. 95Galvanic Corrosion.................................................................................. 95Erosion-Corrosion.................................................................................... 95Intergranular Attack ................................................................................ 95Dealloying............................................................................................... 95Liquid Metal Embrittlement .................................................................... 96Stress Corrosion Cracking........................................................................ 96High-Temperature Corrosion................................................................... 96

Chapter 13: Corrosion of Metals and Alloys...............................99

Aluminum and Its Alloys .............................................................................. 99

Iron and Steel .............................................................................................. 100Cast Irons .............................................................................................. 100Carbon and Low-Alloy Steels ................................................................ 101

Stainless Steels............................................................................................. 106Ferritic Grades....................................................................................... 106Precipitation-Hardening Grades ............................................................ 109Duplex Grades ...................................................................................... 109Austenitic Grades .................................................................................. 111Cast Stainless Steels............................................................................... 113

High-Performance Austenitic Alloys ........................................................... 115

Nickel and Its Alloys ................................................................................... 121Chromium-Free Alloys .......................................................................... 122Chromium-Bearing Alloys..................................................................... 126

Copper and Its Alloys ................................................................................. 128

Titanium and Its Alloys ............................................................................... 129

Zirconium and Its Alloys............................................................................. 130

Other Nonferrous Metals and Alloys........................................................... 131

x Materials Selector for Hazardous Chemicals

Chapter 14: Resistance of Nonmetallic Materials ....................137

Elastomers................................................................................................... 137

Plastics ........................................................................................................ 138Thermoplastics ...................................................................................... 138Thermoset Resin Materials .................................................................... 140

Carbon and Graphite................................................................................... 141

Ceramics ..................................................................................................... 142

Chapter 15: Corrosion in Contaminated Caustic andMixtures......................................................................145

Contaminants in Caustic Soda..................................................................... 145Chlorates ............................................................................................... 146Chlorides............................................................................................... 148Chlorine/Hypochlorite.......................................................................... 149Mercury................................................................................................. 149Sulfur .................................................................................................... 150Iron ....................................................................................................... 150

Sodium Hydroxide Treatments .................................................................... 151Petroleum Refining................................................................................ 151Bauxite Refining .................................................................................... 151Soap Manufacture ................................................................................. 152Sodium Hydrosulfide Production .......................................................... 152Caustic Fusion Reactions ....................................................................... 152Metal Finishing ..................................................................................... 153Pulp and Paper...................................................................................... 153

Caustic Contamination ................................................................................ 154Contamination of Steam ........................................................................ 154Contamination of Organic Media .......................................................... 155Contamination of Molten Sodium.......................................................... 155

Chapter 16: Related Chemicals ....................................................159

Soda Ash..................................................................................................... 159

Potassium Hydroxide .................................................................................. 160

MS-6: Ammonia and Caustic Soda xi

Chapter 17: Summary of Corrosion of Materials in CausticSoda.............................................................................163

Chapter 18: Specific Production Equipment...............................167

Production Equipment ................................................................................ 167Pressure Vessels..................................................................................... 167Brine Circulation Piping ........................................................................ 169Evaporators and Crystallizers................................................................ 169Salt Separators....................................................................................... 170

Caustic Soda Handling................................................................................ 170Heat Exchangers.................................................................................... 171

Heaters............................................................................................ 171Coolers ............................................................................................ 171

Storage Tanks ........................................................................................ 171Piping.................................................................................................... 173Pumps................................................................................................... 175Valves.................................................................................................... 175Gaskets, Seals, and O-Rings................................................................... 176

Shipping of Caustic Soda............................................................................. 177

Appendix A. Nominal Composition of Alloys...........................181

Appendix B. Approximate Equivalent Grade of Some Cast andWrought Alloys........................................................185

Appendix C. Glossary of Corrosion and Materials Terms ......187

Appendix D. Glossary of Acronyms and Abbreviations..........191

Index ..................................................................................................193

xiii

List of Figures

Section I—Ammonia

Figure 3.1 General View of Ammonia Production Plant. (Photo courtesy of SasolLtd, Secunda, RSA).

Figure 3.2 Block Diagram of the Steam/Air Reforming Process for Ammonia Pro-duction.

Figure 3.3 Block Diagram of the Partial Oxidation Process for Ammonia Produc-tion.

Figure 5.1 Operating Limits for Steels in Hydrogen Service, API 941.Figure 5.2 Metal Wastage Rates of Nickel Alloys in a Strongly Carburizing At-

mosphere at Elevated Temperature.Figure 5.3 Corrosion of Various Copper Alloys in Deaerated Ammonia.Figure 5.4 Corrosion of Various Copper Alloys in Aerated Ammonia.Figure 5.5 The Effect of Grain Size on the Time to Cracking of Yellow Brass

(C26800) in Ammonia.Figure 6.1 Wastage Rates (in mm/y) of Glass-Lined Steel in Ammonia. Vol: Sur-

face Area � 20.Figure 8.1 Part of a Modern Ammonia Plant Using Coal as a Feedstock. (Photo

courtesy of Sasol Ltd, Secunda, RSA).Figure 8.2 Ammonia Production Flow Sheet Showing Principal Items of Equip-

ment. (Courtesy M. P. Sukumaran Nair, Fact Ltd, Cochin, India).Figure 8.3 Effect on Service Life of Overheating an HK 40 Tube Above the Pre-

scribed Base Temperature.Figure 8.4 Stress to Rupture Data for HpModified Alloys Compared with HK 40.

Section Ii—Caustic Soda

Figure 9.1 Chlor-Alkali Plant in Australia. (Courtesy of CHEMETICS—A divi-sion of Aker Kvaerner Canada Inc.).

Figure 10.1 Range of Boiling and Freezing Temperatures of NaOH.Figure 11.1 Schematic View of Membrane Cell Showing Inputs and Outputs.Figure 11.2 Block Diagram of the Production of Caustic Soda from the Various

Types of Electrolytic Cells.Figure 11.3 Flow Diagram of Triple-Effect Caustic Soda Evaporator.

xiv Materials Selector for Hazardous Chemicals

Figure 13.1 Effect of pH on the Corrosion Rate of Aluminum in NaOH at 30�C(left) and 60�C (right).

Figure 13.2 Corrosion Rates of Gray Cast Iron ComparedwithNiResist� in CausticSoda.

Figure 13.3 Temperature and Concentration of Caustic Soda that can Cause SCCof Carbon Steels.

Figure 13.4 Temperature and Concentrations of Caustic Soda that Require StressRelief to Prevent SCC of Carbon Steel.

Figure 13.5 Corrosion Resistance of Duplex Stainless Steels in Boiling NaOH So-lutions.

Figure 13.6 Isocorrosion Curves (inmpy) for 304 and 316 Stainless Steels in CausticSoda also Showing Limits of SCC.

Figure 13.7 Isocorrosion Curve for a Corrosion Rate of 0.1 mm/y for 904 L(N80904), other Stainless Steels, and Titanium.

Figure 13.8 Isocorrosion Curves for CF 8 (J92600) inNaOHat EquilibriumPressure(left) and Atmospheric Pressure (right).

Figure 13.9 Effect of Molybdenum and Nickel in Various Alloys on Corrosion Ratein NaOH Solutions.

Figure 13.10 Effect of Nickel Content on Corrosion in NaOH of Various AlloysWithand Without Oxygen in the Atmosphere.

Figure 13.11 Effect of Molybdenum, Nickel, and Chromium on the Threshold Tem-perature at Which the Corrosion Rate Exceeds 5 mpy (0.127 mm/y) in50% NaOH.

Figure 13.12 Isocorrosion Data for Cn 7M (J92700) Compared with CD 4MCu(J93370) in NaOH Solutions.

Figure 13.13 Isocorrosion Curves for CN 7M (J92700) in NaOH at Equilibrium Pres-sure (left) and Atmospheric Pressure (right).

Figure 13.14 Isocorrosion Curves (in Mm/y) for Alloy 28 (N08028) and StandardAustenitic Stainless Steels in NaOH Solutions.

Figure 13.15 The Effect of Nickel on The Corrosion of Iron-Nickel-Chromium Al-loys in Caustic Soda.

Figure 13.16 SCC Regions for Nickel and Other Alloys in NaOH.Figure 13.17 Isocorrosion Curve (in mpy) for Alloy 200 (N02200) and Alloy 201

(N02201) in Caustic Soda.Figure 13.18 Isocorrosion Curves for Alloy 600 (N06600) and Alloy 201 (N02201) in

Caustic Soda.Figure 14.1 Wastage Rates (in mm/y) of Glass-Lined Steel in Caustic Soda. Vol:

Surface Area � 20.Figure 14.2 Effect of Temperature on the Corrosion of Borosilicate Glass in 50%

NaOH.Figure 15.1 Effect of Chlorates on Various Alloys in 50% Caustic at 150�C (302�F).Figure 15.2 Effect of Nickel Content on Corrosion of Various Materials in Simu-

lated First-Effect Liquor at 185�C (365�F).Figure 17.1 Isocorrosion Curve (at 1 mpy) for Stainless Steel and Nickel in Caustic

Soda.Figure 17.2 Summary Curves for SCC Regions for Carbon Steel, Stainless Steel,

Nickel-Rich and Nickel-Based Alloys.Figure 17.3 Range of Use of Various Nickel Alloys in Caustic Soda.

xv

List of Tables

Section I—Ammonia

Table 1.1 Industrial Uses of Ammonia.Table 2.1 Commercial Grades of Anhydrous Ammonia.Table 2.2 Grades of Commercial Ammonia Solutions.Table 2.3 Typical Properties of Anhydrous Ammonia.Table 2.4 Typical Properties of Nominally 30% Aqua Ammonia.Table 2.5 Human Physiological Response to Ammonia in Air.Table 5.1 Corrosion Rates of Aluminum in Ammonium Hydroxide Solutions at

20�C (68�F).Table 5.2 Corrosion Data for NiResist� and Cast Iron in Ammonia Solutions and

Environments.Table 5.3 Ammonia SCC Mitigation Measures.Table 5.4 Chromium Equivalents for Alloys Commonly Used in Ammonia

Plants.Table 5.5 Nitrogen Absorption in Nickel Alloys Exposed to Flowing Ammonia

at 1200�F (648�C).Table 5.6 Effect of Temperature on Nitriding Depth in Various Nickel Alloys.Table 5.7 Nitriding Tests in an Ammonia Converter.Table 5.8 Corrosion of Alloy 200 in 1 N Ammonium Hydroxide (1.7% NH3).Table 5.9 Corrosion of Alloy 200 in Agitated Ammonium Hydroxide Solutions

at Room Temperature.Table 5.10 Corrosion of Alloy 400 in Agitated Ammonium Hydroxide Solutions

at Room Temperature.Table 6.1 Estimated Temperature Limits (�C [�F]) for Various Elastomers That

May Be Suitable in Ammonia.

xvi Materials Selector for Hazardous Chemicals

Table 6.2 Suggested Temperature Limits (�C [�F]) for Various Plastics in Am-monia.

Table 6.3 Recommended Temperature Limits for Plastic-Lined Pipe in Ammo-nia.

Table 6.4 Suggested Temperature Limits (�C [�F]) for Various Plastics in DualLaminate Construction in Ammonia.

Table 6.5 Suggested Temperature Limits for FRP in Ammonia Service.Table 6.6 Estimated Temperature Limits (�F [�C]) of Commercial FRP Piping.Table 8.1 O-Ring Materials Compatible with Ammonia and Ammonium Hy-

droxide.Table 8.2 DOT Classifications for Ammonia.

Section II—Caustic Soda

Table 9.1 Typical Uses of Caustic Soda by Industry.Table 10.1 Physical Constants of Sodium Hydroxide (NaOH).Table 10.2 Boiling Points of Strong NaOH Solutions.Table 10.3 Hydrogen Ion Concentration of Various-Strength Caustic Soda Solu-

tions at 25�C (77�F).Table 13.1 Effect of Nickel Additions on Corrosion of Cast Irons in Boiling 50%

to 65% NaOH.Table 13.2 Corrosion Data for NiResist� and Cast Iron in Caustic Soda.Table 13.3 Corrosion of Cast Irons by Molten NaOH at 510�C (950�F).Table 13.4 Corrosion of Steel by Sodium Hydroxide Solutions.Table 13.5 Corrosion of Stainless Steels in NaOH Solutions.Table 13.6 Corrosion Rates of Stainless Steels in 50% NaOH at 140�C (290�F).Table 13.7 Corrosion Rates (mpy [mm/y]) in Boiling 50% NaOH Solution.Table 13.8 Corrosion Rates of 17-4PH (S17400) in Hot, Concentrated Caustic.Table 13.9 Lowest Temperature at Which Corrosion Rate Exceeds 5 mpy (0.127

mm/y).Table 13.10 Corrosion Rates (mpy [mm/y]) for Various Alloys in Boiling 50% So-

dium Hydroxide.Table 13.11 Corrosion Rates (mm/y [mpy]) of Various Alloys in NaOH and

NaOH/NaCl.Table 13.12 Corrosion Rates (mm/y [mpy]) of Stainless Steels in Sodium Hydrox-

ide at Various Conditions.Table 13.13 Static Corrosion Rates (mpy [mm/y]) in Molten Caustic Soda.Table 13.14 Corrosion Rates (mpy [mm/y]) of Nickel and OtherMaterials in Caus-

tic Evaporation Plants.Table 13.15 Corrosion Rates of Commercially Pure Titanium in Various Solutions

of NaOH.Table 13.16 Corrosion Rates of Zirconium (R60702) in Sodium Hydroxide Solu-

tions and Mixtures.Table 14.1 Dry, Hot Air Temperature Limits for Various Elastomers.Table 14.2 Estimated Elastomer’s Maximum Temperature (�C [�F]) in Various-

Strength Solutions of NaOH.

MS-6: Ammonia and Caustic Soda xvii

Table 14.3 Thermoplastics Maximum Temperature (�C [�F]) in Various Strengthsof NaOH.

Table 14.4 Temperature Limits for Plastic-Lined Pipe in Caustic Soda.Table 14.5 Temperature Limits (�C [�F]) for Various Plastics in Dual Laminate

Construction in Up to 10% Caustic Soda Solutions.Table 14.6 Thermosets as FRP Maximum Temperature (�C [�F]) in Various

Strengths of NaOH.Table 14.7 Corrosion Rate in mm/y of Borosilicate Glass in NaOH at Various

Temperatures.Table 14.8 CorrosiveWeight Loss (mg/cm2/y) of Various Ceramics in 50%NaOH

at 100�C (212�F).Table 15.1 Resistance of E-Brite�Alloy to Caustic Solutions Containing NaCl and

NaClO3.Table 15.2 Corrosion Rates (mm/y) of Various Alloys in Hot Caustic Solutions.Table 15.3 Corrosion Rates (mm/y) of S32906 and N02200 in Boiling NaOH So-

lutions Simulating Membrane and Diaphragm Cell Liquors.Table 15.4 Corrosion Rates (mm/y) of Various Alloys in NaOH/NaOCl Exposed

to Liquid and Vapor.Table 16.1 Laboratory Corrosion Tests in Potassium Hydroxide.Table 16.2 Corrosion Rate (mm/y [mpy]) of Metals and Alloys in KOH Solutions.Table 17.1 Alloys for Caustic Soda Service by Corrosion Resistance Category.Table 18.1 Nickel Alloys Used in Caustic Soda Production.Table 18.2 DOT Classifications for Caustic Soda.

xix

Foreword

The original basis for the first part of this monograph on materials selection for andcorrosion in the manufacture, handling, and utilization of ammonia was the NACE/MTI Materials Selection Advisor (MSA) “CHEMCOR 12,” prepared by Michael J.Conley of Det Norske Veritas Industry, Inc., Houston, Texas. The basis for the secondpart on materials selection for and corrosion in the manufacture, handling, andutilization of caustic soda was the NACE/MTI Materials Selection Advisor (MSA)“CHEMCOR 6,” prepared by Dr. Peter Elliott of Corrosion &Materials ConsultancyInc., Colts Neck, New Jersey.A first draft of both sections was prepared and edited by C.P. Dillon and W.I.

Pollock but was not published. Both sections have now been completely reviewed,updated, rewritten, and combined into this one volume by M. Davies of CARIADConsultants. Most of the figures were prepared by P.J.B. Scott, also of CARIADConsultants. Technical input and comments have been provided by a number ofpeople, including Jim Alexander (DuPont Dow Elastomer), Daniel Leander (Sand-vik), Jim McCoy (Special Metals), Sasol, and Doug Shaw (Chemetics).Information is provided in this monograph on the properties of ammonia and

caustic soda, production methods, health and safety issues, forms of corrosion spe-cific to these chemicals, definitions, and relevant specifications for materials ofconstruction as well as supporting laboratory and field corrosion data. Alloys areidentified by their UNS number together with generic or trade name where appro-priate.The materials of construction preferred for manufacturing, storing, transporting,

and handling ammonia, aqueous ammonia solutions, and caustic soda as commer-cial products are described in detail.

Section IAmmonia

3

1Introduction

Ammonia is one of the most important industrial and naturally occurring chemicalsin the world. It is an important link in the chain of nitrogen fixation whereby at-mospheric nitrogen is converted for use by living organisms. It is the fourth mostwidely produced industrial chemical in the United States (after sulfuric acid, nitro-gen, and ethylene).

Production statistics for the United States in 1996 were 13,800, imports were 3,500,and exports were 500, all in thousand metric tons of nitrogen. The world total pro-duction of ammonia for the same year was 93,500 thousand metric tons of nitrogen.Approximately 80% of the U.S. apparent domestic ammonia consumption was forfertilizer use, including anhydrous ammonia for direct application, urea, ammoniumnitrate, ammonium phosphates, and other nitrogen compounds. Ammonia was alsoused to produce plastics, synthetic fibers, and resins, 10%; explosives, 4%; and nu-merous other chemicals, 6%.1

Although ammonia’s primary use is in fertilizers, it has many other uses as shownin Table 1.1.2,3

The use of ammonia as a refrigerant to replace banned CFCs in, for example, airconditioning and household refrigerators is being actively explored.4 There is alsoa growing tendency to use ammonia in large, centralized applications like districtcooling and heating, including heat pump applications. There is scope to introduceammonia into the automotive industry since flammable or toxic products cannot beused in direct cooling systems for safety reasons. The use of ammonia in commercialrefrigeration and in medium-size air-conditioning systems is likely to increase.5

Ammonium hydroxide, the monohydrate of ammonia, is widely used for injec-tion into combustion air to reduce NOx emissions. It is also used instead of anhy-drous ammonia in many of the applications listed previously, partly to avoid theregulatory threshold limits placed on anhydrous ammonia as an extremely hazard-ous substance.6

References

1. G.A. Rabchevsky, “Nitrogen (Fixed)—Ammonia,” U.S. Geological Survey, Min-eral Commodity Summaries (1997), http://minerals.usgs.gov/minerals/pubs/commodity/nitrogen/480397.pdf.

4 Materials Selector for Hazardous Chemicals

Table 1.1 Industrial Uses of Ammonia

Industry Uses of Ammonia

Agriculture For direct application to crops; to provide proteinfor ruminating animals; as a preharvest cottondefoliant; an antifungal agent on some fruits; as apreservative for the storage of high-moisture corn

Fertilizer To produce ammonium nitrate, urea, ammoniumhydroxide, ammonium, and nitrate salts

Metal treating In nitriding, carbonitriding, bright annealing,furnace brazing, sintering, sodium hydridedescaling, and atomic hydrogen welding

Chemical process In the manufacture of nitric acid; certain alkalissuch as soda ash; dyes; pharmaceuticals such assulfa drugs, vitamins, and cosmetics; synthetictextile fibers such as nylon, rayon, and acrylics; andplastics such as phenolics and polyurethanes

Petroleum In neutralizing the acid constituents of crude oiland for protection of equipment from corrosion

Mining For extracting metals such as copper andmolybdenum from their ores

Water and wastewater To control pH; in solution form to regenerate weakanion exchange resins; with chlorine to producepotable water; as an oxygen scavenger in boilerwater treatment

Pollution control In stack emission control systems to neutralizesulfur oxides from combustion of sulfur-containingfuels; to control NOx in both catalytic andnoncatalytic applications; to enhance the efficiencyof electrostatic precipitators for particulate control

Photochemical As the developing agent in white printing and blueprinting and in the diazo duplication process

Petrochemical and cold storage In industrial refrigeration systemsRubber For the stabilization of natural and synthetic latex

to prevent premature coagulationPulp and paper For pulping wood; as a casein dispersant in the

coating of paperFood and beverage In industrial refrigeration systems; as a source of

nitrogen for yeast and microorganisms; in thehydrogenation of fats and oils

Fuel cells As a source of hydrogenLeather As a curing agent; a slime and mold preventative

in tanning liquors; as a protective agent for leathersand furs in storage

Household As commercial and household cleaners anddetergents

MS-6: Ammonia and Caustic Soda 5

2. Anon, “Ammonia (Anhydrous) Chemical Backgrounder,” NSC, National SafetyCouncil (2003), http://www.nsc.org/library/chemical/ammonia.htm.

3. Anon, “Uses of Ammonia” (Mt. Laurel, NJ: RM Technologies Inc., 2003),http://www.rmtech.net/uses_of_ammonia.htm.

4. Anon, “Two Japanese Companies Team Up to Provide CFC Alternatives,”zonAction, 41 (April 2002): p. 3, http://www.uneptie.org/ozonaction/library/oan/oan41/oan41e.pdf.

5. P. de Larminat, “Expanding the Use of Ammonia,” ASHRAE Journal 42, 3 (2000):pp. 35–40.

6. Anon, “Aqua Ammonia Properties” (Mt. Laurel, NJ: RM Technologies Inc.,2003), http://www.rmtech.net/Aqua%20Ammonia.htm.

7

Table 2.1 Commercial Grades of Anhydrous Ammonia

Property Commercial Refrigeration Metallurgical

Water content �5,000 ppm 75 ppm max 33 ppm maxOil content �5 ppm 4 ppm max 2 ppm max

2Properties of Ammonia

Ammonia (CAS 7664-41-7), also known as liquid ammonia or anhydrous ammonia,is a colorless gas with a pungent, suffocating odor. It is a major raw material formany nitrogen-bearing compounds and as a constituent in processes other than itsown manufacture (Chapter 3). It can develop characteristics related to the natureand degree of contamination by other chemicals (Chapter 7).Pure, anhydrous ammonia is a colorless, pungent, suffocating gas comprising

three atoms of hydrogen and one atom of nitrogen (NH3). Its odor is familiar fromthe use of “household ammonia,” a dilute aqueous solution, or other cleaners con-taining low levels of ammonia. It is also the smell in smelling salts, sometimes calledammonia but are, in fact, ammonium carbonate crystals.Ammonia liquefies readily under pressure at room temperature and is usually

stored and shipped in the liquid state. A relatively stable compound, NH3 requiresa high heat source for ignition in air. Even in the presence of a suitable ignitionsource, there is a relatively narrow flammable concentration range (16%–25% byvolume in air). Consequently, conditions favorable for ignition in air are seldomencountered in normal operation and handling. However, care must be taken withammonia/air mixtures in confined spaces.Anhydrous ammonia is available in a number of different commercial grades as

shown in Table 2.1.1

Ammonium hydroxide (CAS 1336-21-6), the monohydrate of ammonia, is a weakbase or alkali. It is an excellent acid neutralizer. It also comes in different commercialstrengths and grades, some of which are shown in Table 2.2.1

Ammonium hydroxide is also known as ammonia solution, aqueous or aquaammonia, ammonia water, or ammonia monohydrate. It has a boiling point of


Table 2.2 Grades of Commercial Ammonia Solutions

Constituent Commercial Agricultural

Ammonia as NH3 19%–29% (� 0.5%) 24.5%–25.3%Chlorides �1.0 ppm N/ACarbonate as CO2 �1.0 ppm N/AAppearance Clear ClearNitrogen content 15.6%–24% 20.0%–20.8%

Table 2.3 Typical Properties of Anhydrous Ammonia

Property Data

Molecular weight 17.03Boiling point at 1 atm �33.4�C (�28.17�F)Freezing point at 1 atm –77.7�C (�107.9�F)Critical temperature* 113�C (270.32�F)Critical pressure* 112.5 atm (11.4 MPa)Latent heat of evaporation 1.37 kJ/g (588.7 Btu/lb)Relative density vs. air 0.597Vapor density at 1 atm, �33.3�C (�28�F) 0.888 kg/m3 (0.055 lb/ft3)Liquid density at �33.3�C (�28�F) 682 kg/m3 (42.6 lb/ft3)Specific volume at 1 atm, � 0�C (32�F) 1.297 m3/kg (20.78 ft3/lb)Autoignition temp. (iron catalyst) 651�C (1,204�F)Autoignition temp. (quartz container) 850�C (1,562�F)Specific heat at 1 atm, 25�C (77�F) 35.652 K/molConstant pressure (CP) 2.189 kJ/kg �C (0.5232 Btu/lb �F)Constant volume (CV) 1.672 kJ/kg �C (0.3995 Btu/lb �F)Vapor pressure:at 25.7�C (78.3�F)at �77.7�C (�107.9�F)

1,013 kPa (10 atm)6.077 kPa (0.067 atm)

*Critical temperature is the temperature above which a given gas cannot be liquefied. Criticalpressure is the pressure at which a gas may just be liquefied at its critical temperature.4

�77�C (�107�F) and is soluble in water at ambient temperatures to about 90 partsin 100 (@47%). Ammonium hydroxide is corrosive to aluminum, copper, lead, nickel,silver, tin, zinc, and various alloys of these metals and galvanized surfaces.

Physical Properties

The physical properties of anhydrous ammonia are shown in Table 2.3.1,2,3

Similar properties are given for commercial ammonium hydroxide solution inTable 2.4.5,6


Table 2.4 Typical Properties of Nominally 30% Aqua Ammonia

Property Data

Molecular weight 35.06Boiling point 27.2� C (81�F)Melting/freezing point �72.4�C (�98.3� F)Vapor pressure at 20�C (68� F) 475 mm Hg (162 kPa)Specific gravity at 15�C (59� F) 0.895 g/mlVapor density (air � 1) at 15� C (at 0�C, 101.3 kPa) 0.618 0.7714 g/LLiquid density (at 0�C, 101.3 kPa) 0.6386 g/cm3

Bulk density 896 kg/m3

Solubility Completely soluble in water

Chemical Properties

Ammonia is alkaline and caustic. A 17.03-g/L solution (1 N) has a pH of 11.6; a 1.7-g/L solution (0.1 N) has a pH of 11.1, and a 0.17-g/L solution (0.01 N) has a pH of10.6. A 5% solution has a pH of 12.2, a 10% solution is pH 12.4, and at 30% the pHis 13.5. It is highly soluble in water (529 g/L at 20�C) and soluble in alcohol, chloro-form, and ether. It is easily liquefied under pressure. It is incompatible or reactivewith strong oxidizers, acids, halogens, and salts of silver and zinc. It is corrosive tocopper and galvanized steel; liquid ammonia will attack some plastics, rubber, andcoatings.7

Materials that should not be brought into contact with ammonia or ammoniumhydroxide include the following:6

• Oxidizing agents (e.g. perchlorates, chlorates, hydrogen peroxide, chromic tri-oxide, nitrogen oxides, calcium, or sodium hypochlorite)—can react violently orexplosively.

• Heavy metals and their salts (e.g. silver, gold, lead, mercury, or zinc, especiallyhalide salts)—may form shock-sensitive compounds that may explode when dry.

• Halogens (e.g. chlorine, bromine, fluorine, or iodine) or interhalogens (e.g. bro-mine pentafluoride, or chlorine trifluoride)—can react violently or form explosivechemicals.

• Nitromethane—increases the sensitivity of nitromethane to detonation; formssalts, which are explosive when dry.

• Acids, acid anhydrides, and acid chlorides—can react violently or explosively.• Calcium—reacts with evolution of heat; may ignite at higher temperatures.• Acrolein, propiolactone, and propylene oxide—mixing with 28% ammonium hy-droxide in a closed container caused a violent reaction.

Safety and Health Considerations

Ammonia is a naturally occurring chemical in all mammalian species, with a normalblood level of about 1.0 mg/liter. Because it is highly water soluble, ammonia vapors


Table 2.5 Human Physiological Response to Ammonia in Air

Vapor Concentration(ppm) Limit or General Effect Exposure Period

5 Odor detectable by mostpersons

Unlimited

25 NIOSH/ ACGIH REL 8-hour TWA35 ACGIH STEL 15-minute TWA50 OSHA PEL 8-hour TWA300 NIOSH IDLH —400–700 Immediate nose and throat

irritation30 minutes to 1 hour causesserious effect

1,700 Severe coughing; severe eye,nose, and throat irritation

Could be fatal after 30minutes

2,000–5,000 Severe coughing; severe eye,nose, and throat irritation

Could be fatal after 15minutes

5,000–10,000 Respiratory spasms; rapidasphyxia

Fatal within minutes

irritate and can damage eyes and mucous membranes of the nose, throat, and lungs.At higher concentrations, ammonia is corrosive to tissue and exposure can be fatal.Anhydrous liquid ammonia produces second-degree burns on the skin and exten-sive destruction of the anterior chamber in the eye. Liquid ammonia can freeze thesurface of the skin, causing thrombosis of surface vessels, ischemia, and necrosis.7

The human response to various concentrations of ammonia in air is shown inTable 2.5.1,8 This table also shows the current limits set by various regulatory bodiesto protect humans that might come into contact with ammonia.

First Aid Procedures

The following specific steps are recommended to alleviate the results of physicalcontact with ammonia:

• Eyes—Flush with copious quantities of water for at least 15 minutes. If irritationpersists, obtain medical attention.

• Skin—Wash off with water. If irritation persists, obtain medical attention.• Inhalation—Remove from exposure. If breathing is difficult or discomfort persists,obtain medical attention.

• Ingestion—Rinse mouth with water. Give copious amounts of water to causedilution in stomach. Do not induce vomiting.

Recommended Protective Equipment

A NIOSH/MSHA acid-gas respirator with full face piece or, in other severe expo-sures, a self-contained breathing apparatus is recommended. Eyes should be pro-


tected by chemical goggles. Rubber, neoprene, or other resistant elastomer glovesshould be used when in contact with ammonia. A rubber apron and boots are sug-gested, especially in release situations.When storing or handling ammonia goggles and/or face shields, rubber aprons,

gloves, and boots should be worn. When unloading bulk vehicles, personnel shouldwear chemical goggles and rubber or neoprene gloves. All fittings should be prop-erly secured prior to energizing the unloading system. Care should be taken to avoidphysical contact when disconnecting lines/hoses after unloading.

Disposal, Spill, or Leak Procedures

In case of spills or leaks, wash with copious amounts of water.Ventilation is required in enclosed areas handling ammonia. Should a release

occur in an enclosed area, eyes and skin should be protected from contact.All stationary storage installations shall have at least two suitable gas masks in

readily accessible locations. Full face masks with ammonia canisters, for example,as approved by the Bureau of Mines, U.S. Department of the Interior, are suitablefor emergency action for most leaks, particularly those that occur outdoors. Forprotection in concentrated ammonia atmospheres, self-contained breathing air ap-paratus is required.Stationary storage installations shall have an easily accessible shower or a 55-

gallon drum of water available.

Fire

Ammonia is considered to be nonflammable, but a large and intense energy sourcecan cause ignition and/or explosion. Lower flammable limit for ammonia is 15.5%volume in air; the upper flammable limit is 27.0% volume in air; and the autoignitiontemperature is 1204�C (651�C). In case of fire, the best procedure is to stop the flowof gas. If the fire is small, use dry chemical or CO2; for large fires, use water spray,fog, or foam. Use water to keep fire-exposed containers cool and water fog or foamto reduce vapor concentrations if necessary. Full protective equipment, including aself-contained breathing apparatus, should be worn in a fire involving thismaterial.5

References

1. Anon, “Ammonia and Ammonia Solution—Handling and Storage” (Sioux City,IA: Terra Industries Inc., 2002), 28 pp.

2. Anon, “Ammonia,” MSDS G-11 (Murray Hill, NJ: BOC Gases, 1996), http://www.vngas.com/pdf/g11.pdf.

3. J.R. Jennings, ed., Catalytic Ammonia Synthesis (New York, NY: Plenum Press,1991), pp. 393–394.


4. T.C. Collocott, A.B. Dobson, eds., Chamber Science and Technology Dictionary(Edinburgh, Scotland: W&R Chambers Ltd, 1984), p. 288.

5. Anon, “Aqua ammonia 19–30% NH3,” MSDS 2050 (Sioux City, IA: Terra Indus-tries Inc., 2002), http://www.terraindustries.com/our_products/ammonia/msds/aqua_ammonia_19–30.pdf.

6. Anon, “Aqua ammonia,” MSDS 11_01 (North York, ON, Canada: MARSU-LEX Inc., 2002), http://www.marsulex.com/PDF_MLX/MSDS/AmmoniumHydroxide.msds.pdf.

7. Anon, “Ammonia (Anhydrous) Chemical Backgrounder,” NSC, National SafetyCouncil (2003), http://www.nsc.org/library/chemical/ammonia.htm.

8. R.J. Lewis, ed., Sax’s Dangerous Properties of IndustrialMaterials, 10th ed. (NewYork, NY: John Wiley & Sons, 2000), pp. 228–229.

13

3Production of Ammonia

Modern industrial synthesis of ammonia was first developed by Fritz Haber, a pro-fessor at Karlesruhe University, and Karl Bosch, an engineer with Badischer Anilin-und Soda-Fabrik (BASF). Production of 30 metric tons per day began in 1913 nearBASF’s Ludwigshafen complex. The Haber-Bosch process involved the reaction ofgaseous nitrogen and hydrogen at high temperatures and pressures over iron cat-alysts. This development was important in that it replaced nitrogen sources such asChilean nitrates and the limited production of ammonia from coke-oven gas. Localsources of nitrogen were needed at that time not only for agricultural uses but alsoin the manufacture of explosives.1

Ammonia is basically produced from water, air, and energy. The source of theenergy is normally hydrocarbons, thus providing hydrogen as well as energy, butcoal or electricity can also be used. Steam reforming of light hydrocarbons is themost efficient route, with about 77% of world ammonia capacity being based onnatural gas. There are, however, many developments taking place largely to reduceenergy use, increase production, and decrease plant size. Materials developmentshave greatly improved operation in the high-temperature parts of the process. Otherinnovations include gas-heated reforming and isobaric production, made possibleby improvements in process design and catalysts.2,3

The typical size of a large single-train ammonia plant is 1,000–1,500 t/d, althoughcapacities of 1,800 t/d and above are not uncommon for new plants. The processand energy systems are integrated to improve overall energy efficiency. The am-monia plant may stand alone or be integrated with other plants, such as a urea ornitric acid plant. A typical modern ammonia plant, showing the major items ofequipment, is shown in Figure 3.1.

Two main types of production process for ammonia synthesis gas are currentlyin operation in Europe:

• Steam reforming of natural gas or other light hydrocarbons (natural gas liquids,liquefied petroleum gas, naphtha). Modified steam reforming using excess air inthe secondary reformer and heat exchange autothermal reforming are also beingused to some extent. This process is shown schematically in Figure 3.2.4


Figure 3.1 General View of Ammonia Production Plant (photo courtesy of Sasol Ltd,Secunda, RSA)

Figure 3.2 Block Diagram of the Steam/Air Reforming Process for AmmoniaProduction


• Partial oxidation of heavy fuel oil or vacuum residue. This process is shown sche-matically in Figure 3.3.5

The ammonia synthesis process is largely independent of the type of synthesisgas production process, but synthesis gas quality influences the design and oper-ating conditions. About 85% of world ammonia production is based on conventionalsteam reforming. The theoretical process conversions, based on methane feedstock,are given in the following approximate formulae:

0.88CH � 1.26Air � 1.24H O r 0.88CO � N � 3H4 2 2 2 2N � 3H r 2NH (1)2 2 3

The synthesis gas production and purification normally take place at 25–35-bar pres-sure. The ammonia synthesis pressure is usually in the range from 100 to 250 bar.

Since most of the catalysts used in the process are sensitive to them, sulfur andsulfur compounds are removed in the primary reformer section. Here the sulfur inthe feed gas is hydrogenated to H2S.

The desulfurized gas is mixed with process steam, and the steam/gas mixture isthen heated to 500�C–600�C in the convection section before entering the primaryreformer. The primary reformer consists of a large number of high–nickel chromiumalloy tubes filled with nickel-containing reforming catalyst.

The overall reaction is highly endothermic, and additional heat is required to raisethe temperature to 780�C–830�C at the reformer outlet.

Figure 3.3 Block Diagram of the Partial Oxidation Process for Ammonia Production


Only 30% to 40% of the hydrocarbon feed is reformed in the primary reformerbecause of the chemical equilibria at the actual operating conditions. The tempera-ture must be raised to increase the conversion. This is done in the secondary re-former by internal combustion of part of the gas with the process air, which alsoprovides the nitrogen for the final synthesis gas.

The process air is compressed to the reforming pressure and heated further in theprimary reformer convection section to around 600�C (1112�F). The process gas ismixed with the air in a burner and then passed over a nickel-containing secondaryreformer catalyst. The reformer outlet temperature is around 1000�C (1832�F), andup to 99% of the hydrocarbon feed (to the primary reformer) is converted, giving aresidual methane content of 0.2% to 0.3% (dry gas base) in the process gas leavingthe secondary reformer. The process gas is cooled to 350�C to 400�C (662�F–752�F)in a waste heat steam boiler or boiler/superheater downstream from the secondaryreformer.

The CO, CO2, and steam are removed from the gas leaving the secondary re-former. Remaining CO or CO2 is poisonous to the ammonia synthesis catalyst, so itis converted to methane by catalytic reaction with hydrogen. Water is also formedand is condensed/absorbed before the hydrogen/nitrogen syngas is compressedbefore ammonia synthesis that takes place on an iron catalyst at pressures usuallyin the range 100–250 bar and temperatures in the range 350�C to 550�C (662�F–1022�F). Only 20–30% is reacted per pass in the converter because of the unfavorableequilibrium conditions. The ammonia that is formed is separated from the recyclegas by cooling/condensation, and the reacted gas is replaced with fresh makeupsynthesis gas, thus maintaining the loop pressure. Since the reactions are exothermicand there is a large temperature range in the loop, extensive heat exchange is re-quired. A newly developed ammonia synthesis catalyst containing ruthenium on agraphite support has a much higher activity per unit of volume and has the potentialto increase conversion and lower operating pressures. Other catalysts based on co-balt and ruthenium are being investigated.

The high-purity anhydrous ammonia from this stage is either used directly indownstream plants or transferred to storage tanks. From these, the ammonia can betransferred to road tankers, rail tank cars, or ships (see Chapter 8).

The partial oxidation process is used in the gasification of heavy feedstocks suchas residual oils and coal. Extremely viscous hydrocarbons and plastic wastes mayalso be used as fractions of the feed. An air separation unit is needed for the pro-duction of oxygen for the partial oxidation step. The nitrogen is added in the liquidnitrogen wash to remove impurities from the synthesis gas and to get the requiredhydrogen/nitrogen ratio in the synthesis gas.

The partial oxidation gasification is a noncatalytic process taking place at highpressure (�50 bar) and temperatures around 1400�C (2552�F). Some steam is addedfor temperature moderation. The gas is treated to remove minerals as ash, sulfur ashydrogen sulfide, soot, and carbon dioxide. The ammonia synthesis is similar to thatused in steam reforming plants but simpler and more efficient because of the highpurity of synthesis gas from liquid nitrogen wash units and the synthesis loop notrequiring a purge.6

Showa Denko of Japan has constructed a 65,000 t/a ammonia unit in the indus-


trial city of Kawasaki near Tokyo based on gasification of municipal plastic wastes.As well as ammonia production, the plant will recycle sulfur, metallic impuritieswill be recovered and sold, and chlorine will be captured.7 This plant started pro-duction in 2003.8

References

1. Anon, “Ammonia Synthesis,” University of Idaho (2003), http://www.chem.uidaho.edu/�honors/ammonia.html.

2. P. Agarwal, “Ammonia: The Next Step,” Chemical Engineers Resource Page(2002), http://www.cheresources.com/ammonia.shtml.

3. Anon, “KAAP plus�—Advanced Technology for Low-Cost Ammonia Produc-tion,” brochure no. HO2369 (Houston, TX: Kellogg Brown & Root, 2000), 2 pp.

4. Anon, “Production of Ammonia,” vol. 1 (Brussels, Belgium: EFMA, EuropeanFertilizer Manufacturers’ Association, 2000), p. 10.

5. Anon, “Production of Ammonia,” vol. 1 (Brussels, Belgium: EFMA, EuropeanFertilizer Manufacturers’ Association, 2000), p. 11.

6. Anon, “Production of Ammonia,” vol. 1 (Brussels, Belgium: EFMA, EuropeanFertilizer Manufacturers’ Association, 2000), 44 pp.

7. Anon, India Fertilizer News (Aug. 2003), http://www.fertindia.com/International%20News-.htm.

8. Anon, “Message from the Management—Annual Report 2002” (Japan: ShowaDenko, Aug. 2003), www.sdk.co.jp/contents/investment_info.

19

4Corrosion by Ammonia

Corrosion is the deterioration of a material by reaction with its environment. Formetals and alloys, this is mostly an electrochemical process involving anodic andcathodic reactions.Corrosion rates in aggressive chemicals usually decrease as the pH increases. In

alkaline solutions, the hydrogen ion is present in very low concentrations. However,many metals pass through a minimum corrosion rate at some pH, usually basic,and then suffer increased corrosion as pH continues to rise. Aluminum can liberatehydrogen ions from basic solutions. Since hydrogen ions are in short supply, it islikely that the cathodic reaction in alkaline media involves absorbed water mole-cules, such as

�H O � e r OH � H (1)2

while the anodic reaction remains the same:

�M r M � e (2)

The metal ion is removed from solution by forming a basic salt such as ferrate,aluminate, or zincate. In ammonium hydroxide, nickel and copper will dissolve toform complex amines.Quite often, corrosion by alkalis leads to pitting and other localized attack because

they tend to form cathodic films, and attack is concentrated at susceptible anodicareas.

Passivity

Stainless steels and some other iron-chromium and nickel-chromium alloys, as wellas aluminum, titanium, and zirconium, develop a relatively inert, tenacious surfaceoxide layer. Interruptions to this film by inclusions, embedded metal particles, ormechanical breaks provide unprotected sites for corrosion to occur.


In the case of austenitic stainless steels, inclusions and other foreign materials canbe removed from the surface by immersion in 10% nitric acid with 3% hydrofluoricacid at about 70�C (158�F). The purpose of this treatment is to clean the metal surfaceand remove embedded particles before allowing the oxide film to reform in a morecontinuous, thicker, and more tenacious form than might occur naturally. A similarprocess involving the use of caustic solutions is applied to aluminum alloys. Theprotective surface oxide reforms spontaneously on reexposure to air.The terms “active” and “passive” are used to describe the corrosion behavior of

metals that form these protective films. When active, the protective oxide film isbreached or removed, and corrosion proceeds on the base material. When passive,the protective oxide film remains intact and minimizes corrosive activity.

Forms of Corrosion

Several types of corrosion are encountered in ammonia production plants, includinguniform corrosion, galvanic corrosion, acid attack, crevice corrosion, intergranularcorrosion, cavitation corrosion, high-temperature corrosion, and corrosion relatedto hydrogen, embrittlement, metal dusting, and so on. These corrosion problemshappen at various stages in the production process.1

General Corrosion

General or uniform corrosion is the common form of metal loss in most corrodentsin the absence of passivating films. In this form of corrosion, the metal is removeduniformly over the entire exposed surface, and a corrosion allowance can be builtinto the design based on expected corrosion rate.

Pitting Corrosion

Pitting is a localized type of corrosion usually taking the form of small but deepcavities. Pits tend to originate at points where the protective oxide film is breached.Mechanical damage and surface deposits are two reasons for the development ofactive areas leading to pits. Once started, the pit develops its own corrosive envi-ronment and proceeds independently of the bulk solution. Chlorides are often in-volved in pitting corrosion of stainless steels.Pitting in aluminum initiates at flaws in the oxide film, usually associated with

intermetallic phase particles in the aluminum. Chlorides are often but not alwaysassociated with pitting corrosion in aluminum.2


Crevice Corrosion

Crevice corrosion is induced by a similar mechanism to pitting, that is, breakdownof the passive film leading to active localized corrosion. Crevice corrosion occurs onclosely abutting surfaces, such as flange faces and under gaskets where the bulkliquid cannot penetrate. Chemical changes within the crevice due to depletion oraccretion of aggressive species cause pitting and attack within the crevice. The areaoutside the crevice comprises the cathode, while that internal to the crevice com-prises the anode in the circuit. Crevice corrosion is often associated with oxygendepletion within the crevice.

Intergranular Attack

Intergranular attack (IGA) is a form of accelerated corrosion in grain boundary re-gions caused by compositional differences. IGA occurs in austenitic stainless steelsbecause of a depletion of chromium in this region. These chemical differenceswithinthe alloy result from thermal processing in a temperature range, usually 425�C to815�C (800�F–1500�F) favoring chromium carbide precipitation at the grain bound-aries. The stainless steel is then said to be “sensitized” (i.e. susceptible to IGA).

Erosion-Corrosion

This is a form of corrosion resulting from the loss of protective surface oxide fromexcessive velocity or turbulence of contacting fluids. It can also be caused or accel-erated by the presence of entrained particles. Centrifugal pumps often exhibit thistype of corrosion. This problem is controlled by alloy selection.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) is an environmentally assisted form of attack thatresults from interaction between the environment and a specific alloy system undertensile stress. It is a temperature-sensitive phenomenon. Classical examples are SCCof copper alloys in ammonia, steel in caustic soda, and stainless steels in chlorideenvironments. Carbon and low-alloy steels can suffer from SCC in ammonia, andsteps must be taken to avoid or control this problem (see Chapter 5 for details).

Vapor-Phase Attack

Most corrosion data are derived from laboratory tests made in the liquid phase.However, in actual plant equipment, there is often a vapor space above the liquid


acid where vapor composition may differ from the bulk liquid. This vapor maycondense on the equipment inner surface and be either more or less corrosive thanthe bulk liquid. The condensing liquid may not contain constituents that inhibitattack in the bulk liquid phase. This occurs in anhydrous ammonia storage in whichsmall quantities of water prevent SCC in the liquid but may be absent in the vapor(see Chapter 5 for details).

High-Temperature Corrosion

Since many parts of ammonia production plant operate at elevated temperatures,they are prone to a number of high-temperature corrosion and embrittlementmech-anisms. These include oxidation, nitriding, hydrogen attack, carburization, metaldusting, hydrogen sulfide corrosion, and temper embrittlement. These forms of cor-rosion are normally controlled by a combination of selected operating conditionsand materials that are used in the parts of the plant where these forms of deterio-ration can occur (see Chapter 5 for details).

Dealloying

Dealloying is the term used to describe the preferential loss of one component of amulticomponent alloy. In anhydrous ammonia, the phenomenon is not common,but some copper-based alloys are susceptible to dealloying in ammonia solutions.

References

1. M.P. Sukumaran Nair, “Tackling Corrosion in Ammonia Plants—Selecting theProper Materials,” Chemical Processing 12, 1 (2001).

2. E.E. Stansbury, R.A. Buchanan, Fundamentals of Electrochemical Corrosion(Materials Park, OH: ASM International, 2000), p. 325.

23

5Corrosion of Metals and Alloys

This chapter discusses the corrosion behavior of metals and alloys in nominally pureammonia and ammonium hydroxide. Corrosion in contaminated solutions andmixtures is discussed in Chapter 7. Materials of construction for specific plant itemsand types of equipment are described in Chapter 8.

Ammonia and ammonium hydroxide are not particularly corrosive in themselves,but some problems arise with specific materials, particularly in the presence of con-taminants. Air or oxygen contamination is a factor in many instances, causing gen-eral corrosion of some materials and localized corrosion, specifically stress corro-sion cracking, in others. Contamination with carbon dioxide can lead to corrosiondue to carbamates, sometimes encountered in ammonia recovery systems. High-temperature corrosion will occur in hot dissociated ammonia.

Aluminum and Its Alloys

Aluminum and its copper-free alloys show good resistance to dry, gaseous ammoniaat ambient or elevated temperatures. Corrosion rates of �0.025 mm/y (1 mpy) at21�C and �0.05 mm/y (2 mpy) at 100�C are typical.1

Aluminum may be used for cargo tanks for the anhydrous product. If moistureis present, there is some attack on aluminum, but a protective film soon forms andcorrosion stops. Aluminum tubing is used in ammonia refrigeration operating inliquid ammonia containing 5% water. In moist ammonia, vapor corrosion is low,below about 50�C (120�F). Under condensing conditions of steam and ammonia,aluminum can be attacked, and the rate does not decrease with time. Attack isprevented if the CO2:NH3 ratio is at least 2.5:1. Hydrogen sulfide also inhibits cor-rosion under condensing conditions. Aluminum is used for compressors, heat ex-changers, evaporators, condensers, and piping in the production of ammonia. Alu-minum pressure vessels are used in the storage and transport of ammonia.2

There is mild action on aluminum in ammonium hydroxide solutions at tem-peratures below about 50�C (120�F). The greatest attack occurs in concentrated so-


lutions (around 25%) and at about 5% concentration. For aluminum to perform wellin ammonium hydroxide, the solution must be free from heavy metals and halogenions. Attack is limited if the solution is saturated with aluminum ions before ex-posure takes place. As with anhydrous ammonia, aluminum is attacked at highertemperature, but this attack stops as a protective film forms.3,4 Even at ambienttemperatures, corrosion rates decrease with time of exposure (see Table 5.1).5

The initial attack of aluminum by dilute ammonia solutions (up to �10%) iscontrolled by the diffusion of OH ions to the surface and is a function of pH. Thesurface is passivated once sufficient corrosion product has been produced to forma protective film. Under some exposure conditions, the corrosion product may con-tinue to dissolve rather than form a protective film.2

Aluminum alloys are highly susceptible to the effects of contaminants and cansuffer pitting attack. Any application for aluminum should include testing of sam-ples under the exact service conditions expected.

Table 5.1 Corrosion Rates of Aluminum in Ammonium Hydroxide Solutions at 20�C(68�F)

Concentration 2% 5% 10% 22%

Corrosion rates in g/m2/d (mm/y)1-day test 130 (16.9) 176 (22.9) 159 (20.7) 63 (8.2)7-day test 29 (3.8) 35 (4.6) 33 (4.3) 10 (1.3)

Iron and Steel

Ferrous alloys are generally not corroded by ammonia or ammonium hydroxide atambient or elevated temperatures. Corrosion can, however, occur in the presence ofcontaminants, and they can be subject to stress corrosion cracking in storage atambient temperatures and can become embrittled in low-temperature handling.

Cast Irons

Cast iron has been widely used in ammonia production and handling. Since it isgenerally thick walled, somewhat higher corrosion rates can be tolerated as long asthe iron in solution is acceptable. Cast iron strippers have been used to concentratecrude gas liquors with about 1–2% ammonia up to 25% ammonium hydroxide. Acorrosion rate of 1 mm/y (40 mpy) has been found in the vapor space of a nodularcast iron ammonia pressure distillation plant operating at about 88�C (190�F). Whitecast iron with �18% chromium content, ASTM A532 Grade IID, has been used insome applications.


High-nickel cast irons have been used in valves, pumps, and so on in ammoniumhydroxide solutions. The austenitic nickel cast iron NiResist� cooling coils corrodedat about 0.005 mm/y (0.1 mpy) at 25�C (77�F) and 0.15 mm/y (5.9 mpy) at 70�C(158�F) in flowing ammonia gas.1 Laboratory testing of NiResist� cast irons andunalloyed cast irons gave the results shown in Table 5.2.6 This table also shows datafrom ammonia-containing environments in industrial applications. In most appli-cations, unalloyed cast iron is at least as good as the NiResist� irons, but in theconcentrated and contaminated solutions, the alloyed irons are superior. In mostcases in this table, the recommended NiResist� are types 1 (F41000) and 2 (F41002)with type 3 (F41004), also appropriate for the dilute solutions.

Table 5.2 Corrosion Data for NiResist� and Cast Iron in Ammonia Solutions andEnvironments

Average Corrosion Rate(mpy [mm/y])

MediumTemperature

(�F [�C]) Cast Iron NiResist�

5% ammonium hydroxide 60 (15.6) Nil 0.0110% ammonium hydroxide 60 (15.6) Nil 0.2 (�0.01)25% ammonium hydroxide 60 (15.6) Nil 0.18 (�0.01)50% ammonium hydroxide 60 (15.6) Nil Nil75% ammonium hydroxide 60 (15.6) Nil NilConcentrated ammonium hydroxide 60 (15.6) 2 (0.05) NilAmmonia 5%–6% by vol; 150 ppmphenol in H2O vapor

215.6 (102) 2 (0.05) 0.9 (0.02)

Ammonia liquor separator tank — 0.09 (�0.01) 0.05 (�0.01)Ammonia liquor 6.5 g/L ammonia 215.6 (102) 3 (0.08) 0.6Ammonia liquor with sulfates,sulfides, etc.

100 (37.8) 0.1 0.01

Carbon Steels

Ammonia is essentially noncorrosive to steels at ambient temperatures, which ac-counts for their widespread use. Carbon steels (commonly referred to as “steels” inmany standards and regulations pertaining to ammonia storage and transport) in-clude both carbon- and carbon-manganese steels commonly specified for processequipment. These alloys are by far the most commonly used materials for the storageand handling of ammonia. The standard used to specify the steel is selected on thebasis of the mechanical properties required by the application, with considerationof environmental factors.

Even at elevated temperatures, corrosion rates in anhydrous ammonia are low,less than 0.05 mm/y (1.9 mpy) in the range from 297 to 589 K (24�C–316�C).7 One


very important exception relative to corrosion resistance of steels is the tendency ofanhydrous ammonia to cause stress corrosion cracking (see discussion later in thischapter).

Ordinary carbon and alloy steels are satisfactory in ammonium hydroxide service,although a superficial rusting will occur in the vapor space.

Stress Corrosion Cracking of Steels Ammonia stress corrosion cracking (SCC) incarbon-steel vessels was first reported in the mid-1950s in agricultural service tanks.Cracking occurred in areas of high residual stress, such as welds and cold-formeddished heads. Hot forming or stress relieving the heads considerably reduced theoccurrence of cracking, as did the addition of a small amount of water to the am-monia.

Throughout the 1960s and early 1970s, cracking problems appeared to be asso-ciated mainly with high-strength quenched and tempered steels. Later there werereports of cracking occurring in spheres containing anhydrous ammonia with wateradditions and also in spheres that had been stress relieved after finding and repair-ing cracks.

The cause of this cracking is now accepted to be high local stresses and the pres-ence of air contamination, although nitrogen and carbon dioxide are also thoughtto play a role. Cracking is accelerated by the use of high-strength steels, the presenceof hard welds, and air contamination.2,8 The highest susceptibility to SCC has beenfound to be in liquid ammonia with 3 to 10 ppm oxygen and a water content �100ppm. SCC can occur, however, in ammonia with an oxygen content down to 0.5ppm when the water content is very low.9

Possible ways to control or reduce liquid ammonia stress corrosion cracking incarbon steels include the following:10

• Eliminate oxygen• Add around 0.2% water to the ammonia• Use steel with an actual yield strength less than 300 MPa• Reduce residual stress by stress relieving• Inspect often enough to detect cracks before they grow to dangerous proportions• Use a sacrificial anode such as a zinc spray• Cathodically protect the steel• Use a different material such as stainless steel• Paint the steel with a suitable protective coating

Some of these options can be eliminated for the following reasons:

• There is no commercially available paint suitable for ammonia submersion duty.• Using stainless steel would increase the capital cost of vessel construction by at

least 100% and increase steel mass.• Cathodic protection has not yet been shown to be reliable in refrigerated instal-

lations and would not protect in vapor spaces (e.g. in road tankers).• Zinc spraying of ammonia tank internals has been used, largely on a trial basis.

More experience is required.


The remaining options that are currently used for storage vessels or road tankersare evaluated in Table 5.3.10

Despite all the research and the available information on the subject of SCC ofsteels in ammonia, sudden failures do still occur. A 20-year-old liquefied ammoniagas cylinder with a charged capacity of 25 kg operating at 12-bar pressure fracturedinto four pieces. These cylinders are hydraulically tested annually at 30 kg/cm2. Thefailure was found to have initiated in the upper dished head, near to the girth weld.A similar unfailed cylinder was destructively examined and found to have SCCcracks in the same location. It was recommended that all cylinders with similaroperating history should be scrapped, that this type of cylinder should not be ex-posed to direct sunlight, and that they should be regularly inspected to applicablestandards.11

SCC of carbon steel occurred in ammonia receiver tanks used for recycling am-monia in a urea plant. These tanks (SA 516 grade 70) suffered extensive cracking inwelds and HAZ in spite of the addition of 0.2% water to the system. It was concludedthat the water was not uniformly distributed throughout these vessels and was,anyway, probably inadequate to protect the vapor space and in the condensing am-monia.12

Nitriding of Steels At high temperatures, ammonia may dissociate into hydrogenand nascent nitrogen. The nascent nitrogen has a high affinity for iron and reacts toform a very hard, brittle metal nitride. This is sometimes a desirable reaction, andwear-resistant surfaces are commonly produced on steel parts by nitriding in anammonia atmosphere.

Although commercial nitriding is performed at temperatures above the normalservice temperature for steels (495�C–565�C or 925�F–1,050�F), significant nitridingcan occur at lower temperatures, resulting in loss of ductility. Ammonia dissociationis catalyzed by iron, which also contributes to the damage potential. For these rea-sons, steels are restricted to use at temperatures below 300�C (600�F) in ammoniaservice. In ammonia converters, nitriding layers can develop over time to a depthof several millimeters, and these hard layers can cause brittle, surface cracks to form.Austenitic steels (e.g. in converter baskets) develop thin, hard nitrided layers thattend to flake off.13

Hydrogen Attack At high temperatures and pressures, hydrogen that is present inammonia synthesis can dissociate, the atomic hydrogen entering the steel lattice toreact with carbon to form methane. This weakens the steel in two ways: by removingcarbon from the steel (decarburization) and by forming blisters or fissures in sodoing. This can eventually cause the vessel or pipe to rupture, often without anyobvious prior deformation. Areas around weld seams are particularly prone to thisphenomenon.

The risk of attack may exist at temperatures as low as 200�C (392�F) and hydrogenpartial pressure as low as 7 bar. Selection of appropriately resistant materials canlargely eliminate this problem. The classic Nelson curves give guidance on the sta-bility limits of various alloys in terms of temperature and hydrogen partial pressure.These original guidelines have been modified in recent years on the basis of ongoing


Table 5.3 Ammonia SCC Mitigation Measures

Corrosion MitigationScheme Refrigerated Tanks Road Tankers

Eliminate oxygen This is a normalcommissioning procedure,but regular inspectionreintroduces oxygen. Thenumber of inspectionsshould be minimized; on-line inspection is desirable.

Not usually practical.

Passivate ammonia with0.2% water

This is now normalpractice.

Vapor space not protected.

Use steel with �300 MPaactual yield strength

Most tanks are constructedof grade 490 steel (ASTMA515 or equivalent), soactual yield easily exceeds300 MPa.

Tankers in Australia areoften made of quenchedand tempered steel whereyield �600 MPa to reducetare weight.

Stress relieve Not practical. Mandatory, especiallywhere yield �300 MPa,which is almost always.

Inspect regularly Traditionally tanks wereshut down to inspect.Inspection online nowbecoming more possibleand used.

Inspection is mandatory,usually every 1 or 2 years.

experience. For example, low-alloy steels with 0.25% and 0.5% molybdenum arenow classed as unalloyed steels from the point of view of resistance to hydrogenattack.13

Steels with low levels of molybdenum and no chromium failed in a number ofcases in catalytic reforming service, so care was recommended in the use of thesealloys for that application. However, failures caused by hydrogen also occurred inhydrodesulferization, ammonia synthesis, and other parts of ammonia productionplants, leading to their removal from these safe operating charts.14 Curves that in-corporate findings from the extensive operating experience are available that pro-vide safe operating conditions for carbon and low-alloy steels (see Figure 5.1).15,16

The standard chromium-molybdenum steels are being modified and improvedso that they can be used at higher temperatures. A modified 21/4% Cr, 1% Mo hasbeen developed that is usable at temperatures up 484�C (903�F) instead of beinglimited to 454�C (849�F) as are conventional steels of this type, and this higher tem-perature operation is permitted under ASME II rules. The modification included theaddition of 1/4% V. The API 941 standard has placed this modified steel at the samelimits in terms of temperature and hydrogen as the 3% Cr, 1% Mo steel. The first


Figure 5.1 Operating Limits for Steels in Hydrogen Service, API 941

vessel made from this steel was an ammonia converter fabricated in 1995. The va-nadium modified steel also has a better resistance to temper embrittlement thandoes the standard, unmodified version.16

Hydrogen damage can also occur from the dissociation of molecular hydrogeninto atomic hydrogen at elevated temperatures. Atomic hydrogen can diffuse intometal structures and recombine into molecular hydrogen at defects or discontinui-ties within the metal. This is also known as hydrogen embrittlement and is usuallyassociated with welds that have not received correct PWHT. In contrast to the phe-nomenon described previously, this effect of hydrogen is reversible, and hydrogencan diffuse back out of the structure if held at atmospheric pressure at around 300�C(572�F). Slow cooling from elevated temperature and pressure operation is oftenrecommended to permit this diffusion process to occur.

Temper Embrittlement If heat-resistant steels are held at tempering temperatures,that is, above about 400�C (752�F), for long periods, their impact properties candecline. The transition temperature between ductile and brittle behavior can be el-evated to 60�C (140�F) from the normal zero or below. This tendency to embrittlecan be reduced by controlling the level of trace elements (silicon, phosphorus, man-ganese, and tin) in the steel. In this respect, modern steels are generally much cleanerthan some of the older steels and so are less prone to this phenomenon. Vessels orpipes in which temper embrittlement is anticipated should not be pressurized atlow temperatures.

Hydrogen Sulfide Attack Most high-temperature steels are attacked by hydrogensulfide in the gas stream in partial oxidation plants. The use of austenitic stainless


steels eliminates this problem, but stress relief of welds is advised in these plants toavoid SCC by chlorides sometimes present in the feed oil.13

Alloy Steels

Steels alloyed with small amounts of chromium, molybdenum, nickel, or other ele-ments are referred to as low-alloy steels, and they can provide an economical alter-native to high-alloy steels at both high and low temperatures. Low-alloy steels havebeen used effectively in ammonia storage vessels. These steels are also subject toSCC, with high stresses and air contamination being the main factors causing crack-ing. The addition of 0.1% to 0.2% water inhibits this attack.2 Chromium molybde-num steels are the most common steels of this type used in ammonia applications.

Chromium-Molybdenum Steels Alloy chromium-molybdenum steels are com-monly used at elevated temperatures where the alloying additions provide resis-tance to hydrogen attack and increase the strength of the alloy. Typical alloys are1.25% Cr, 0.5% Mo (K11597); 2.25% Cr, 1% Mo (K21950); 5% Cr, 0.5% Mo (K41545);7% Cr, 0.5% Mo (S50300); and 9% Cr, 0.5% Mo (S50400). Their use in ammoniaservice is chiefly in ammonia synthesis, the alloy selection being based on API Pub-lication 941.15 Old editions of this standard have curves showing the resistance ofcarbon, 1/2% molybdenum alloys (with no chromium addition). As mentioned pre-viously under hydrogen resistance, service experience has shown that this alloy haslittle more resistance to hydrogen attack than carbon steel, so the API publicationwas updated in 1991 to reflect this experience. Other uses for these chromium-molybdenum steels are in high-strength parts such as fasteners.

Nickel Alloy Steels Addition of nickel to steel greatly enhances the low-temperature toughness (impact properties). For this reason, these materials aresometimes specified for low-temperature ammonia service, especially as weld fillermetals. Typically, the nickel alloy steels contain 3.5% (K32025), 6%, or 9% nickel(K81340).

Stainless Steels

Stainless steels are not normally required in ammonia, from the point of view ofcorrosion, but they do find many applications. All grades of stainless steel are re-sistant to ammonium hydroxide solutions at up to the atmospheric boiling point.

Stainless steels can be classified into three groups according to metallurgical struc-ture and response to heat treatment. These are the martensitic, ferritic, and austeniticgroups. Further subdivisions include duplex alloys with austenitic/ferritic micro-structures and precipitation-hardening (PH) grades strengthened by an age-hardening treatment.


Ferritic Grades

Ferritic stainless steels show a transition from ductile to brittle behavior over a nar-row temperature band. This transition can occur above room temperature in steelswith high levels of carbon, nitrogen, or chromium. This effect, combined with atendency to sensitize from heating, limited the usefulness of the early ferritic stain-less steels and restricted them to thin sections.

The modern low-carbon, low-nitrogen grades (with or without stabilizing addi-tions) have limited toughness and are still usually restricted to thin sections. Theferritic stainless steels are resistant and sometimes immune to chloride stress cor-rosion cracking. This type of alloy can be subject to 475�C (857�F) embrittlementcaused by precipitation of �� chromium–rich phase. The ferritic grades are also par-ticularly prone to r-phase precipitation because of their high chromium and mo-lybdenum contents. Both types of embrittlement can be removed by heating andrapid cooling.

The first superferritic steels were based on 26% Cr, 1% Mo (S44627) and the co-lumbium stabilized XM-27 (S44627). These were developed to provide better resis-tance to chloride SCC than the austenitic 300 series. These ferritic steels have lowinterstitial content with high chromium and very low carbon levels. Most modernsuperferritic stainless steels are based on a 29% Cr, 4% Mo alloy, and they need lowC � N levels (i.e. less than 0.025%) to avoid intergranular corrosion caused bychromium depletion from precipitation of carbides and nitrides. Some of the currentferritics contain higher levels of C � N and have additions of titanium or niobiumas carbon/nitrogen stabilizers.

Superferritic steels include AL 29-4C� (S44735), AL 29-4-2� (S44800), Sea-Cure�(S44660), and Monit� (S44635). The superferritic AL 29-4C� (S44735) has been usedsuccessfully in an ammonia stripper reboiler handling water, ammonia, H2S, andsteam.17

Precipitation-Hardening Grades

The PH stainless steels see limited service in ammonia environments but play animportant role nonetheless. These alloys exhibit very high strengths combined withgood notch toughness and corrosion resistance properties. For this reason, they areoften the preferred material for such parts as valve stems and certain critical fasten-ers. Some of these alloys are susceptible to hydrogen embrittlement in corrosive orhydrogen-rich environments.

Duplex Stainless Steels

Duplex stainless steels have a controlled balance between austenite- and ferrite-bearing constituents. The “duplex” structure contains approximately 50/50 austen-ite and ferrite phases, resulting in higher strength as compared to the 18-8 grades,


as well as improved corrosion resistance in some environments (e.g. chloride-bearing aqueous solutions).

Duplex stainless steels are not commonly used in ammonia service except wheretheir resistance to chloride SCC is useful from the water side of heat exchangers.The modern grades, such as alloy 2205 (S31803), typically also contain molybdenumto achieve a composition of about 22% Cr, 5.5% Ni, 3% Mo, 0.03% C max. They arestrengthened and stabilized by nitrogen. The mixed austenite-ferrite structure im-parts strength and resistance (but not immunity) to chloride pitting and SCC. Theduplex 3RE60 (S31500) has been used to replace standard austenitic stainless steelsin this type of situation. The duplex stainless steels have higher strength than thelower austenitic grades such as type 304 (S30400) but are subject to temper embrit-tlement at about 475�C (885�F).

Welding tends to lead to variations (20%–80%) in the austenite/ferrite ration inthe as-cast weld bead and fusion zone. To minimize this effect, they are welded witha nickel-rich, overmatching rod.

Austenitic Stainless Steels

The austenitic stainless steels constitute a large, diverse body of alloys developedfrom the original 18% Cr, 8% Ni stainless steel, type 302 (S30200). The standardcommercial grade is type 304 (S30400), approximately 18% Cr, 8% Ni, �0.08% C.Additions of titanium or columbium protect against sensitization and chromiumcarbide precipitation by precipitating carbon as titanium or columbium carbides,thus preventing chromium depletion. Types 321 (S32100) and 347 (S34700) are thecommon stabilized grades.

A low-carbon type 304L (�0.03% C max.) version is available and is widely usedto overcome the problems associated with chromium carbide precipitation and chro-mium depletion. A molybdenum-containing variant is type 316L (S31603), contain-ing about 17% Cr, 12% Ni, and 2% to 3 % Mo. The molybdenum addition improvescorrosion resistance in many environments, such as in chloride-containing solutions.There is also a stabilized version of the molybdenum-containing grade that is com-monly used in Europe and is becoming more common in North America. This grade,316Ti (S31635), is a high-carbon stainless steel with the carbon stabilized by theaddition of a titanium addition equal to at least five times the carbon content. Itsmain application is in situations where it is exposed to temperatures between 550�Cand 800�C (1022�F and 1471�F) for prolonged periods.

Modern steelmaking practice has made it possible to produce L grade stainlesssteels with controlled nitrogen additions that have the mechanical properties of theequivalent high-carbon, non–L grade. Dual grade stainless steels 304/304L and 316/316L are now available and permitted for use in many applications. ASME has ruledthat a dual grade steel may use the straight non–L grade allowable stresses for allforms up to 540�C (1004�F). Improved steelmaking control also means that 316 canbe made more cheaply with the molybdenum content at the low end of the permittedrange of 2% to 3%. This can have a serious effect on the corrosion resistance of thistype of steel. In applications where molybdenum is a key factor in corrosion resis-


tance, a minimum level should be specified, or type 317 or 317L, with 3% to 4% Mo,should be used.18

The standard austenitic grades, such as types 304 (S30400) and 316 (S31600), findwidespread use in ammonia service since they are resistant to general corrosion andammonia stress corrosion cracking. Stabilized and low-carbon grades are not gen-erally needed in ammonia applications. Type 304 (S30400) stainless steel was foundto be resistant to intergranular attack in 28% ammonia solution at room temperatureeven after sensitizing for 1 hour at 677�C (1251�F).19 Stainless steels can, however,fail by SCC that is initiated by sensitization formed during heating, for example,when welding. Such a failure in a reformer tube occurred at the welded junctionbetween the type 321 flange and HK 40 catalyst tube. The weld and HAZ wassensitized when joined using a high-carbon welding rod. The high–nickel alloy weldintended to protect the cast steel (HK 40) from corrosion had inadvertently beenmachined off, exacerbating the corrosion.20

The high cost of stainless steels relative to carbon and low-alloy steels often pre-cludes their use for large equipment, but their many advantages lead to their selec-tion for smaller parts and components. They are used in low temperature servicesince they have an extremely low Nil Ductility Transition Temperature (NDTT) andexhibit excellent notch toughness at temperatures far below the atmospheric boilingtemperature of ammonia. In elevated temperature services in ammonia synthesis,they are used because of their resistance to hydrogen attack and nitriding.

Some shallow nitriding does occur after years of service in aggressive environ-ments, but because of the excellent notch toughness of the base material, this effectdoes not reduce the structural integrity.

Another common area of application of the basic grades is in heat exchangertubes, where the material selection is influenced by the fluid used to heat or coolthe ammonia.

These standard grades can suffer from chloride SCC, particularly in partial oxi-dation plants in which the feed oil contains chlorides. Chloride SCC failures canalso occur under upset condition. An example of such a failure occurred in type 321(S32100) and 310S (S31008) tubes in a waste heat boiler in an ammonium synthesisconverter. The direct cause of this SCC was the failure of a feedwater boiler pumpthat was not repaired or replaced and permitted a buildup of deposits in the system.General and intergranular corrosion was also observed on these tubes.21

There are many more specialty grades of stainless steels that see service wheresome factor limits the use of other materials. A very common example is the use ofspecial ferritic grades or duplex grades of stainless steels as heat exchanger tubeswhere austenitic stainless steels are subject to pitting or chloride stress corrosioncracking from cooling waters. The martensitic grades of stainless steel are usedwhere high strength is required, especially at elevated temperatures, as in compres-sor rotors.

Cast Stainless Steels

The equivalent cast version of types 304 (S30400) and 304L (S30403) are CF8 (J92600)and CF3 (J92700), respectively, and they exhibit approximately the same corrosion


response as the wrought alloys. However, castings can have surface layers contain-ing more than the maximum allowable carbon content of 0.08%, which can signifi-cantly reduce corrosion resistance of the surface.

In the cast form, the difference between CF8 (J92600) or CF3 and the correspond-ing molybdenum-bearing grades CF8M (J92900) and CF3M (J92800) is insignificant.CF 3 or CF 8 is not particularly common, and manufacturers of cast pumps andvalves tend to standardize on CF 8M, which has a broader range of applications.The molybdenum-grade castings are often more available and cheaper than the 304-or 304L-grade equivalents. Since the cast version of these alloys is unlikely to bewelded, there is rarely a justification for specifying the low-carbon grades in thiscase. This assumes that the valves or pumps, if weld repaired by the manufacturer,are properly reheat treated (solution annealed) to restore optimum corrosion resis-tance. Availability and price are likely to favor the non–L grade, and a properlyheat-treated casting in CF 8 or CF 8M is likely to be as corrosion resistant as theirlow-carbon cast or wrought equivalents.

Alloys for Use at Elevated Temperatures

Many of the operations in ammonia production take place at elevated temperatures.Some of the materials used for these applications are conventional iron-based ornickel-based alloys that have been discussed previously. However, these specializedelevated-temperature duties in ammonia production, petroleum refining, and so onhave generated a demand for ever better materials specific to these duties. Many ofthe materials used here are proprietary and have been developed to have highstrength and good creep resistant properties in aggressive, gaseous environments.

The standard stainless steels, types 304, 310, and 347 (S30400, S31000, and S34700),were commonly used but often failed by cracking at elevated temperatures. A betteralloy, HK40 (J94204) with 25% Cr, 20% Ni, was developed, and this became theindustry standard.22

A later material development produced the HP (N08705) alloy with 26% Cr, 35%Ni, which was modified by the addition of alloy elements such as molybdenum,niobium, or tungsten. These modified alloys have improved creep resistance butstill possess good ductility and weldability. Various HP-modified alloys are used inammonia applications, one of the most common being HP 45Nb.5 A more recentdevelopment is the production of HP microalloys in which trace quantities of tita-nium, zirconium, and rare earths are added during casting. These alloys have betterresistance to carburization and better high-temperature creep-rupture properties.22

Other new alloys, many of them proprietary, exist, such as 25% Cr, 35% Ni, 15%Co, 5% W; 28% Cr, 48% Ni, 5% W; and 35% Cr, 45% Ni plus additional elements,and these offer higher strength and good high-temperature properties.23

There has also been some development of coatings to resist high-temperatureattack by ammonia. Uncoated types 304 (S30400) and 316 (S31600) and the samesteels coated with silica applied by a sol-gel procedure were tested in anhydrous


ammonia at high temperature. The uncoated samples were attacked and formed anitride scale that embrittled the metal. After 115 hours of testing at 500�C (932�F),uncoated samples were completely degraded, while the coated samples were onlylightly attacked. Multilayer coatings were most effective, and the stainless steel sub-strates were not sensitized to intergranular attack by the high-temperature coatingprocess.24

Creep Resistance

Apart from metal loss due to corrosion or oxidation processes, the other importantfactor governing use of alloys is their mechanical properties. Creep is the processwhereby strain or deformation occurs (usually at elevated temperature) under theapplication of stress levels below yield. A component under creep loading mayeventually fail by a process known as stress rupture if the creep stress is not relievedby strain. Alloys discussed previously that have adequate resistance to this me-chanical phenomenon and to high-temperature corrosive attack have been devel-oped and are used in ammonia furnaces and other similar applications.

Metal Dusting

The phenomenon of metal dusting occurs during high-temperature operation, suchas in steam superheaters downstream of the secondary reformer. It is related to theprocess of carburization, in which carbon migrates into the structure, forming hardcarbides. Carburization occurs above about 800�C (1471�F) in the presence of hy-drocarbons that crack to provide the carbon.

Metal dusting occurs at 500�C to 800�C (932�F–1471�F) on iron-nickel or iron-cobaltalloys in gases containing carbon monoxide. Carbon formation is catalyzed by iron,nickel, or cobalt, and the effect is to produce a surface dust layer consisting of amixture of metal, oxides, and carbon. This dusting is usually observed as pitting orgeneral corrosion attack. Theoretically, alloys that form protective films of the oxidesof chromium, silicon, or aluminum should be more resistant. Virtually all high-temperature alloys can be prone to this attack, but higher steam/CO levels help, asdoes coating with aluminum. In alloys 601, 625 (N06601, N06625), or similar alloys,the attack is tolerable in normal operation.13

Hydrogen sulfide in the gas offers some protection from metal dusting since theadsorbed sulfur blocks the surface for the adsorption of CO or CH4 and other hy-drocarbons and the molecules cannot adsorb and dissociate if their adsorption sitesare occupied by sulfur. Similarly, dense oxide scales can prevent ingress of carboninto the structure. If sulfur cannot be tolerated in the process, nickel alloys with highchromium and aluminum or silicon additions are the best materials to resist metaldusting.25

Alloy resistance to metal dusting is dependent on its ability to form a protectivechromium-oxide scale and can be ranked according to its chromium equivalence:


Table 5.4 Chromium Equivalents for Alloys Commonly Used in Ammonia Plants

Nominal %

Alloy Cr Ni Si Al Cr Equiv.Expected

Performance

Wrought alloys304 18 10 — — 18 Poor800/800H 20 32 0.3 0.3 22 Poor803 25 35 0.3 0.3 27 Fair310 25 20 0.3 — 26 Fair600 15 72 — — 15 Fair601 22 60 — 1.5 27 Good617 22 52 — 1.2 26 Good214 16 76 — 4.5 30 GoodAPM 22 — — 6.0 440 Best

Cast alloysHK-40 25 20 1.0 — 28 GoodHP-Mod 26 35 1.5 — 30 GoodXTM 35 48 1.5 — 40 Best

Chromium equivalent � Cr% � 3 � (Si% � Al%) (1)

Values of chromium equivalent for alloys (some of which are proprietary) commonlyused in high-temperature applications in ammonia plants are shown in Table 5.4.23

There are also nickel alloys that are said to have exceptional resistance to metaldusting and elevated-temperature corrosion. The standard alloys, such as alloy 600and alloy 601, have been used in these environments, but other alloys have beendeveloped with superior high-temperature behavior. Alloy 693 (N06693) was testedfor a year in CO and 20% hydrogen at 621�C (1150�F). Pit depths measured on thisalloy were only 0.031 mm, while in the same tests pit depths were 8.164, 3.451, 0.033,0.34, and 0.293 mm, respectively, on alloy 800 (N08800), alloy DS, alloy 601 (N06601),alloy 620, and alloy 690 (N06690).26

An example of the failure of alloy 601 (N06601) occurred in a heat exchanger onan ammonia plant after only two and a half years of service. It was determined thatthis failure had been caused by contamination of the process side with steam. Thiscaused mineral deposition that destroyed the protective film within about half ayear after the steam ingress into the process side.27

Another nickel alloy developed for resistance to metal dusting and carburizingis alloy 602CA (N06025). The metal wastage rate of this alloy is compared with othernickel alloys in a strongly carburizing CO-H2-H2O gas (see Figure 5.2).28 This alloyhas been tested in an ammonia plant in Europe and showed no metal dusting attemperatures of 450�C to 850�C (842�F–1562�F). Alloy 601 had some attack, and alloy800H was severely attacked.

There are also coatings being developed to resist metal dusting. Diffusion coatingsbased on oxide formers such as silicon, titanium, chromium, and aluminum have


Figure 5.2 Metal Wastage Rates of Nickel Alloys in a Strongly CarburizingAtmosphere at Elevated Temperature

been tested and found to show good potential to extend the lifetime of iron- andnickel-based alloys under metal dusting conditions.29

Nickel and Its Alloys

The nickel alloys are seldom used in ammonia service except at elevated tempera-tures. They are very resistant to dry ammonia but can be attacked by gaseous am-monia if more than about 1% water is present. They are resistant to anhydrousammonia and exhibit good resistance to nitriding. The resistance to nitriding in hot,flowing ammonia of various nickel alloys is compared with standard stainless steelsin Table 5.5.30

Further data in Table 5.6. show the effect of temperature on nitrogen absorptionand nitriding depth for nickel alloys compared with a type 310 (S31000) stainlesssteel.31 Samples were exposed to pure ammonia for 168 hours.

Long-term nitriding tests on alloy 800H/800HT compared with standard stainlesssteels showed the increased resistance of the high-nickel alloy (see Table 5.7).32 Testconditions were 65% hydrogen, 35% nitrogen, at 11,000 psi (75.8 MPa) and 1000�F(540�C).

Nickel-chromium alloys with 50% to 80% nickel resist pure NH3 to about 500�C(932�F). Nickel is attacked by wet ammonia in the presence of air in a manner anal-ogous to the reaction with copper.


Table 5.5 Nitrogen Absorption in Nickel Alloys Exposed to Flowing Ammonia at1200�F (648�C)

Alloy Nitrogen Absorption (mg/cm2)

230 0.7600 0.8S 1.3X 1.7800H 4.3310 7.4304 9.8

Table 5.6 Effect of Temperature on Nitriding Depth in Various Nickel Alloys

Nitrogen Absorption (mg/cm2)Nitriding

Depth (lm) Mils

Alloy1200�F(650�C)

1800�F(980�C)

2000�F(1090�C)

1800�F(980�C)

2000�F(1090�C)

214 1.5 0.3 0.2 1.4 (35.6) 0.7 (18)230 0.7 1.4 1.5 4.9 (124) 15.3 (389)617 1.3 1.5 1.9 15.0 (381) �22 (559)601 1.1 1.2 2.6 6.6 (168) �23 (584)X 1.7 3.2 3.7 7.4 (188) �23 (584)556 4.9 6.7 4.2 14.7 (373) �20 (508)800H 4.3 4.0 5.5 11.1 (282) �30 (762)310 7.4 7.7 9.5 15.1 (384) �31 (787)

Table 5.7 Nitriding Tests in an Ammonia Converter

Depth of Nitriding (mm [in.])

Alloy 1-Year Exposure 3-Year Exposure

800H/800HT 0.137 (0.0054) 0.135 (0.0053)310 stainless steel 0.224 (0.0088) 0.234 (0.0092)309 stainless steel 0.241 (0.0095) 0.244 (0.0096)446 stainless steel 1.059 (0.0417) 1.151 (0.0453)304 stainless steel 1.085 (0.0427) 1.118 (0.0440)


Table 5.8 Corrosion of Alloy 200 in 1 N Ammonium Hydroxide (1.7% NH3)

Test Conditions Corrosion Rate (mpy [mm/y])

Total immersionQuiet 0.8 (0.020)Air agitated �0.1 (�0.001)

Alternate ImmersionContinuous 2.7 (0.069)Intermittent 0.4 (0.01)Spray—4–30 days �0.1 (�0.001)

Nickel alloy 200 (N02200) will resist ammonium hydroxide only up to about 1%concentration. Dissolved oxygen may maintain passivation up to about 10% con-centration. Higher concentrations are highly corrosive to nickel even in the presenceof air (see Table 5.8).33

The corrosion rate of alloy 200 (N02200) in agitated solutions of various strengthsof ammonium hydroxide at room temperature is shown in Table 5.9.34 These datashow that with agitation, nickel is attacked by moderate concentrations of ammo-nium hydroxide even at room temperature.

Alloy 400, with about 30% copper content, is more resistant than alloy 200 asshown in Table 5.10.34 In solutions of �3% ammonium hydroxide, corrosion rate isincreased considerably by aeration and agitation.

Table 5.9 Corrosion of Alloy 200 in Agitated Ammonium Hydroxide Solutions atRoom Temperature

NH4OH Concentration (%) Corrosion Rate (mm/y [mpy])

1.1 012.9 14.2 (560)20.2 9.4 (370)27.1 4.6 (180)

Table 5.10 Corrosion of Alloy 400 in Agitated Ammonium Hydroxide Solutions atRoom Temperature

NH4OH Concentration (%) Corrosion Rate (mm/y [mpy])

2.7 03.6 1.8 (70.9)5.5 7.6 (299)8.2 8.1 (319)

11.1 8.3 (327)18.3 5.9 (232)25.8 0.9 (35.4)


Figure 5.3 Corrosion of Various Copper Alloys in Deaerated Ammonia

The corrosion rate of alloy 800 in 5% and 10% ammonium hydroxide at 80�C(176�F) was �0.003 mpy (�0.1 mpy) in 7-day laboratory tests.32

The nickel-chromium-molybdenum alloys such as N06625, N10276, and so on areresistant but find no application because of the adequate resistance of lesser alloys.

Copper and Its Alloys

Copper alloys are generally to be avoided in ammonia service. Although resistantto pure, dry NH3, contamination by water and oxygen will cause SCC and generalcorrosion. Corrosion of various copper alloys in deaerated ammonia is shown inFigure 5.3. and is compared with the behavior of the same alloys in aerated ammoniain Figure 5.4.35 These data clearly show the corrosive effect of the presence of air inthe solution. Carbon steel (A-285) is included in these data for comparison, and itscorrosion behavior is also adversely affected by the presence of oxygen at loweroxygen levels.

Ammonia and copper typically react to form an intensely blue copper/ammo-nium complex. All copper-based alloys can be made to crack in ammonia vapor,ammonia solutions, ammonium ion solutions, and environments in which ammoniais formed. It is generally true that any metal with a small grain size is more resistant


Figure 5.4 Corrosion of Various Copper Alloys in Aerated Ammonia

to SCC regardless of whether the cracking is transgranular or intergranular. Theeffect of grain size on the time to cracking of yellow brass (C26800) in ammonia isshown in Figure 5.5.36

While some copper-based alloys are far superior to others in their resistance anddry anaerobic ammonia does not cause corrosion, it is general industry practice toavoid all use of copper-based alloys in ammonia and related services. It can be notedin passing that copper in solid solution in ferrous metals (generally less than 3%)added to attain certain physical, mechanical, or corrosion properties does not posea problem in ammonia applications.

Copper alloys C11200 and C26000 were penetrated at a rate of 5 lm/y (0.2 mpy)in anhydrous ammonia at atmospheric temperature and pressure. Corrosion rateswere also low if small amounts of water were present, but oxygen was also probablyexcluded.2

Copper alloy tubes in utility condensers have often been attacked by ammoniaon the steam side. The ammonia in this case comes from decomposition of aminesor hydrazine added to the boiler feed to control oxygen and passivate the boilersurface. Admiralty brass is commonly used to tube such condensers and has beensubject to this attack by ammonia. Copper alloy condensers form a surface layer ofcuprous oxide when placed in surface. If excess oxygen is present, this oxide layeris converted to cupric oxide, which is readily complexed by ammonia. Attack byammonia in this type of utility condenser is accelerated by air in-leakage. In caseswhere boiler chemistry cannot be altered or controlled to avoid ammonia formation,a more resistant alloy, such as copper-nickel, is used.37,38


Figure 5.5 The Effect of Grain Size on the Time to Cracking of Yellow Brass(C26800) in Ammonia

All copper-based alloys are attacked by ammonium hydroxide unless air is rig-orously excluded, which is not feasible in plant practice. The deep royal blue of theammonium/copper complex is immediately obvious.

Dealloying can also occur in some copper alloys in ammonia solutions. It wasfound that dezincification occurred in 70/30 brass in 10 N ammonium hydroxidesolution at 32�C (90�F). Corrosion and dezincification were increased by the appli-cation of increasing stress to the alloy. The mechanism of this increased attack wasshown to be due to an increase in open-circuit potential and a shift in the polari-zation curve under the influence of applied stress.39

Titanium and Its Alloys

Titanium in not attacked by atmospheres containing ammonia but can be corrodedat elevated temperatures. The protective oxide film is effective in ammonia up to atleast 300�C (572�F).2 At higher temperatures, ammonia will decompose into nitrogenand hydrogen that may cause hydrogen embrittlement of titanium. Titanium cor-roded at 440 mpy (11.2 mpy) in an ammonia-steam mixture at 431�F [221�C]). Thishigh corrosion rate was thought to be associated with hydriding. Titanium showsexcellent resistance to corrosion in up to 70% ammonium hydroxide up to the boilingpoint.40 The corrosion rate of titanium in 100% anhydrous ammonia is �0.13 mm/yat 40�C (104�F). In 28% ammonium hydroxide solution at room temperature, the rateis 0.0025 mm/y.41


Zirconium and Its Alloys

Zirconium is resistant to ammonia even at elevated temperatures. The corrosion rateof zirconium Zr702 (R60702) in wet ammonia at 38�C (100�F) is less than 5 mpy(�0.127 mm/y) and in 28% ammonium hydroxide at up to 100�C (212�F) is �1 mpy(�0.025 mm/y).42 Zirconium is stable in ammonia up to about 1000�C (1832�F).2

Niobium

Niobium is not attacked by 13% and 25% ammonium hydroxide solutions at 20�Cto 100�C (68�F–212�F).1

Tantalum

Tantalum is resistant to anhydrous liquid ammonia but should not be exposed tothe gaseous mixtures encountered in ammonia synthesis at elevated temperature.Above about 250�C (482�F), it reacts rapidly with hydrogen to form brittle hydrides.Tantalum is not corroded by 10% aqueous ammonium hydroxide solutions up to100�C (212�F) but is attacked by hot, concentrated ammonium hydroxide solutions.1

Other Metals and Alloys

Lead is resistant in ammonia at temperatures up to 60�C (140�F), and hard lead issatisfactory up to 100�C (212�F) in dry ammonia. Lead is also resistant to ammoniumhydroxide at room temperature.43 The corrosion rate is very sensitive to the presenceof air and agitation. In 27% ammonia solution at 20�C (68�F), lead had a corrosionrate of 110 g/m2/d (3.3 mm/y) with rapid agitation and only 21 g/m2/d (0.63mm/y) without agitation.5

Tin is resistant to dry ammonia and saturated ammonia solutions, but dilutedammonia solutions corrode tin.

Zinc is not resistant to ammonium hydroxide.Magnesium is not attacked by wet or dry ammonia at ordinary temperatures, but

attack may occur if water vapor is present.The precious metals are also not used. In fact, there is a potential hazard if silver

is exposed under some conditions because explosive azides may be formed. Preciousmetals are not employed, and silver or silver-rich alloys are not to be employed inammonia or ammonium hydroxide.


References

1. Anon, Dechema Corrosion Handbook, Ammonia and Ammonium Hydroxidesection, CD-ROM (Frankfurt, Germany: Dechema aV, 2001).

2. B.D. Craig, D.B. Anderson, eds., Handbook of Corrosion Data (Materials Park,OH: ASM International, 1997), pp. 128–135.

3. Anon, Aluminium with Food and Chemicals (London, UK: Alcan Industries Ltd,1966), p. 18.

4. F.L. LaQue, H.R. Copson, Corrosion Resistance of Metals and Alloys, 2nd ed.(New York, NY: Reinhold Publishing Corp., 1963), 365 pp.

5. E. Rabald, Corrosion Guide (Amsterdam, Netherlands: Elsevier Scientific Pub-lishing Co., 1968), pp. 46–50.

6. R. Covert, J. Morrison, K. Rohrig, W. Spear, Ni-Resist and Ductile Ni-Resist Al-loys, reference book no. 11018 (Toronto, ON, Canada: NiDI, 1998), 42 pp.

7. P. Ludwigsen, H. Arup, “Stress Corrosion Cracking of Mild Steel in AmmoniaVapor above Liquid Ammonia,” Corrosion 32, 11(1976): pp. 430–431.

8. A.W. Loginow, “Stress-Corrosion Cracking of Steel in Liquefied Ammonia Ser-vice,” MP 25,12 (1986): pp. 18–22.

9. L. Lunde, R. Nyborg, “Stress Corrosion Cracking of Carbon Steel Storage Tanksfor Anhydrous Ammonia,” Proc. International Fertiliser Society—Proceeding307 (1991).

10. P. McGowan, “Managing Ammonia Stress Corrosion Cracking,” Plenary Ad-dress to the Australasian Corrosion Association Conference (Auckland, NewZealand: Australasian Corrosion Association, 2000).

11. M.M. Ghanem, A.M. Elbatahgy, “Catastrophic Failure of Liquefied AmmoniaGas Cylinder,” MP 42, 4 (2003): pp. 52–55.

12. K.C. Pattnaik, M.P. Gupta, “Stress Corrosion Cracking of Ammonia ReceiverTank,” Br. Corr. J. 30, 1 (1995): p. 80.

13. M. Appl, Ammonia: Principles and Industrial Practice (Weinheim, Germany:Wiley-VCH, 1999), pp. 209–221.

14. K.L. Baumert, G.V. Krishna, D.P. Bucci, “Hydrogen Attack of Carbon-0.5 Mo-lybdenum Piping in Ammonia Synthesis,” MP 25, 7 (1986): pp. 34–37.

15. API 941, “Steels for Hydrogen Service at Elevated Temperatures and Pres-sures in Petroleum Refineries and Petrochemical Plants” (Washington, DC: API,latest ed.).

16. L.P. Antalffy, G.T. West, “The New Generation Vanadium Modified Steels,” min-utes of EFC WP15 meeting, Total Fina Elf (Paris, France, 2002).

17. Anon, “AL 29-4C,” Technical Data Sheet B-151-Ed5/7.5M/793/GP (Pittsburgh,PA: Allegheny Ludlum Steel Corporation, 1993), 8 pp.

18. G. Kobrin, J. Lilly, J. Mac Diarmid, B. Moniz, “Stainless Steels for ChemicalProcess Equipment,” NiDI reprint series no. 14 038 (Toronto, ON, Canada: NiDI,1998), pp. 1–9.

19. J.M. Stone, “Corrosion Resistance of Nickel and Nickel-Containing Alloys inCaustic Soda and Other Alkalies,” CEB-2 (New York, NY: International NickelCompany Inc., 1973), p. 20.


20. S.K. Bhaumik, R. Rangaraju, M.A. Parameswara, T.A. Bhaskaran, M.A. Venka-taswamy, A.C. Raghuram, R.V. Krishnan, “Failure of Reformer Tube of an Am-monia Plant,” Engineering Failure Analysis 9 (2002): pp. 553–561.

21. K.M. Verma, H. Ghosh, K.C. Pattnaik, “Stress Corrosion Failure of Waste HeatBoiler Tubes in an Ammonia Synthesis Converter,” Br. Corr. J. 15, 4 (1980): pp.175–178.


23. S.B. Parks, C.M. Schillmoller, “Improve Alloy Selection for Ammonia Furnaces,”Hydrocarbon Processing (Int. ed.) 76, 10 (1997): pp. 93–98.

24. O. de Sanctis, L. Gomez, N. Pellegri, A. Durfi, “Behaviour in Hot AmmoniaAtmosphere of Sio2-Coated Stainless Steels Produced by a Sol-Gel Procedure,”Surface and Coatings Technology 70 (1995): pp. 251–255.

25. H.J. Grabke, “Metal Dusting,” Materials and Corrosion 54, 10 (2003): pp. 736–746.

26. S. McCoy, “Inconel Alloy 693 Exceptional Metal Dusting and High TemperatureCorrosion Resistance,” minutes of EFC WP15 meeting, Total Fina Elf (Paris,France, 2002).

27. H.J. Grabke, M. Spiegel, “Occurrence of Metal Dusting—Referring to FailureCases,” Materials and Corrosion 54, 10 (2003): pp. 799–804.

28. D.C. Agarwal, L. Stewart, M. McAllister, “Alloy 602CA (UNS N06025) SolvesPig Tail Corrosion Problems in Refineries,” CORROSION/2003, paper no. 03495(Houston, TX: NACE International), 17 pp.

29. C. Rosado, M. Schutze, “Protective Behaviour of Newly Developed Coatingsagainst Metal Dusting,” Materials and Corrosion 54, 11 (2003): pp. 831–853.

30. Anon, “Haynes 230 Alloy for Industrial Heating Applications—Data Summary,”Brochure H-3033G (Kokomo, IN: Haynes International, 2002), p. 19.

31. F.G. Hodge, “High Performance Alloys Solve Problems in the Process Indus-tries,” CORROSION/91, paper no. 173 (Houston, TX: NACE, 1991), p. 9.

32. Anon, “Solutions to Materials Problems,” CD-ROM (Huntington, WV: Inco Al-loys International, 1997).

33. Anon, “Corrosion Resistance of Nickel and Nickel-Containing Alloys in CausticSoda and Other Alkalies,” CEB-2 (New York, NY: International Nickel CompanyInc., 1973), p. 21.


35. N.W. Polan, et al. (1981), in Metals Handbook—Corrosion, vol. 13, 9th ed., ed.J.R. Davis (Metals Park, OH: ASM International, 1987), p. 622.

36. H.H. Uhlig (1971), in Metals Handbook—Failure Analysis and Prevention, vol.11, ed. G.W. Powell, S.A. Mahmoud (Metals Park, OH: ASM International, 1986),p. 208.

37. B.J. Buecker, “Watch Out for Steam-Side Corrosion in Utility Condensers,” MP31, 9 (1992): pp. 68–70.

38. B.J. Buecker, E. Loper, “Steam Surface Condenser Tubes: Watch Out for SneakyCorrosion,” MP 39, 5 (2000): pp. 60–64.


39. T.K.G. Namboodhiri, R.S. Tripathi, “The Stress-Assisted Dezincification of 70/30 Brass in Ammonia,” Corrosion Science 26, 10 (1986): pp. 745–756.

40. Anon, “Corrosion Resistance of Titanium,” brochure no. TMC-0105 (Denver,CO:Timet, 1999), p. 22.

41. Anon, “Corrosion Resistance of Titanium,” ref. no. 1431531969 (Birmingham,UK: IMI Kynoch Ltd, 1969), pp. 28, 36.

42. Anon, “Zircadyne Corrosion Data,” bulletin no. TWCA-8101ZR (Albany, OR:Teledyne Wah Chang, 1987), 25 pp.

43. Anon, “Corrosion of Lead” (London, UK: Lead Development Association, 1971).

47

6Resistance of NonmetallicMaterials

Nonmetallic materials (i.e. elastomers, plastics, ceramics, and carbon products) findfew applications in ammonia systems except occasionally as O-rings and gaskets.Some nonmetallic materials do, however, find limited applications in ammoniumhydroxide service.

Elastomers

Some elastomeric materials perform well in strong bases such as ammonia. Satis-factory materials include perfluoroelastomers, special grades of fluorohydrocarbonelastomers (e.g. Viton�TBR-S), polychloroprene rubber, acrylonitrile rubber (Buna-N�), and butadiene-styrene rubber (Buna-S�). Ethylene propylene diene monomerrubber (EPDM) is resistant to ammonia but may be attacked by oils present in com-pressed gas systems. Buna-N� elastomer are suitable for cold ammonia gas, but theirperformance is only fair to poor in hot gas. Neoprene is suitable in cold ammoniawhile PTFE is acceptable across the temperature range in ammonia.1

Satisfactory materials in ammonium hydroxide include perfluoroelastomers andpolychloroprene rubber. Some elastomers that are attacked and are unsatisfactoryare butyl, general purpose FKM, hard rubber, isoprene, and natural rubber.

Temperature limits for various elastomers are given in Table 6.1. These data area compilation from various sources. Testing is recommended for applications in-volving elastomers in specific ammonia environments.

Plastics

Most plastics are chemically resistant to ammonia and ammonium hydroxide atambient temperatures. However, because of the hazardous nature of ammonia, the


Table 6.1 Estimated Temperature Limits (�C [�F]) for Various Elastomers That May BeSuitable in Ammonia

Elastomer FamilyGaseousAmmonia

LiquidAmmonia

SaturatedAmmoniaSolution

Chloroprene (CR, neoprene) 60 (140) 20 (68) 80 (176)Chlorosulfonated polyethylene (CSM, Hypalon�, etc.) 60 (140) 20 (68) —Ethylene propylene (EPR, EPDM, Nordel�, etc.) 60 (140) 20 (68) 80 (176)Fluoroelastomer (FKM, Viton� TBR-S, ETP-S) 50 (122) 50 (122) 50 (122)Natural rubber (NR) 60 (140) 20 (68) 60 (140)Nitrile rubber (NBR, Buna-N�, etc.) 40 (104) 20 (68) 60 (140)Perfluoroelastomers (FFKM, Kalrez� 6375, 1050LF, etc.) 260 (500) 240 (464) 260 (500)

Table 6.2 Suggested Temperature Limits (�C [�F]) for Various Plastics in Ammonia

PlasticGaseousAmmonia

LiquidAmmonia

10% AmmoniaSolution

Polyethylene (PE) 60 (140) 60 (140) 60 (140)Polybutylene (PB) 60 (140) 20 (68) 60 (140)Polypropylene (PP) 60 (140) 60 (140) 60 (140)Polyvinyl chloride (PVC) 60 (140) 60 (140) 60 (140)Chlorinated PVC (CPVC) 40 (104) fair — 40 (104) fairPolyvinylidene fluoride (PVDF) 80 (176) 40 (104) 100 (100)Polytetrafluoro ethylene (PTFE) 120 (248) 120 (248) 120 (248)

use of plastics is generally not recommended except in seals and gaskets, wherefluoropolymers are typically used.

Thermoplastics

Resistant grades of fluoropolymers are ECTFE (Halar�), ETFE (Tefzel�), PVDF(Kynar�), FEP, PFA, and PTFE (Teflon�). Some plastics that are generally unsatis-factory are ABS, epoxy, certain polyesters, polyisobutylene, and polystyrene. Dataon temperature limits for various common thermoplastics are shown in Table 6.2.These data are a compilation from various sources. Various manufacturers list ele-vated temperature limits for plastics in liquid ammonia, and these are included inthis table. They are not, however, of very practical interest since it is highly unlikelythat plastics would be used in elevated temperature (therefore, also high pressure)liquid ammonia. In the unlikely event that an application for a plastic was being


considered in liquid ammonia, the subzero properties would be more relevant andshould be investigated.

One area of interest is the difference in behavior of PVC and CPVC. In manyenvironments, CPVC is more resistant than PVC and can withstand higher tem-peratures. This is not the case, however, in ammonia and amines. PVC has generallygood resistance to ammonia and some amines, even at somewhat elevated tem-peratures, while CPVC has extremely poor resistance to ammonia or ammoniumhydroxide and limited resistance to most amines, even at ambient temperatures.This is due to the extremely high reactivity of amines and chlorine, the higher avail-ability of chlorine on the CPVC, and its lower bond strength on CPVC versus PVC.Even at fairly low concentrations and temperatures, ammonia and many amines arecapable of rapid dehydrochlorination of CPVC. One CPVC pipe handling 28% am-monium hydroxide at ambient temperature failed after only one year in service.2

If polyethylene is used to store concentrated aqueous ammonium solutions, thereis a weight loss due to outward diffusion through the plastic. A solution of 27%ammonium hydroxide solution kept in a 500-cm3 bottle, 1-mm wall at 20�C (68�F)for 54 days lost 3.6% of its weight.1

Use of many of these thermoplastics is confined to linings for pipe and vesselsrather than as solid construction items because of the hazardous nature of am-monia. Recommended temperature limits for plastic-lined steel pipes are given inTable 6.3.3

Thermoplastics are also used as the resistant liner in dual laminate constructionin which fiber-reinforced plastic (FRP) is used as the reinforcing, structural element.The corrosion resistance depends on the resistance of the thermoplastic liner, al-though resistant resins are often used in the FRP reinforcement in case of permeationand leaks in the thermoplastic liner. Most common thermoplastics are used in thistype of construction, and many of them are suitable for use in ammonia applications(see Table 6.4).4

Table 6.3 Recommended Temperature Limits for Plastic-Lined Pipe in Ammonia

Ammonia or Hydroxide PP PVDF PTFE

Anhydrous gas 150�F (65�C) NR 450�F (230�C)Anhydrous liquid 225�F (110�C) NR 450�F (230�C)1% ammonium hydroxide 225�F (110�C) 225�F (110�C) 450�F (230�C)10% ammonium hydroxide 225�F (110�C) 225�F (110�C) 450�F (230�C)Concentrated Ammonium hydroxide 225�F (110�C) 225�F (110�C) 450�F (230�C)

Note: NR � not recommended

Thermoset Resins

The fiberglass-reinforced thermoset composites, commonly called FRP, have a resis-tance determined by the polymer used. Some suggested concentration and tem-


Table 6.4 Suggested Temperature Limits (�C [�F]) for Various Plastics in Dual Lami-nate Construction in Ammonia

Plastic Gaseous Ammonia(Technically Pure)

Aqueous AmmoniumHydroxide (�25%)

PVC 60 (140) 40 (104), 60* (140)CPVC (chlorinated PVC) — 40* (104)PE 60 (140) 60 (140)PP 60 (140) 60 (140)PVDF 40 (104), 100* (212) —ECTFE (ethylenechlorotrifluoroethylene)

20 (68) 100 (212)

ETFE (ethylenetrifluoroethylene)

150 (302) 150 (302)

FEP (fluorinated ethylenepropylene)

150 (302) 150 (302)

PFA (perfluoro alkoxy) 150 (302) 150 (302)

*Conditionally resistant at this temperature. The medium can attack the material or causeswelling. Restrictions must be made in regard to pressure and/or temperature, taking theexpected service life into account. The service life of the installation can be noticeablyshortened.4

Table 6.5 Suggested Temperature Limits for FRP in Ammonia Service

Ammonia orAmmonium Hydroxide

Bisphenol AFumurate Vinyl Ester Epoxy Furane

Liquefied ammonia NR NR NR NRAmmonia gas, dry 140�F (60�C) 200�F (93�C) — —Ammonia gas, wet 200�F (93�C) 200�F (93�C) — —5% ammonium hydroxide 180�F (82�C) 180�F (82�C) 180�F (82�C) 100�F (38�C)10% ammonium hydroxide 140�F (60�C) 140�F (60�C) 160�F (71 oC) 100�F (38�C)20% ammonium hydroxide 140�F (60�C) 140�F (60�C) 151�F (66�C) NR29% ammonium hydroxide 100�F (38�C) 100�F (38�C) 126�F (52 oC) NR

Note: NR � not recommended*Limiting allowable concentration

perature limitations of FRP are as shown in Table 6.5. Data are a compilation fromvarious sources.

Standard FRP piping is also available in a number of different resins and typesof construction. The recommended limits of application of one commercial supplierare shown in Table 6.6.5,6 Some of these pipes are cast, and others are filamentwound, depending on the resin system and application.


Tab

le6.

6Estim

ated

Tem

perature

Lim

its

�F(�C)of

Com

mercialFR

PPiping

Ammonia

Epo

xy(Red

ThreadII)

Epo

xy(Green

Thread)

Chem

Thread

Prop

rietary

(Z-Core)

Epo

xy(RB-2530,

RB-1520)

Vinyl

ester

(CL-2030)

Prop

rietary

(CL-1520)

Various

Resins

(F-CHEM

6

F-CHEM-V)

Dry,anh

ydrous

gas2,4

150(66)

225(107)

100(37.8)

275(135)

150(66)

1100(37.8)

100(37.8)

1

Wetgas

——

——

150(66)

1100(37.8)

100(37.8)

Liquid

NR

NR

NR

NR

NR

NR

NR

5%solution

120(49)

150(66)

180(82)

200(93)

150(66)

150(66)

1803

,5

10%

solution

120(49)

150(66)

165(74)

200(93)

150(66)

150(66)

150(66)

3,5

20%

solution

100(37.8)

125(52)

150(66)

200(93)

150(66)

150(66)

150(66)

3,5

28%

solution

100(37.8)

125(52)

125(52)

200(93)

100(37.8)

100(37.8)

100(37.8)

3,5

Saturatedsolution

——

—175(79)

——

—

Not

e:NR

�notrecommended

1.Maxim

umtemperature

forwhich

inform

ationisavailable;couldbe

serviceableathigh

ertemperatures.

2.Dry

gasesun

der

pressure

cancond

ense

toliq

uidsin

cool

weather.T

hissituationshou

ldbe

avoided.

3.Adou

blesyntheticveillin

erisrecommended.

4.Pipelin

eforallp

ressurized

gasapplications

shou

ldbe

buried

atleast3feetdeep.

Not

recommended

forabovegrou

ndpressurizedgaslin

esif

theop

eratingpressuresexceed

25psig

for1-to

6-inch

pipe,14psig

for8-inch

pipe,9

psig

for10-inchpipe,6

psig

for12-inchpipe,5

psig

for14-

inch

pipe,4

psig

for16-inchpipe,and

1psig

for18-inchandlarger

sizes.

5.Abisphenolv

inyl

esteror

epoxyresinispreferredforthisapplication.

6.Based

onstandardbisphenolA

viny

lester

resin.


Figure 6.1 Wastage Rates (in mm/y) of Glass-Lined Steel in Ammonia.Vol: Surface Area � 20

Carbon and Graphite

Carbon and graphite are resistant to ammonium hydroxide. However, with imper-vious graphite heat exchangers, an epoxy resin should be specified rather than phe-nolic, which is attacked by alkaline chemicals. Carbon and graphite are resistant toall solutions of ammonia up to their limiting temperature, which depends on theindividual grade and formulation. Graphite is resistant to anhydrous ammonia overthe full range of concentration up to the temperature limit of the graphite.Fluorocarbon-bonded graphite, Diabon F100�, is resistant in 20% ammonia/causticammonia at up to 40�C (104�F).7

Ceramic Materials

Glass is resistant to about 60�C (140�F) in very dilute solutions of ammonium hy-droxide (perhaps 1%) but will withstand solutions to pH 14 at room temperature.See Figure 6.1 for more details on rate of attack on glass-lined steel.8


References

1. Anon, “Corrosion Resistance Guide” (Chicago, IL: Yamada Pumps Inc., 2001),p. 6.

2. M.L. Knight, “Failure Analysis Of PVC and COVC Piping Materials,” COR-ROSION/2003, paper no. 03606 (Houston, TX: NACE International), 7 pp.

3. Anon, “Chemical Resistance Guide” (Bay City, MI: Dow Chemical, 1991), 20 pp.4. Anon, “Chemical Resistance of Thermoplastics Used in Dual Laminate Con-

structions,” DLFA (2002), 143 pp., http://www.dual-laminate.org/html/corrosion_guide.html.

5. Anon, “Chemical Resistance Guide,” bulletin no. E5615 (Little Rock, AR: SmithFibercast, 2003), 16 pp.

6. Anon, “Smith Fibercast FRP Pipe & Fittings” (Sunshine, LA: CorPro Inc., 2003).7. Anon, “The Chemical Industry Builds on Graphite,” brochure no. PE 200/07

(Meitingen, Germany: SGL Carbon Group, 2001), 24 pp.8. Anon, “Worldwide GLASTEEL 9100,” brochure no. SB95-910-5 (Rochester, NY:

Pfaudler Reactor Systems, 2000), p. 5.

55

7Corrosion in ContaminatedAmmonia

There are various kinds and degrees of contamination encountered in ammoniaproduction and usage, and these may profoundly affect the corrosion behavior ofthe materials of construction commonly used in this service. As mentioned in Chap-ter 3, there are some impurities inherent in the manufacturing process that are sub-stantially removed during the refining process. In other instances, the ammoniabecomes contaminated with other impurities or may be mixed with other chemicalsdeliberately for specific purposes. The presence of specific contaminants in ammoniaand ammonium hydroxide streams, other than the oxygen-induced stress corrosioncracking andwater inhibition previously discussed, can cause unexpected corrosion.

Ammonium Chloride

Organic chlorides in naphtha are converted to hydrogen chloride (HCl) in the NHT(naphtha hydrotreater), where they combine with ammonia (NH3) to form ammo-nium chloride (NH4Cl). The amount and rate of NH4Cl formation is controlled bythe HCl level because of the overabundance of ammonia in the NHT effluent comingfrom the conversion of nitrogen in the feed. Depending on the chloride level in thefeed, the location of NH4Cl deposition can shift. Solid ammonium chloride tends todeposit at temperatures as high as 150�C to 170�C, a temperature range often cor-responding to the coldest effluent exchangers. While the dry NH4Cl salt is not cor-rosive, it can cause fouling and plugging of heat exchanger tubes. It is also veryhygroscopic, and the wet salt can be extremely corrosive. Most corrosion problemsin hydrotreaters result from the deposition of wet ammonium chloride.1

Carbon Dioxide—Carbamates

If carbon dioxide, ammonia and large quantities of water are together at ambienttemperatures, noncorrosive ammonium carbonate or bicarbonate is formed. At el-


evated temperatures (�60�C) at atmospheric pressure or above, carbamates such asammonium carbamate NH4CO2NH2 (or NH2COONH4) can be formed. These arevery corrosive to carbon steel (rates of 3 mm/y [120 mpy]) and to stainless steels athigher temperatures and pressures.2 Ammonium carbamate can hydrate to the non-corrosive ammonium carbonate or thermally decompose back to ammonia and car-bon dioxide.

These various reactions may compete in different parts of ammonia strippingstills and give very local areas of corrosion. The corrosive carbamate may be tran-sient, depending on variations in CO2 content, temperature, and so on.

Ammonium carbamate is also a transient intermediate that exists during the pro-duction of urea by the following reactions:3

2NH � CO r NH COONH (ammonium carbamate) � heat3 2 2 4

NH COONH r CO(NH ) (urea) � H O (1)2 4 2 2 2

Carbamates can cause severe corrosion in urea production, and oxygen injection hasbeen used to maintain passivity of stainless steels in this process. Other manufac-turers have used titanium or zirconium to avoid this type of corrosion. In laboratorytests, the vapor-phase corrosion rate of steel increased by an order of magnitudewhen CO2 was bubbled through 28% ammonium hydroxide instead of air.

Severe corrosion has occurred in localized sections of a stripping column in theproduction of alkylamines, in which an alcohol and ammonia are reacted catalyti-cally:

C H OH � NH r C H -NH � H O (2)2 5 3 2 5 2 2

Carbamate corrosion of the steel column and type 410 (S41000) trays occurred in theammonia column, while feed and tails piping, reflux area and reboiler, and 304(S30400) trays were unattacked. The aggressive carbamates form only in local areasunder specific conditions, so this problem is often very localized and severe. Cor-rosion of steel ammonia stripping columns has occurred in alkanolamine andethylene-amine processes. In the former process, replacement of the carbon steel bytype 304 (S30400) stainless steel was successful; in the later process, type 316 (S31600)was attacked, and titanium was found to be necessary.4

Extensive corrosion was found within six months of startup of type 304 (S30403)components in a intercooler and aftercooler of a CO2 gas cleaning circuit in a ureaplant. The failure of the shell, fins, demisters, and sealing strips was identified asbeing due to corrosion by CO2 and/or the reaction product of CO2, ammonia, andwater vapor (probably ammonium carbamate). This attack was aided by the gal-vanic effects between the type 304 (S30400) stainless steel and the 3RE60 (S31500)duplex stainless steel tubes. The use of type 304 for all components and the additionof oxygen to the CO2 have been recommended to avoid this problem with the re-placement exchangers.5

Contamination problems with ammonium hydroxide are rare, although it willabsorb carbon dioxide from air to form ammonium carbonate. Usually, aqueoussolutions do not see high enough temperatures to cause significant corrosion via the


carbamate mechanism. However, steel coupons in the vapor phase over 28% am-monium hydroxide corroded at about 20 mpy when a stream of carbon dioxide waspassed through the vapor space. This test indicated formation and replenishmentof ammonium carbamate corrosion.6

Chlorides

Chloride contamination in ammonium hydroxide does not cause increased corro-sion of ferrous alloys or even significant pitting or crevice corrosion of stainlesssteels. This is presumably due to the high pH of the solution. However, reactionscan occur to produce traces of corrosive species that will cause corrosion problemsif constantly replenished.

References

1. S. Kapusta, F. van den Berg, R. Daane, M.C. Place, “The Impact of Oil FieldChemicals on Refinery Corrosion Problems,” CORROSION/2003, paper no.03649 (Houston, TX: NACE International), 12 pp.

2. C.P. Dillon, Corrosion Control in the Chemical Process Industries, 2nd ed. (St.Louis, MO: MTI, 1994), p. 287.

3. Anon, “Production of Urea and Urea Ammonium Nitrate,” vol. 5 (Brussels, Bel-gium: EFMA, European Fertilizer Manufacturers’ Association, 2000), 44 pp.

4. C.P. Dillon, “Carbamates Can Cause Corrosion Problems,” MP 38, 12 (1999): pp.74–75.

5. H. Shaikh, R.V. Subba Rao, R.P. George, T. Anita, H.S. Khatak, “Corrosion Fail-ures of AISI Type 304 Stainless Steel in a Fertiliser Plant,” Engineering FailureAnalysis 10 (2003): pp. 329–339.

6. C.P. Dillon, Materials Selection for the Chemical Process Industries (New York,NY: McGraw-Hill Inc., 1991), pp. 325–326.

59

8Specific Production Equipment

The corrosion resistance of various materials under a variety of conditions has beendiscussed in previous chapters. Materials of construction that are being or have beenused for specific equipment are detailed in this chapter, which also covers aspectsof design and operation that are relevant to particular types of equipment. Onecritical aspect of design is the specification of suitable materials of construction toadequately resist the aggressive environment. Mechanical design is the other essen-tial design step, and the basic requirements are described in several standards, suchas API 650 for Storage Tanks and ASME Section VIII, Division 1 for Pressure Ves-sels.1,2 All pressure vessels should be built and tested to the requirements of theconstruction code to which they are built. Details on ammonium salts and organicderivatives (amines) are beyond the scope of this monograph.

Production Stages

Different materials are used at various stages of ammonia production. A modernammonia plant is shown in Figure 8.1, and a schematic production flow sheet isshown in Figure 8.2. The items of equipment can conveniently be discussed underthe relevant process stages.

Desulfurization Section

Stainless steel types S30400, S31600, or S32100 are often used in the construction ofequipment items in this part of the process to resist high-temperature attack byhydrogen sulfide and carbonyl sulfide. External corrosion and thinning of firedheater coils and interior deposition of carbon resulting from coking (leading to over-heating) can cause failures. Fuel gas lines containing hydrocarbon vapors and hy-drogen sulfide must be of type 304 (S30400) material and preferably have heat trac-ing to avoid condensation.3


Figure 8.1 Part of a Modern Ammonia Plant Using Coal as a Feedstock (photocourtesy of Sasol Ltd, Secunda, RSA)

Primary Reformer

Steam-hydrocarbon reforming over a nickel catalyst usually takes place in a tubularexternally fired furnace operating at pressures from 25 to 35 bar and at temperaturesfrom 780�C to 820�C (1436–1508�F). The reformer tube can suffer from carburization,oxidation, overheating, stress-corrosion cracking, sulfidation and thermal cycling.The standard stainless steels, types 304 (S30400), 310 (S31000), and 347 (S34700),have failed by cracking, so a better alloy (HK 40, 25% Cr, 20% Ni) was developedand proved satisfactory for vertical reformer tubes, permitting plant capacity toreach 600 t/d. This material has a design life of 100,000 operating hours, but this isconsiderably shortened if overheating occurs. If an HK 40 tube is subjected to aservice temperature of 55�C (100�F) above the prescribed base temperature, its ser-vice life will be reduced by about 90%, as shown in Figure 8.3.4

HP (chromium, nickel) alloys were modified to have improved creep resistancewith good ductility and weldability. The modifications of the HP alloys include

Fig

ure

8.2

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iaPr

oduc

tion

Flow

Shee

tSh

owin

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inci

palI

tem

sof

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ipm

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ourt

esy

M.P

.Suk

umar

anN

air,

FAC

TLtd

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ochi

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Figure 8.3 Effect on Service Life of Overheating an HK 40 Tube above thePrescribed Base Temperature

additions of some combination of molybdenum, niobium, and/or tungsten of whichthe HP 45Nb is one of the most commonly used. Stress to rupture values for thesealloys is compared with the original HK 40 in Figure 8.4.5 These data clearly showthe improvements in elevated temperature properties obtained from these alloys.

When these stronger alloys are specified, thinner-walled tubes can be used, re-ducing tube supports and increasing heat transfer. A number of plants all over theworld have been revamped with HP-modified tubes in their primary reformers,increasing plant capacities by up to 30%. A more recent development is the pro-duction of HP microalloys that have better resistance to carburization and betterhigh-temperature creep-rupture properties.3 Alloys such as 25% Cr, 35% Ni, 15%Co, 5% W; 28% Cr, 48% Ni, 5% W; and 35% Cr, 45% Ni plus additional elementshave been developed to offer higher strength and good high-temperatureproperties.These strong alloys either can provide a higher operating temperature with betterproduct yield or are offered as a substitute for HK-40 or HP-Mod and improve runlength and tube life.5

Originally, HK, HT, and HU alloys were used for risers, manifolds, and transferheaders, and many of these failed from thermal cycling during startup and shut-down. Wrought alloy 800H (N08800) was the normal replacement alloy used. Al-though it is not as strong as the cast alloys, it is more ductile and resistant to thermalshock.5 The cast version of alloy 800H is a cheaper alternative with a higher creep-rupture strength, a low tendency for embrittlement, and good ductility but is stillrarely used in this application. Hot reformed gas transfer lines usually are refractorylined with an interior alloy 800 sheathing.3


Figure 8.4 Stress to Rupture Data for HP Modified Alloys Compared with HK 40

As part of an upgrade at the Ruwais Fertilizer Industries (FERTIL) plant in AbuDhabi, the primary reformer tubes that were IN-519 (24% Cr, 24% Ni) were replacedwith an HP-modified material (KHR 35 CT, Kubota, Japan). The design life of theoriginal tubes was 100,000 hours, and they were replaced in 1998, when they hadcompleted 116,000 operating hours. Because of the improved material characteris-tics, it was possible to increase the ID of the tubes, which accommodated about 24%extra catalyst and allowed increased throughput. At the same time, the secondaryreformer (refractory lined chromium, 1/2% Mo) was replaced by a refractory-lined11/4% Cr, 1/2% Mo unit.6

Secondary Reformer

The secondary reformer operates at 957�C to 1025�C (1755�F–1877�F) and is arefractory-lined vessel with an outer shell of a low-alloy (0.5% Mo) steel. Metaldusting can occur at this stage of the process, and type 304 stainless or alloy 800(N08800) is very susceptible to this type of attack in the temperature range 500�C to800�C (932�F–1471�F). Process controls to keep the CO/CO2 ratio low, the steam/hydrogen ratio high, and temperatures outside the critical range can help avoid thisattack.3

Metal dusting typically occurs on channels, outlet cones, bypass or shell liners,ferrules, and so on in the waste heat boiler or between the waste heat boiler and


steam superheater. If this corrosion is experienced, the alloy can be upgraded ac-cording to the guidelines (see Table 5.4 under metal dusting) and the severity of theattack.5

Hydrogen embrittlement is common in reformed gas pipelines, which are usuallylow-alloy steel downstream of the feedwater heaters where the temperature is�400�C (�752�F). This problem can be avoided by selecting conservative materialsbased on the curves defining safe temperature and hydrogen partial pressure con-ditions (see Figure 5.1).3

High-Temperature Converter

The high-temperature converter, where CO is converted into CO2, is normally madefrom a low-alloy, 1% Cr, 0.5% Mo steel. The converted gas pipeline is usually type304 (S30400) stainless steel to avoid corrosion by the acidic condensate.

Carbon Dioxide Removal System

The materials used in this part of the plant depend on the solvent used to removeCO2. Type 304 or 316 (S30400 or S31600) are commonly used here, but stress-relievedcarbon steel is also used for piping and other equipment. Iron in the circulatingsolution can cause erosion, especially in pumps and bends. In the case of somesolvents, such as methyl diethanolamine (MDEA), additives limit corrosion fromthe carbamates present. In potassium carbonate systems, arsenic and vanadium saltsare added to reduce corrosion. A passive layer of magnetite on steel surfaces ismaintained by the addition of small amounts of air. If stress corrosion cracking (SCC)of carbon steel is a problem, here duplex stainless steels can provide a solution.3

Waste Heat Recovery System

Various grades of ASTM materials such as A213, A312, A335, and A351 provide auseful service life for the boilers and steam superheaters that operate under highheat flux. Control of water quality is also an important factor.3

Ammonia Synthesis

If the synthesis gas contains traces of carbon monoxide and carbon dioxide on mix-ing with the ammonia in the recirculating gas from the synthesis loop, ammoniumcarbamate will be produced. This material will clog and corrode the downstreamequipment. This problem can be avoided only by controlling the carbon oxides levelin the fresh makeup gas to less than 5 ppm.3


The ammonia synthesis converter operates at 150 to 200 bar and at about 515�C(959�F). Nitriding and hydrogen embrittlement can occur in this part of the plant.The pressure shell is usually a multilayer or multiwall carbon-steel vessel. The in-ternal catalyst baskets are usually type 321(S32100) stainless steel. Alloys of chro-mium (12%–20%), nickel (5%–25%) with molybdenum, vanadium, and tungsten areused for the ammonia converter internals, and ASTM A 213, T22 material of the 21/4% Cr, 1% Mo type or their improved versions are used in the boiler. Hydrogenattack can occur in this converter.3

Distillation Columns

Distillation columns are often found in recovery systems in chemical processes in-volving ammonia. Ammonia stripping stills should be fabricated to ASME Coderequirements, usually of carbon steel with type 410 (S41000) valve trays. Higher-alloy column sections and trays may be required if CO2 contamination occurs toavoid problems of carbamate formation.

Heat Exchangers

Heat exchangers are used in ammonia production, in ammonia refrigeration units,and in chemical processing.

Heaters Tube-and-shell heaters using good-quality steam or an inert heat transfermedium are usually of carbon-steel construction, unless the ammonia or ammoniumhydroxide is contaminated.

The cooler part of the combustion air preheater at the tail end of the flue gas heatrecovery train is likely to corrode as a result of sulfur dioxide condensation from inthe flue gas. In this area, cast iron or glass will resist the acid attack. Carbon-steelpreheater tubes, with 1.5 to 2 meters of type 304 (S30400) tubes at the cold end ofthe tube sheet, can also perform well.

The high-pressure feedwater heaters are prone to leaks at the tube-to-tube sheetjoints. A lining or overlay of the tube sheet with a material such as alloy 600 (N06600)and the use of tube materials such as chromium-molybdenum types reduce this riskconsiderably.

Startup heaters, either electrical or directly fired, are used to heat the synthesisgas feed to the ammonia converter during plant startup. Hydrogen-induced cracks,overheating, flame impingement, thinning at the bends, furnace explosions, and soon are problems encountered in this equipment. Type 321 (S32100) stainless steel isoften suitable for startup heater coils and downstream pipeline.3

A number of heaters in ammonia plants have failed and been replaced with3RE60� (S31500). These include carbon-steel heat exchangers such as feedwater


heaters and waste heat boilers that have failed by corrosion and stainless steel ex-changers such as shift conversion heaters that failed by SCC.7

Coolers and Condensers Carbon or low-alloy steels may be used, depending onthe temperature involved. If the exchanger is water cooled, a stainless steel or otheralloy appropriate to the water chemistry should be employed. Copper alloys shouldnot be used in this service. Aluminum-finned steel tubes are used in air-cooledexchangers in ammonium hydroxide service.

The compressor interstage coolers are usually made of carbon steel and use wateras the cooling medium. Low-velocity areas in the water passage are prone to scalingand microbial corrosion, leading to tube failures. This can be avoided by usingimproved exchanger design to avoid low-velocity areas and through a propercooling-water treatment program.3

A number of coolers in ammonia plants have failed and been replaced with du-plex 3RE60� (S31500). These include carbon-steel heat exchangers, such as convertereffluent cooler, syngas compressor coolers, and compressor coolers that have failedby corrosion and stainless steel exchangers such as raw gas compressor coolers thatfailed by SCC.7

Storage Tanks

Ammonium hydroxide solutions are usually stored and otherwise handled incarbon-steel tanks or vessels. There are no special requirements, such as heat treat-ment or impact testing of cylindrical storage vessels, for aqueous ammonia. Becauseof possible rusting or carbamate corrosion in the vapor phase, type 304 (S30400)may be used where contamination by particulate iron salts is objectionable.

Ammonia is stored as a liquid in one of three ways:8,9

• Flat-bottomed steel storage tanks are used for storing very large volumes of am-monia, with typical capacity of 10,000 to 30,000 tonnes (up to 50,000). These tanksare designed for low pressure, so the anhydrous ammonia must be fully refrig-erated at atmospheric pressure and at the atmospheric boiling point of –33�C.

• Semirefrigerated tanks or spheres are used where larger volumes of ammoniamust be stored. These are held at some intermediate temperature, such as –12�C(10�F), between ambient and fully refrigerated (–28�F, –33�C) conditions. Cylin-drical storage vessels (“Bullets”) are also commonly employed.

• Ammonia can be pressurized (around 2 MPa, 300 psig) at ambient temperaturein spheres or horizontal cylinders up to about 1700 tonnes.

Refrigerated Storage Vessels Ammonia storage tanks are usually made from car-bon steel after a stress-relief heat treatment to protect against possible SCC (seeChapter 5) and for improved impact properties. Stainless steel, such as type 304(S30400), is also an acceptable material of construction but is not commonly used.The 31/2% nickel steel has good low-temperature toughness properties and is fre-quently used as a weld filler metal and sometimes as plates and forgings.


There are several construction types of refrigerated storage vessels. The mostimportant are the following:

• Single containment: A single-wall insulated tank, normally with a containmentbound around it.

• Double containment: This type of storage tank has two vertical walls, both ofwhich are designed to contain the stored amount of liquid and withstand thehydrostatic pressure of the liquid. The roof rests on the inner wall.

• Full containment: The two walls of this closed storage tank are also designed tocontain the stored amount of liquid, but in this case the roof rests on the outerwall.

The tank must be constructed in conformity with relevant codes for the construc-tion of pressure vessels or storage tanks based on its design pressure and tempera-ture. The storage tank must be safeguarded against high pressure and in the case ofrefrigerated liquid ammonia also against a pressure below the minimum designpressure. The ingress of warm ammonia into cold ammonia must be avoided toeliminate risk of excessive evaporation. Tanks must be completely insulated.9 TheU.S. standard governing the design, construction, location, installation, and opera-tion of anhydrous ammonia systems, including refrigerated ammonia storage sys-tems, is contained in 29 CFR standard no. 1910.111.

The tensile strength of the steel used for construction should be restricted. ANSIstandard K-61.110 requires that steels used in fabricating pressure-containing partsof a container shall have a tensile strength no greater than a nominal 480 MPa (70,000psi). In the United Kingdom, the strength of steels used for spheres and cylindricalvessels is restricted to those having a minimum specified yield (not tensile) strengthno greater than 350 N/mm2 (50 ksi). The design, construction, and operation ofstorage containers is regulated in most countries.

Both welding procedures and welders must be qualified to appropriate standardsand the tank or vessel thoroughly inspected after fabrication.

Tanks and vessels used for containment of anhydrous ammonia must be postweldheat treated as prescribed in the ASME Code. Although some construction codescall for a stress-relief temperature as low as 520�C (968�F), it is recommended thatthe minimum temperature should be 593�C (1100�F) for prevention of SCC. In noevent should the PWHT be less than 587�C (1050�F) tank metal temperature. Impacttesting for low-temperature service (e.g. for 31/2% Ni steels) is normally requiredfor cryogenic storage. Type 304 (S30400) is acceptable without PWHT or impacttesting.

When cryogenic tanks are empty, their inspection can be carried out by a numberof techniques. However, emptying these tanks to carry out periodic inspections isdifficult and expensive. It is also known that most SCC is initiated during start-upand that even small quantities of residual oxygen can initiate cracking. In some casesit has been found that acoustic emission sensors permanently fixed to such tankshave been useful in detecting defects needing further investigation or remedial ac-tion.11

Pressurized Storage Vessels These are also usually carbon steel and designed forup to 2.5 MPa with the pressure of larger cylindrical tanks being limited to about


1.6 MPa to avoid the use of steel thicknesses greater than about 30 millimeters.Reflective paint or insulation is added to the outside to avoid solar heating. Theshape of the vessel used in this type of storage is largely controlled by its capacity.Cylindrical, usually horizontal, tanks are used for up to about 150 tonnes of am-monia. Spheres are used for 250 to 1500 tonnes.8

Piping

For process piping (not gas pipelines) handling anhydrous ammonia, carbon steelis the most common material of construction. As with vessels, stress-relief heat treat-ment of carbon steels is recommended (or required by regulations) both for protec-tion against possible SCC and for improved impact properties. Types 304 (S30400)or 304L (S30403) are becoming more frequently used as an economical alternativeto carbon steel, especially in complex piping systems. The higher material cost canbe offset by the elimination of stress-relief heat treatment, impact tests, and painting.

Carbon-steel piping is the most common material of construction for ammoniumhydroxide and is usually satisfactory if the line is always full. Even if occasionallydrained, more resistant alloys are not required unless particulate iron oxides areobjectionable. In such cases, aluminum or stainless steel piping may be selected.Type 304 (S30400) or 304L (S40303) is also used where erosion-corrosion is likely.

Pumps

Anhydrous ammonia or ammonium hydroxide may be pumped in liquid form,while gaseous ammonia is moved by compressors. A pump for liquid anhydrousammonia is usually of cast steel construction for both impeller and volute. For low-temperature service, an all–stainless steel pump is preferred. For off-the-shelf items,the standard material is CF8M (J92900), which is usually the most readily available,but low-carbon variants as well as molybdenum-free grades are also suitable.

Single-stage centrifugal pumps made from low-temperature steel castings withstainless steel shafts and couplings are used for handling liquid ammonia at –33�C.12

Cast steel pumps are routinely used for ammonium hydroxide service. Stainlesssteel CF8M may be selected to avoid iron contamination.

Compressors

Cast steel is commonly used, except at temperatures above 300�C (570�F), wherenitriding may occur. At these elevated temperatures, cast austenitic stainless steels(e.g. CF8M) are employed. For steel compressors, the various components may beas follows:

Rotor impellers: Type 410 (S41000) stainless steelStationary parts: Type 410 (S41000) or 304 (S30400) stainless steel


Valves

As with compressors, cast steel is commonly used except at temperatures above300�C (570�F). At elevated temperatures, cast austenitic stainless steels, such asCF8M (J92900), are used. Impact testing of carbon steels for low-temperature serviceis normally required.

Valves for ammonium hydroxide are usually also of cast steel or iron, sometimeswith a type 410 (S41000) seat. For throttling valves, stainless steel may be employed.

Gaskets, Seals, O-Rings, and Hoses

Gaskets are used to seal the metallic or nonmetallic flange faces of pipes. A varietyof gasket materials are suitable for ammonia or ammonium hydroxide service,including spiral-wound stainless steel and PTFE, flexible graphite, and compressedasbestos fiber (CAF). For environmental reasons, CAF is restricted or prohibitedin many countries but is serviceable where permitted. A gasket design that limitscold flow of PTFE is required at elevated temperatures. This constraint can bemechanical or be achieved by adding resistant fillers to the PTFE, such as glass,silica, or graphite.

Another gasketing option is an envelope gasket. This consists of a core ofextension-resistant elastomer, sheathed in a thin sheet of fluoroplastic to resist theammonia.

PTFE gaskets used with titanium or zirconium should be made from virgin PTFE,not recycled. There have been cases of fluoride corrosion in gasket areas when re-cycled PTFE gaskets were used. A newer development provides a soft, easily com-pressible chemically inert 100% PTFE material with a unique combination of chem-ical resistance and low-torque requirements. Most grades of Gylon� PTFE gasketsare suitable for ammonia and ammonium hydroxide at all strengths and tempera-tures from subzero up to 260�C (500�F).13

Materials that can be used for O-rings, gasket seals, and hoses in ammonia andammonium hydroxide service are shown in Table 8.1.14 Ratings given are up to themaximum service temperature unless otherwise noted. Testing is always recom-mended for seals to be used in specific ammonia environments.

It should be noted that some of these elastomers, such as ethylene propylenerubber, may be attacked by entrained oils in compressed gas systems. Naturalrubber, hard rubber, isoprene, butyl rubber, and Viton� A should not be employed.Viton� Extreme (TBR-S and ETP-S) may be considered at temperatures below 50�C(122�F).

Ammonia should be transferred through articulated arms rather than throughhoses wherever possible. Where hoses are used, they should be externally reinforcedwith type 304 (S30400) stainless steel, while the hose materials themselves may beEPDM, CPR, Buna-N�, or Buna-S�.

Fluoropolymers (FEP, PTFE, PFA) and flexible graphite are suitable packing ma-terials.


Table 8.1 O-Ring Materials Compatible with Ammonia and Ammonium Hydroxide

Elastomer FamilyAnhydrousAmmonia

ColdAmmonia

Gas

HotAmmonia

Gas

ConcentratedAmmoniumHydroxide

Butyl (IR) 4 4 1 4Chloroprene (CR,neoprene)

4 4 3 4

Chlorosulphonatedpolyethylene (CSM,Hypalon�, etc.)

0 4 3 4

Epichlorohydrin (CO,ECO)

0 0 0 3

Ethylene-propylene (EPR,EPDM, Nordel�, etc.)

4 4 3 2

Fluoroelastomer (FKM,Viton� TBR-S, ETP-S)

0 0 0 2

Fluorosilicone (FSI) 1 0 1 4Natural rubber (NR) 4 4 1 0Nitrile (NBR, Buna-N�,etc.)

3 4 3 2

Perfluoroelastomer (FFKM,Kalrez� 6375m 1050LF,Chemraz�, etc.)

4 4 4 4

Polyacrylate (ACM) 1 1 1 1Polysulfide (T) 1 4 1 1Polyurethane 1 3 1 1Silicone (SI) 2 4 4 4Styrene butadiene (SBR) 1 4 1 1Polytetrafluoroethylene(PTFE, Teflon�, etc.)

4 4 4 4

Key to compatibility:4 Good, both for static and dynamic seals3 Fair, usually OK for static seals2 Sometimes OK for static seals; not OK for dynamic seals1 Poor0 No data

Bolting

Standard bolting appropriate to the strength and temperature requirements is em-ployed (e.g. B7, B8) in both ammonia and ammonium hydroxide service.

Transportation Equipment

Equipment used for transporting anhydrous ammonia is regulated in most coun-tries. Advice on such regulations can usually be obtained from the manufacturer or


Table 8.2 DOT Classifications for Ammonia

Concentration DOT No. Classification

Anhydrous 1005 125 Nonflammable compressed gas10%–35% solution 2672 154 Corrosive liquid�35%–50% solution 2073 125 Corrosive liquid�50% solution 1005 125 Corrosive liquid

supplier of ammonia, often contained in Material Safety Data Sheets (MSDS) or fromthe local regulatory body. In the United States, the relevant regulations are in theCode of Federal Regulations (CFR) under Title 29 (Labor), Title 33 (Navigation andNavigable Waters), Title 40 (Protection of Environment), Title 46 (Shipping), andTitle 49 (Transportation).15 The relevant health and safety rules are incorporated intoregulations of the U.S. Department of Labor and Industries.16

With little exception, containers for transportation of anhydrous ammonia arefabricated from carbon or low-alloy steels with some specific requirements as tostrength, stress relief, PWHT, and so on to avoid SCC as discussed previously.

For shipping containers (including cylinders and DOT-portable containers), cargotanks, and barges, carbon steel is the most common material of construction. Aswith storage facilities, type 304 stainless steel (S30400) is also acceptable but notcommonly used. The agency regulating transportation in the United States is theDepartment of Transportation (DOT). The DOT classifies all transportable materialsby number and class. Table 8.2 shows this information for ammonia and ammoniasolutions.

Tanks and containers, approved for the transport of ammonia, must be con-structed in accordance with DOT specifications DOT-51, DOT-106A, and DOT-110A.

Ammonium hydroxide (NH4OH) is rarely transported in large volumes becauseit is more economical to transport the compressed gas and make the hydroxidesolution by adding water. Small quantities of the reagent grade (28%) is shipped inglass bottles. Polyethylene canisters or metal casks are used for shipping 25% aque-ous ammonia.8

However, there will be occasions in chemical processing where ammonium hy-droxide needs to be transported. Carbon steel is used, and there are no specialrequirements, such as heat treatment or impact testing of transportation vessels, foraqueous ammonia. Aluminum and type 304 stainless steel are also suitable. Copper-based alloys and nickel-based alloys lacking a chromium constituent must not beused in this service.

Railcar Transport (Tank Cars) For rail transport tank cars, carbon steel is the mostcommon material of construction. Stainless steel (e.g. type 304) is also acceptablebut not commonly used. The section of the CFR dealing specifically with ammoniafor rail car transport is 49 CFR 173.314. In addition to the federal regulatory require-ments, ANSI standard K-61.110 and CGA pamphlet G-2.110 have specific require-ments. The design must also be approved by the Association of American RailroadsCommittee on Tank Cars before the cars are placed in service.


Pressure vessels are used for anhydrous ammonia at pressures of about 2.5 MPa.Atmospheric pressure vessels are used for 25% aqueous ammonia with higher pres-sure vessels (up to 1.6 MPa) for higher concentration ammonia solutions.8

The use of copper, silver, zinc, or their alloys is prohibited. Baffles made fromaluminum may be used only if joined to the tank by a process not requiring PWHTof the tank.

Tank Trucks For over-the-road transportation in cargo trailers used with motorizedvehicles, carbon steel is the conventional material. The use of high-strength low-alloy steels has been restricted or eliminated in some countries. In 1968, a road tankerruptured in France, killing five people. The investigation into the incident revealedthat the cause was stress corrosion cracking, aggravated by fatigue. The material ofthe tanker was “T-I” steel (A-517) with a tensile strength of 758 to 779 MPa (110–113 ksi). As a result of this incident, French authorities prohibited the use of T-I steelfor storing or carrying ammonia under pressure.17

In addition to federal regulatory requirement, ANSI Standard K-61.110 definessome specific stipulations.

Marine Transport For marine transport, the relevant sections of CFR 46 (part 54)apply, specifying applicable pressure/temperature conditions for different shippingmodes. These regulations provide limitations and modifications to the requirementsof ASME Division 1, Section VIII, for pressure vessels used for marine shipping.

Pipelines If the geography permits, transport of large quantities of ammonia bypipeline is more economical over long distances than by river barge or rail. The MidAmerican Pipeline System (MAPCO) has a total length of 1745 kilometers and ex-tends from Texas to Minnesota, while the Gulf Central Pipeline is a pipeline systemwith a total length of 3,057 kilometers and connects producers in Texas and Loui-siana with terminals in Arkansas, Iowa, Illinois, Indiana, Nebraska, and Missouri.This pipeline was constructed from Grade X42 steel pipe (minimum yield strength42,000 psi) manufactured to AP1 standard 5LX.18 This grade of pipe was chosenover higher-strength grades (X46 or X52) in recognition of the relationship betweenammonia stress corrosion cracking and residual stresses. Ammonia in the pipelinecontains a minimum of 0.2% water as an additional precaution.

The world’s longest ammonia pipeline has been in operation since 1983 in Russiaand connects the producers at Togliatti/Gordlovka with the terminals of Grigor-oswski/Odessa some 2424 kilometers away. Ammonia is transported in these pipe-lines at a temperature of 2�C (pressure is between 22 and 100 bar), so it needs to bewarmed up at the supplier end and cooled again at the receiver end of the pipe.8

References

1. API 650, “Welded Steel Tanks for Oil Storage” (Washington, DC: API, latest ed.).2. ASME Section V111, “Pressure Vessels” (New York, NY: ASME International,

latest ed.).



4. G.W. Powell, S.A. Mahmoud, eds., Metals Handbook—Failure Analysis and Pre-vention, vol. 11 (Metals Park, OH: ASM International, 1986), p. 290.

5. S.B. Parks, C.M. Schillmoller, “Improve Alloy Selection for Ammonia Furnaces,”Hydrocarbon Processing (Int. ed.) 76, 10 (1997): pp. 93–98.

6. S.A. Al-Ghafli, H.M. Lari, B. Bousmaha, H.I. Bukhari, “Energy ConservationMeasures: Energy Audit, Process Optimization,” IFA Pub. (2003), http://www.fertilizer.org/ifa/publicat/pdf/tech0016.pdf.

7. Anon, “List of References—Heat Exchangers in Ammonia and Urea Plants,” S-12311-ENG (Sandviken, Sweden: Sandvik Steel, 1996), 10 pp.

8. M. Appl, Ammonia: Principles and Industrial Practice (Weinheim, Germany:Wiley-VCH, 1999), pp. 209–221.

9. Anon, “Production of Ammonia,” vol. 1 (Brussels, Belgium: EFMA, EuropeanFertilizer Manufacturers’ Association, 2000), 44 pp.

10. Anon, “Safety Requirements for the Storage and Handling of Anhydrous Am-monia,” ANSI K61.1 (New York, NY: ANSI, latest ed.).

11. Anon, “Cryogenic Ammonia Tanks” (Houston, TX: Matrix Inspection and En-gineering, Inc., 2000), http://www.matrixie.com/cryogen.pdf.

12. Anon, Dechema Corrosion Handbook, Ammonia and Ammonium HydroxideSection, CD-ROM (Frankfurt, Germany: Dechema aV, 2001).

13. Anon, “Engineered Gasketing Products,” DPI-8/01 Rev. 0-5M (Palmyra, NY:Garlock Sealing Technologies, 2001), 56 pp.

14. Anon, “O-Ring Compatibilities,: Engineering Fundamentals, Efunda (2002),http://www.efunda.com/DesignStandards/oring.

15. Anon, “Storage and Handling of Anhydrous Ammonia,” Code of Federal Reg-ulations, OSHA (2002).

16. Anon, “Storage and Handling of Anhydrous Ammonia,” Chapter 296-24—PartF-2, U.S. Department of Labor and Industries.

17. A.W. Loginow, E.H. Phelps, “Stress Corrosion Cracking of Steels in AgriculturalAmmonia,” Corrosion 18, 8 (1962): p. 229.

18. API 5LX, “High-Test Line Pipe” (Washington, DC: API, out of print since 1982).

Section IICaustic Soda

77

Figure 9.1 Chlor-Alkali Plant in Australia (courtesy of CHEMETICS—a division ofAker Kvaerner Canada Inc.)

9Introduction

Caustic soda is one of the three most prominent products of the chemical industry,the other two being sulfuric acid and soda ash (sodium carbonate: Na2CO3). By the1990s, over 13 million tonnes were used annually in the United States alone. MajorWest European caustic soda capacity in 2003 was estimated to be 11.3 million ton-nes/y.1 The annual worldwide production of caustic is of the order of 45 milliontonnes.2 As of July 2002, more than 500 companies produced chlor-alkali (a generalterm to cover the coproduction of chlorine and caustic soda) at over 650 sites world-wide, with a total annual capacity of over 51 million metric tons of chlorine. Abouthalf of all plants are located in Asia, but many of these are relatively small.3 A typicalchlor-alkali plant is shown in Figure 9.1. This particular plant in Australia produces85 t/d of caustic using membrane cells.


Although chemists express the concentration as a percentage, the industrial stan-dard for the sale and use of caustic soda is based on the anhydride (Na2O), wherein77.5% sodium oxide is equivalent to 100% NaOH.

It is the most important commercial, caustic chemical, used in a variety of pro-cesses, such as plastics (notably PVC), pulp and paper, soap, glass, aluminum, andso on. It is also used for acid waste neutralization, although soda ash is equallyeffective and less expensive. In waste management programs, caustic soda is ob-served to be about 100 times more soluble than lime, which is another, cheapersubstitute for such applications.

Caustic potash or potassium hydroxide (KOH) is an analogous chemical, some-times used as a substitute in papermaking processes, but is more expensive and oflesser commercial importance. The term “caustic” is often used to describe bothcaustic soda and caustic potash.

Uses of caustic soda include those listed in Table 9.1.4,5

Table 9.1 Typical Uses of Caustic Soda by Industry

Industry Uses of Caustic Soda

Soaps and surfactants In the hydrolysis of oil and fatsChemical process In the manufacture of many chemicals, including amyl

amines, cresol, ethylene amines, formic acid, glycerine, maleicanhydride, phenol, styrene, vinyl chloride monomer

Oil In drilling to control the pH of drilling mud; as a bactericideand calcium remover

Petroleum refining To remove impurities, such as sulfur, sulfur, and acidiccompounds, from the hydrocarbon stream

Water For pH control; regeneration of ion exchange resins; effluentneutralization; descaling of pipe work systems

Food In refining animal and vegetable oils to remove fatty acids; asdry formulations for bottle washing; general cleaningoperations; cleaning of brewery equipment; lye peeling ofpotatoes, fruits and vegetables

Pulp and paper In the chemical treatment of cellulose fiber from wood toproduce cellulose used to make paper; to dissolve lignin frombleached wood pulp as part of the process to produce rayon

Pharmaceutical As a reactant in the manufacture of sodium phenolate used inantiseptics and aspirin

Textile As a scouring agent; bleaching scoured cloth; Mercerizing toimprove luster and dye absorption

Metal refining To solubilize alumina from bauxite in the ore treatment priorto aluminum production

Household As a precursor of commercial and household cleaners, bleach,and detergents

References

1. Anon, “Caustic Soda,” European Chemical News 78, June (2003): p. 18.2. Anon, “Caustic Soda (Sodium Hydroxide)” ChemLink Australasia (1997),

http://www.chemlink.com.au/caustic.htm.3. E. Linak, “CEH Report—Chlorine/Sodium Hydroxide,” SRI Consulting (2002),

http://ceh.sric.sri.com/Enframe/Report.html.4. Anon, “Sodium Hydroxide (Caustic Soda),” Chemistry Store (2002),

http://www.chemistrystore.com/Caustic_Soda.htm.5. Anon, OxyChem Caustic Soda Handbook (Dallas, TX: OxyChem, 2000), 52 pp.,

http://www.oxychem.com/products/handbooks/caustic.pdf.


81

10Properties of Caustic Soda

Caustic soda (CAS 1310-73-2) is the common name for sodium hydroxide (NaOH),an alkali also known as sodium hydrate, lye or white caustic. It is a colorless towhite, odorless solid or is produced in solutions of various strength in water thatare also colorless and odorless.

Caustic soda solutions are coproduced with chlorine by a variety of processes(see Chapter 11). The strength and purity of the caustic solution vary, depending onthe method of production. More concentrated solutions and solid caustic soda areproduced by further evaporative processes. The tolerance range for impurities isdictated by the anticipated service. Different suppliers use different names forgrades, depending on the source or use of their product. Commercial solutions canbe classified from the purest to least pure product in the following way:

• 50% mercury cell grade (also known as rayon or amalgam cell grade) has thelowest salt content of all commercial grades.

• 50% purified grade, made from the diaphragm process, has a salt content �0.01%NaCl and can usually be substituted for mercury cell grade.

• 50% membrane grade is low in salt (�0.01%) and sodium sulfate (�0.01%) butalso has very low levels of trace metals.

• Commercial (or diaphragm cell grade) can be made by diaphragm, mercury, ormembrane cells. Contains around 1% NaCl plus trace metals.

Solutions at 73% are also available either for subsequent dilution but shipped inconcentrated form to save shipping costs or for use in some applications that requirethis more concentrated solution.1,2

Solid product is typically classified by diminishing purity as reagent grade, mer-cury cell grade (MCG), rayon grade (low in iron, copper, and manganese), and com-mercial (i.e. with relatively high levels of impurities).

Physical Properties

The physical constants of pure sodium hydroxide are shown in Table 10.1.3

Aqueous solutions, of course, have different characteristics. At 50% concentration,the solution freezes at ambient temperatures of less than 12�C (54�F), and heated


Table 10.1 Physical Constants of Sodium Hydroxide (NaOH)

Property Data

Molecular weight 40.00Specific gravity 2.13Melting point 318.4�C (605�F)Boiling point 1390�C (2534�F)Latent heat of fusion 40 cal/g (72 Btu/lb)Heat of formation 101 K/cal/molSolubility (g/100 cc water) 42 in cold water (0�C), 109 at 20�C, 347 in hot water (200�C)

tanks and piping are required. In preparing a 40% concentrated solution, the exoth-erm generated by the dilution of solid NaOH can elevate the temperature to abovethe boiling point. Boiling points for various-strength solutions are shown in Table10.2.

The complete range of boiling point and freezing point curves is shown in Figure10.1.4

It will be observed that the minimum freezing point is at about 25% concentration,which is useful for handling strong acid solutions in plant for neutralization, pHcontrol, and so on.

Table 10.2 Boiling Points of Strong NaOH Solutions

Weight (%) Boiling Point (�C [�F])

10 103 (218)20 108 (226)30 116 (241)40 128 (262)

Chemical Properties

Caustic soda is deliquescent, readily absorbing water from the atmosphere to forma film of strong caustic in situ. Solid NaOH reacts vigorously and exothermicallywhen added to water, as in making caustic solutions for various purposes.

The concentration of caustic soda solutions is related to its pH in Table 10.3.3

Because of the difficulty of obtaining accurate pH readings at values above 12, pHis not a valid method to determine concentration of caustic soda solutions.

Caustic soda rapidly absorbs carbon dioxide from the air, forming sodium car-bonate. It should be kept away from heat, sparks, or flames and not stored or mixedwith incompatible materials. Caustic soda is incompatible with the following ma-


Figure 10.1 Range of Boiling and Freezing Temperatures of NaOH

terials: water; acids; flammable liquids; organic halogens; metals such as aluminum,tin, and zinc; and nitromethane.

It is corrosive to some metals and can generate hydrogen gas. Contact with watermay generate sufficient heat to ignite combustible materials. Carbon monoxide gascan form on contact with food and beverage products in enclosed spaces and cancause death.5

Table 10.3 Hydrogen Ion Concentration of Various-Strength Caustic Soda Solutionsat 25�C (77�F)

% NaOH Moles/L pH

7.40 2.0 14.03.83 1.0 13.81.96 0.5 13.60.39 0.1 12.90.20 0.05 12.60.04 0.01 12.0


Safety and Health Considerations

The word “caustic” is defined as “destructive or corrosive to living tissue; an agentwhich burns or destroys living tissue.”6 Given that description and the commonname for sodium hydroxide, it is hardly surprising that it is highly toxic by eitheringestion or inhalation and is a strong irritant to the eyes, skin, and mucous mem-branes; that is, it is “corrosive” in the physiological sense. Beside potential burns tothe eyes or skin, fire may produce irritating or poisonous gases that are harmful ifinhaled.

Various regulatory bodies set exposure limits intended to protect humans whomight come into contact with caustic soda. Current limits are as follows:7,8

OSHA Standards: permissible exposure limit (PEL): TWA: 2-mg/m3 ceilingACGIH threshold limit value (TLV): 2-mg/m3 ceilingNIOSH: recommended exposure limit (REL): 2-mg/m3 ceilingImmediately Dangerous to Life or Health (IDLH): 10 mg/m3

Recommended Protective Equipment

Exhaust ventilation should be provided to maintain airborne concentrations belowrecommended exposure guidelines. When exposure levels could exceed 2 mg/m3,a NIOSH-approved air-purifying, full-face respirator with high-efficiency particu-late filters is recommended. When exposure levels could exceed 10 mg/m3, a self-contained breathing apparatus with a full face piece is recommended.

Protective clothing impervious to caustic such as neoprene or polyvinyl chloride(PVC) should be used when handling caustic. Precautions should be taken to ensurethat all potentially affected body parts are covered, such as taping sleeves and pantlegs to gloves and boots, respectively, and buttoning clothing to the neck. Selectionof specific items such as gloves, coats, pants, boots, aprons, or full-body suits willdepend on operations to be performed. Avoid leather and wool. A safety showershould be located in the immediate work area. Contact lenses should not be worn;they could contribute to severe eye damage. Wear close-fitting chemical splash gog-gles as a minimum. Where there is a possibility of splashes to the face, a full-lengthtransparent face shield should be worn.9

Fire and Explosion

Sodium hydroxide itself will not burn, but fires may occur. If the solution tempera-ture is raised by fire, caustic attacks steel with liberation of hydrogen gas that willburn. Also, other combustibles, such as wood, paper, or oil, may ignite from localoverheating. If caustic soda is exposed to a fire, sodium oxide fumes may be gen-erated. In the event of fire or explosion, the following precautions should be taken:


• Keep unauthorized people away.• Stay upwind and avoid low areas.• Isolate the hazardous area and deny further entry.• Wear self-contained breathing apparatus (SCBA) and full protective clothing and

gear.• If possible to do so safely, remove container from the fire area. If not, cool the fire-

exposed containers with water until flames are fully extinguished. Avoid con-tacting caustic soda with water if at all possible.

Small fires can be extinguished using dry chemicals, carbon dioxide (CO2), waterspray, or foam. Larger fires should be controlled with water spray or foam.

First Aid

Victims should be moved to a safe area with fresh air and an emergency team sum-moned. Contaminated clothing and shoes must be removed and isolated. In case offluid contact, flush skin and/or eyes with fresh, running water for at least 15 min-utes. In case of inhalation, remove to fresh air and supply oxygen and/or artificialrespiration if necessary. In case of ingestion, do not induce vomiting, give largequantities of water if victim is conscious, and seek immediate medical help.

Disposal, Spill, or Leak Procedures

Runoff from fire control or water washing may cause pollution, although the sodiumion itself is nonpolluting. Caustic spills to water are toxic to animal life, so containand neutralize with dilute acetic or hydrochloric acid if possible.

Caustic Dilution

Caustic is often shipped in solid form or in more concentrated solutions than isrequired by the end user. This is done to reduce shipping costs and means that theend user must be able to dissolve or dilute caustic safely to produce the requiredconcentration. The precautions necessary to carry out this dilution process includethe following:

• Always add caustic soda solid or solution to water with constant agitation. Neveradd water to caustic soda solutions.

• The water used should be lukewarm at 80�F to 100�F (27�C–38�C). Never startwith hot or cold water.

Caustic soda, solid or solution, has a high heat of solution so that the addition ofcaustic soda to water or more dilute solution will cause a rise in temperature. If


caustic soda becomes concentrated in one area, is added too rapidly, or is added tohot or cold liquid, the temperature increase can be very rapid. This can result in theformation of dangerous mists or boiling and spattering, which may cause an im-mediate violent eruption.

References

1. Anon, “Caustic Solution Forms” (Midland, MI: Dow Chemical, 2003), 2 pp.2. Anon, “Caustic Soda” (Cleveland, TN: Olin Chlor Alkali Products, undated), 48

pp.3. Anon, “OxyChem Caustic Soda Handbook” (Dallas, TX: OxyChem, 2000), 52

pp., http://www.oxychem.com/products/handbooks/caustic.pdf.4. Anon, “OxyChem Caustic Soda Handbook” (Dallas, TX: OxyChem, 2000), p. 29,

http://www.oxychem.com/products/handbooks/caustic.pdf.5. Anon, “MSDS NAOH50” (Newark, CA: Jones-Hamilton Co., 2002), 8 pp.6. T.C. Collocott, A.B. Dobson, eds., Chambers Science and Technology Dictionary

(Edinburgh, UK: W&R Chambers Ltd, 1984), p. 196.7. R.J. Lewis, ed., Sax’s Dangerous Properties of Industrial Materials, 10th ed. (New

York, NY: John Wiley & Sons, 2000), p. 3253.8. Anon, “NIOSH Pocket Guide to Chemical Hazards,” NTIS no. PB 91-151-183/

A07 (Cincinnati, OH: NIOSH, 1990), 250 pp.9. Anon, “MSDS NAOH50” (Newark, CA: Jones-Hamilton Co., 2002), 8 pp.

87

11Production of Caustic Soda

Sodium hydroxide can bemade in a reversible reaction by treating sodiumcarbonatewith quicklime. The reaction cannot go to completion, so it is difficult to convertmore than about 90% of the carbonate into hydroxide. The majority of caustic sodais made commercially, however, as a by-product of chlorine production by the elec-trolysis of sodium chloride solutions. One ton of salt produces 0.58 tons of chlorineand 0.63 tons of caustic soda.1

There are three major processes used in the manufacture of chlorine and causticsoda. The share of production among these processes, on a worldwide basis, isapproximately as follows:

• Diaphragm cell: 75%• Mercury cell: 19%• Membrane cell: 3%• Other technologies: 3%

This distribution is not uniform throughout the world, however, since the dom-inant technology varies in different regions as follows:

• Western Europe, predominance of mercury cell process (June 2000): 55%• United States, predominance of diaphragm cell process: 75%• Japan, predominance of membrane cell process: �90%

The remaining chlorine production capacity in western Europe consists of dia-phragm cell process 22%, membrane cell process 20%, and other processes 3%.2

In 2003, the technology being employed throughout Europe was distributed asfollows:1

• Diaphragm technology: 25%• Mercury cells: 64%• Membrane process: 11%


The supply of sodium hydroxide (NaOH) is diminishing because of a decreasingdemand for chlorine. At the same time, production of chlorine and caustic soda bymembrane cells is increasing at the expense of mercury cell technology throughoutthe world. There are a number of factors driving the decision about which technol-ogy to use to produce chlorine and caustic, including costs of electricity, environ-mental pressures, and so on.The commercial products vary in purity, the kind and amount of impurities de-

termined largely by the specific process employed in manufacture. For example,diaphragm cell caustic can contain up to 5000 ppm chloride ion compared with 20–30 ppm Cl– in the product from mercury cells.When high-purity grades are required, it is necessary to take precautionary mea-

sures to maintain product quality. The very highest purity caustic, of less than 2ppm iron content, is produced by an advanced membrane gap cell (MGP) technol-ogy in which high-molecular-weight impurities are separated from the caustic. Ironpickup is avoided by using materials other than carbon steel, the conventionalmaterial of construction, in manufacture, storage, and transport. Other materialsutilized for this purpose include nickel-based alloys, stainless steels, and silver (inspecial cases). Storage and transport compartments for liquid products may becoated with epoxy coatings or with electroless nickel plating (ENP). Other devel-opments in membrane cell technology being investigated include the use of oxygenconsumption instead of hydrogen evolution as the cathodic reaction. Related de-velopments include changes to the cathode materials, proprietary coatings, and soon; modifications to the anode structure; and the use of zero gap cell technology.This research is aimed at increasing energy efficiency and product purity.3

Production Processes

Of the three conventional processes, the largest production is derived from dia-phragm cells. Materials of construction for the production and handling of causticsoda are discussed in subsequent chapters. Since brine preparation, construction ofchlorine cells, and handling of chlorine is described in detail in MTI MS-3,4 thoseaspects of caustic soda will not be addressed in this monograph.

Diaphragm Cell Process

In the diaphragm cell, a naturally occurring brine or a salt solution, dissolved inrubber- or brick-lined tanks, is subjected to electrolysis by direct current. In theformer case, the brine must be pretreated to remove certain impurities (e.g. calcium,iron, manganese, and sulfate species).A caustic solution (10%–15%) is produced at an iron cathode. In the anodic re-

action on the other side of the asbestos diaphragm, chlorine and hydrogen areevolved from the solution comprising about 15% each of caustic and sodium chlo-ride.


Since the crude caustic product is heavily contaminated by iron, sodium chlorate,sodium chloride, and dissolved chlorine, further treatment is required. Chemicaltreatments include additions of liquid or other chemicals, evaporation, and salt sep-aration. Asbestos-based diaphragm cells are under pressure to be phased out inwestern Europe.5

Mercury Cell Process

The mercury cell uses a rubber-lined cell in which a solution of approximately 25.5%sodium chloride concentration is electrolyzed, diminishing in concentration to about21% as the reaction progresses.The anode may be graphite or titanium and releases chlorine and hydrogen as

products. The cathode is an inclined steel plate over which runs a molten mercury-sodium amalgam that is bled off to a separate compartment where hydrolysis withdemineralized water produces a 50% caustic of exceptional purity.The mercury cell process is being phased out because of environmental problems

occasioned by release of mercury. European producers have committed to convertmercury plants to membrane technology by 2020.5

Membrane Cell Process

The membrane cell is analogous to the diaphragm cell except that the feed brine ismore highly purified and the perfluorosulfone membrane has lower permeabilitythan a diaphragm. Consequently, a high-quality concentrated sodium hydroxide isproduced. The caustic product frommembrane cells is around 30%NaOH. A typicalmembrane cell is shown schematically in Figure 11.1.6

Figure 11.1 Schematic View of Membrane Cell Showing Inputs and Outputs


Concentrated Solutions and Solid Caustic Soda

The weak caustic solutions from the membrane or diaphragm processes are firstpurified to remove chlorates, chlorides, iron, chlorine, and so on and then concen-trated to produce a salable, usable product. Once the caustic soda leaves whichevertype of chlorine production cell that was used, it must be filtered and purified to alesser or greater extent before being ready for use or sale. Mercury cell caustic isgenerally the most pure and does not usually need to be concentrated further unless73% or solid caustic is being produced. The diaphragm cell caustic is generally themost impure and the weakest. The three processes and the treatment of the causticsoda product are shown schematically in Figure 11.2.7 After purification, multistageevaporators are used to increase the concentration by evaporation (see Figure 11.3).In this figure, showing a triple-stage evaporator, FE is the forced evaporator and CRis the crystallizer.8

Since diaphragm cell caustic soda is the most widely produced, the handling ofthis product will be described further. The product from the other processes istreated similarly with the differences shown in Figure 11.2. Once the caustic sodaleaves the cell, sodium chlorate must be removed to prevent corrosion at elevatedtemperatures. The chlorate is usually extracted with ammonia, but other methods,often proprietary, are also used. Sodium chloride concentrates and crystallizes dur-ing the caustic concentration and must be removed by settling and filtering.The purified caustic is fed to the evaporators, where the solution is concentrated

up to nominally 50% or 73%, depending on the process and the use for which thecaustic is required. If solid product is required, all the water is evaporated from thesolution and the liquid, anhydrous NaOH, is allowed to cool and solidify. Flakesolid is made by passing molten caustic over cooled flaking rolls. The flakes canthen be milled to the required particle size. Caustic soda beads are produced frommolten caustic in a prilling tower that produces beads of uniform shape and size.7

The solids are deliquescent and so require continual protection against exposure toatmospheric humidity. Materials that are used in this concentration and purificationprocess are discussed in Chapter 18.

Impurities

Contaminants in caustic may arise from production processes, storage, and handlingor from chemical processes employing NaOH as a constituent or additive (e.g. forneutralization or emulsification).In the production of caustic by the mercury cell process, the feedstock brine may

introduce calcium, iron, manganese, sulfates, and hypochlorites as well as traces ofmercury. In diaphragm processes, contaminants make up such species as unreactedsodium chloride, sodium chlorate, chlorine, and sodium sulfate. In other processapplications, even a change in chemical grade may lead to unexpected corrosion,and corrosion/materials engineers need good communications channels with sup-


Figure 11.2 Block Diagram of the Production of Caustic Soda from the VariousTypes of Electrolytic Cells

pliers, designers, and operating personnel. The effect of these impurities andmixtures is discussed in Chapter 15.

References

1. Anon, “Keywords—Chlorine,” European Salt Producers’ Association (2003),http://www.eu-salt.com/manufact/chlorine.htm.


Figure 11.3 Flow Diagram of Triple-Effect Caustic Soda Evaporator

2. Anon, “Reference Document on Best Available Techniques in the Chlor-AlkaliManufacturing Industry,” European Commission, Brussels (2001), http://www.envir.ee/ippc/docs/chlor-alkali.doc.

3. J. Chlistunoff, “Advanced Chlor-Alkali Technology,” IMF Program ReviewMeeting, Golden, CO (2003), http://www.oit.doe.gov/imf/pdfs/revpres_17_chlistunoff.pdf.

4. C.P. Dillon, W.I. Pollock, eds., Materials Selector for Hazardous Chemicals: Hy-drochloric Acid, Hydrogen Chloride and Chlorine, vol. MS-3 (St. Louis, MO:MTI, 1995), 200 pp.

5. Anon, “Caustic Soda,” European Chemical News 78, June (2003): p. 18.6. Anon, “Chloralkali Technology” (Vancouver, BC: Chemetics—A Division ofAker Kvaerner, 2003), p. 5.

7. Anon, “OxyChem Caustic Soda Handbook” (Dallas, TX: OxyChem, 2000), p. 4.8. C.M. Schillmoller, “Alloy Selection for Caustic Soda Service,” NiDI technicalseries no. 10019 (Toronto, ON, Canada: NiDI, March 1988), p. 8.

93

12Corrosion by Caustic Soda

Corrosion is the deterioration of a material by reaction with its environment. Formetals and alloys, this is mostly an electrochemical process involving anodic andcathodic reactions.Corrosion rates in aggressive chemicals usually decrease as the pH increases. In

alkaline solutions, the hydrogen ion is present in very low concentrations. However,many metals pass through a minimum corrosion rate at some pH, usually basic,and then suffer increased corrosion as pH continues to rise. Aluminum can liberatehydrogen ions from basic solutions. Since hydrogen ions are in short supply, it islikely that the cathodic reaction in alkaline media involves absorbed water mole-cules, such as

� �H O � e r OH � H (1)2

while the anodic reaction remains the same as in acidic corrosion:

� �M r M � e (2)

The metal ion is removed from solution by forming a basic salt, such as a ferroate,aluminate, or zincate.Quite often, corrosion by alkalis leads to pitting and other localized attack because

they tend to form cathodic films, and attack is concentrated at susceptible anodicareas. Carbon and low-alloy steels, austenitic stainless steels, and some nickel alloysmay suffer either stress-corrosion cracking or general corrosion in hot, concentratedcaustic.

Passivity

Stainless steels and some other iron-chromium and nickel-chromium alloys, as wellas titanium, zirconium, and aluminum, achieve “surface passivity” by developing


a tenacious surface oxide layer that is relatively inert. Interruptions to this film byinclusions, embedded metal particles, or mechanical breaks provide unprotectedsites for corrosion to occur.In the case of austenitic stainless steels, inclusions and other foreign materials can

be removed from the surface by immersion in 20% nitric acid at about 75�C (167�F).The purpose of this treatment is to clean the metal surface and remove embeddedparticles before allowing the oxide film to reform in a more continuous, thicker, andmore tenacious form than might occur naturally. A similar process involving theuse of caustic solutions is applied to aluminum alloys. The protective surface oxidereforms spontaneously on reexposure to air.Sodium hydroxide (NaOH) is a strong alkali and therefore corrosive to ampho-

teric metals (i.e. metals able to form salts with or be corroded by both bases andacids), such as aluminum, lead, and tin, even in dilute solutions at room tempera-ture. These are directly attacked, liberating hydrogen just as one observes with basemetals in a mineral acid and forming salts (i.e. sodium aluminate, plumbate, orstannate) by such reactions as shown here:1

Al � NaOH � H O r NaAlO � 11/2 H (3)2 2 2

A similar type of direct corrosion occurs with, for example, ferrous metals (formingsodium ferroate) but only at higher concentrations and temperatures.Behavior of metals in caustic is governed by their position in the EMF series.

Those anodic to hydrogen (e.g. ferrousmetals) are attacked by caustic with evolutionof hydrogen under certain conditions. Those cathodic to hydrogen (e.g. copper andalloys) are generally resistant in the absence of oxidizing contaminants. As with anycorrosive environment, contaminants in caustic or admixtures with other chemicalscan dramatically change the reactivity with specific materials of construction.

Forms of Corrosion

Of the many forms of corrosion that have been identified, general corrosion pre-dominates with nonresistant metals and alloys, while the resistant alloys are morelikely to suffer some form of localized attack. The most common forms of localizedattack are intergranular corrosion of stainless steels and crevice corrosion. The formsof corrosion described in this chapter are encountered with one material or anotherunder certain conditions of temperature, concentration, or contamination. The cor-rosion behavior of specific materials is discussed in detail further in Chapters 13to 15.

General Corrosion

General or uniform corrosion is the common form of metal loss in most corrodentsin the absence of passivating films. In this form of corrosion, the metal is removed


uniformly over the entire exposed surface. Ferrous alloys can suffer from this formof corrosion, as can nickel, copper, and titanium in the presence of oxidizing species.

Localized Corrosion

Pitting corrosion (in which the attack takes the form of small but deep cavities) orother localized attack (e.g. concentration cell corrosion, crevice attack, and so on)may be encountered under some circumstances, especially with alloys that dependfor their corrosion resistance on a passive film. This is due to the ability of causticto produce cathodic oxide films, concentrating the attack at small anodic areaswherethe film is defective.

Galvanic Corrosion

This does not seem to be a practical problem in caustic soda. Zinc is anodic to copperin this environment, but the usual strong cathode/anode relationship between car-bon and iron is not encountered.

Erosion-Corrosion

This is a form of corrosion resulting from the loss of protective surface oxide fromexcessive velocity or turbulence of contacting fluids. This problem is controlled byalloy selection or by limiting the velocity of fluids in pipes to less than about 2 mps(6 fps). Centrifugal pumps often exhibit this type of corrosion.

Intergranular Attack

Under some circumstances, brasses or bronzes made with zinc may suffer inter-granular attack (IGA). In hot, concentrated caustic soda solutions, carbon steels cansuffer IGA. Austenitic and ferritic stainless steels can suffer IGA due to chromiumdepletion at grain boundaries caused by heating in a critical temperature range.

Dealloying

Although gray cast irons are not prone to graphitic corrosion, high-zinc brasses willsuffer a selective dissolution of the zinc-rich phase.It has recently been shown that high-performance nickel alloys can be subject to

dealloying attack in hot, caustic soda solutions. Field failures from this mechanismhave also been reported.2


Liquid Metal Embrittlement

Liquidmetal embrittlement (LME) is a form of attack inwhichmetals that aremoltenat operating temperatures penetrate the grain boundaries of a solid metal or alloyand cause extensive mechanical damage. It is also known as liquid metal cracking.Mercury deriving from mercury cell production can cause LME of alloys, such asalloy 400. Titanium and zirconium and their alloys, copper and its alloys, aluminumand its alloys, and alloy 200 at elevated temperatures are also known to be at riskfrom this form of attack.3

Stress Corrosion Cracking

Stress corrosion cracking (SCC) is an environmentally assisted form of attack thatresults from interaction between the environment and a specific alloy system undertensile stress. It is a temperature-sensitive phenomenon. Classical examples of SCCare copper alloys in ammonia, steel in caustic solutions, and stainless steels in chlo-ride environments. SCC is the most common corrosion phenomenon associatedwithcaustic. In steels, it is often called “caustic embrittlement,” a misnomer in that duc-tility of the metal is unimpaired in the matrix. SCC is also encounteredwith stainlesssteels under some conditions (see the later discussion). Nickel-base alloys are veryresistant but not totally immune. In the laboratory, SCC has been induced in 70 to30 brasses but only at artificially induced potentials; it is not a problem in actualservice utilizing high-strength copper alloys.The first identified cases of SCC were in riveted carbon-steel boilers that failed

by caustic cracking. The caustic soda used to treat the boiler feedwater becameconcentrated in crevices at the riveted joints, which were highly stressed.4 In themid- to late 1800s, as steam power became more widespread, boiler explosions werecommon, causing a great number of casualties and major damage. Caustic crackingoccurs within a defined range of temperature and caustic concentrations. For ex-ample, at about 10% NaOH, caustic SCC is possible at temperatures �80�C (176�F),while at 20%, it may occur at temperatures �60�C (140�F).

High-Temperature Corrosion

Very hot caustic tends to strip otherwise protective oxide films from alloy systems.At 800�C to 815�C (1470�F–1500�F), molten caustic attacks even high-nickel alloysby selective dissolution of iron, chromium, and molybdenum, leaving a porous sur-face.Heat transfer through a metal to the caustic solution also accelerates corrosion in

many cases. The mechanism is by increasing the caustic concentration locally andby erosion caused by the formation of gas bubbles.


References

1. H.H. Uhlig, R.W. Revie, Corrosion and Corrosion Control, 3rd ed. (New York,NY: John Wiley & Sons, 1985), p. 345.

2. G. Chambers, “Caustic Dealloying Corrosion of High Performance Nickel Al-loys,” Stainless Steel World, October 2003, pp. 57–59.

3. J.R. Davis, ed., Corrosion—Understanding the Basics (Materials Park, OH: ASMInternational, 2000), p. 191.

4. H.H. Uhlig, Corrosion and Corrosion Control (New York, NY: John Wiley &Sons, 1971), p. 129.

99

13Corrosion of Metals and Alloys

The corrosion characteristics of the metals and alloys that might be considered forservice in solutions of nominally pure sodium hydroxide, at various concentrationsand temperatures, are discussed in this chapter. The effect of contaminants ormixtures is discussed in Chapter 15. Materials of construction for specific plant itemsand types of equipment are described in Chapter 18.

Aluminum and Its Alloys

Aluminum is rapidly attacked by even dilute solutions of caustic soda at all tem-peratures. The aluminum ion (Al��) is readily complexed by hydroxyl ions(OH�). Dilute solutions (�1 N) can be inhibited by saturating with potassium di-chromate. Aluminum should not normally be considered for service above aboutpH 8.5.1 The corrosion of aluminum in caustic is controlled by the competing pro-cesses of film growth and dissolution. The film formed consists of an inner compactlayer and an outer crystalline one. At pH around 9 or less, the corrosion rate is low,but at higher pH, cavities form in the outer layer, permitting access of the fluid tothe surface and increasing corrosion rate. At pH 12, these cavities are more promi-nent, and severe localized corrosion occurs. The corrosive effect of increasing solu-tion pH for aluminum at different temperatures is clearly seen in Figure 13.1.2 Theaddition of 1,000 ppm chloride ions decreased the corrosion rate at 60�C (140�F) buthad no appreciable effect at 30�C (86�F).Impurities present in NaOH can have a strong effect on the corrosion of alumi-

num. Impurities such as Fe2� can be reduced on the metal surface, forming pref-erential sites for hydrogen evolution and greatly increasing corrosion current. Alu-minates in solution contribute to a slight decrease in corrosion current.3


Figure 13.1 Effect of pH on the Corrosion Rate of Aluminum in NaOH at 30�C (left)and 60�C (right)

Iron and Steel

Ferrous alloys are very commonly used in caustic soda service, provided iron con-tamination is not objectionable and provided certain restrictions are imposed onservice conditions to avoid stress corrosion cracking (SCC).At ambient temperatures, iron and steel are protected by a passive layer of mag-

netite that is formed by the following reaction:

� � �3Fe � 4 OH r Fe O � 4H � 4e (1)3 4

Magnetite is the least soluble iron oxide in alkaline solutions, so corrosion is largelyprevented under these conditions.4 At elevated temperature, magnetite has beenfound to be either protective or nonprotective, depending on the growth conditions.A protective film grows at the metal/oxide interface with “excess” iron ions formingsoluble ferroates, while the nonprotective magnetite film becomes highly stressedas it grows at the oxide/solution interface. The breakdown of the protective filmalso occurs at more elevated temperatures and under turbulent or erosive condi-tions.5

Cast Irons

Gray cast iron is resistant to caustic but is usually not used because of safety prob-lems associated with its brittle nature. At one time, caustic soda solutions wereconcentrated in cast iron evaporators.Hot alkalis at �30% concentration attack unalloyed irons. Temperature should

be �80�C (175�F) in concentrations up to 70% if the corrosion rate is not to exceed


0.25 mm/y (10 mpy). The corrosion rate of ductile cast iron (DI) is similar to that ofgray cast iron, but DI can be susceptible to cracking in highly alkaline solutions,while gray cast iron is not.6 Ductile cast iron is, however, sometimes used for specificitems, such as valves or pumps.The addition of nickel greatly reduces the corrosion rate of cast iron in boiling

50% to 65% caustic, as shown in Table 13.1.7 This was an 81-day test under a vacuumof 660 mm (26 in.) mercuryThe austenitic nickel cast irons of greater than about 15% nickel content, such as

NiResist� types 1 (F41000) and 2 (F41002) and the ductile NiResist type D2 (F3000),are much more resistant than unalloyed cast iron in caustic solutions up to about70%. Corrosion rate should be �0.25 mm/y (10 mpy) in solutions up to 70% NaOHat temperatures approaching boiling.6 The resistance is roughly proportional to thenickel content unless sulfur or sulfur compounds are present. The nickel-containingcast irons may be susceptible to SCC, especially in the presence of high chlorides,so it is considered a reasonable precaution to stress relieve these alloys before usein hot caustic soda solutions.7 The corrosion rate of NiResist� cast iron is comparedwith that of gray cast iron in Figure 13.2.8 Detailed corrosion data for NiResist� arecompared with those of unalloyed cast iron in Table 13.2.9

The resistance of unalloyed and alloy cast irons in molten caustic soda is shownin Table 13.3.10 These data show that nickel has superior resistance to all these castirons, including the highly alloyed ones.High-silicon cast irons have good resistance to relatively dilute caustic soda so-

lutions at moderate temperatures. This type of iron, such as the 14% silicon cast iron(F47003), is not resistant at higher strengths of caustic at elevated temperature be-cause NaOH reacts with the siliceous film from which it derives its acid resistance.High-chromium cast irons are also not resistant to strong alkaline solutions.

Carbon and Low-Alloy Steels

Some early data on the corrosion of steel in ambient temperature is shown in Table13.4. These data show that corrosion rate decreases with an increase of caustic con-centration at room temperature.11

At ambient temperature, steels are only slightly attacked by caustic soda withsolution strength having little effect on rate. Unalloyed and low-alloy steel corrodeat�0.005mm/y (�0.2 mpy) in up to 20%NaOH at ambient temperature. In strongersolutions, 20% to 50%, the corrosion rate will be �0.01 mm/y (�0.4 mpy), and thesteel is still usually protected by the presence of a passive layer of magnetite.4 Athigher strengths and temperatures, this oxide layer no longer provides effectiveprotection, and corrosion rates increase.The stated limits for the use of carbon and low-alloy steels in caustic soda solution

at temperatures above ambient vary widely. Typical suggested limits on the basisof metal loss by corrosion are 50% concentration at temperatures up to 85�C to 90�C(185�F–194�F)12 and 50% at up to 60�C (140�F).13 One source says that the corrosionrate in 50% will be �1 mpy (�0.025 mm/y) at 40�C (104�F), 5 mpy (0.13 mm/y) at60�C (140�F), and 8 mpy (0.20 mm/y) at 55�C to 75�C (131�F–167�F).14 Yet another


Table 13.1 Effect of Nickel Additions on Corrosion of Cast Irons in Boiling 50% to65% NaOH

Nickel (%)Corrosion Rate(mm/y [mpy])

0 1.9 (73)2.3 (91)2.2 (86)

3.5 1.2 (47)5 1.24 (49)15 0.8 (30)20 0.08 (3.3)20 (�2% Cr) 0.15 (6)30 0.01 (0.4)

Figure 13.2 Corrosion Rates of Gray Cast Iron Compared with NiResist� in CausticSoda


Table 13.2 Corrosion Data for NiResist� and Cast Iron in Caustic Soda

Average Corrosion Rate(mpy [mm/y])

Medium LocationTemperature

�F (�C) Cast Iron NiResist�

50% caustic sodaplus suspendedsalt

Salt tank 180 (82) 6.3 (0.16) 0.01 (�0.01)

50% caustic soda Distributor box tosettler

Hot 20 (0.51) 4 (0.10)

50% caustic soda Evaporator Hot 30 (0.76) 6 (0.91)50% caustic soda High-concentration

evaporatorHot 40 (1.02) 20 (0.51)

Anhydroussodiumhydroxide

Flaker pan 700 (371) 510 (12.95) 13 (0.33)

75% caustic soda Storage tank aftervacuumevaporator

275 (135) 70 (1.78) 4 (0.10)

NaOH and KOHeach 90%

Flaker pan 700 (371) 500 (12.7) 13 (0.33)

Caustic soda anddissolved silicates

Drop kettle inmetal cleanermanufacture

120 (49) 30 (0.76) 5 (0.13)

Table 13.3 Corrosion of Cast Irons by Molten NaOH at 510�C (950�F)

Material Corrosion Rate (mm/y [mpy]) Pit Depth (mm [mils])

Gray iron 2.5–3.4 (97–135) 0.13 (5)Ductile iron 5.3 (207) —White iron 3.8 (151) 0.5 (20)3% nickel-iron 1.8 (71) —Austenitic, type 1 15.9 (628) 1.5 (60)Austenitic, type 2 24.2 (954) 1.8 (70)Ductile austenitic, type 2 11.8 (466) 1.5 (60)Austenitic, type 3 2.2 (87) 0Austenitic, type 4 13.6 (534) 1.0 (40)Wrought nickel 0.23 (9) —


Table 13.4 Corrosion of Steel by Sodium Hydroxide Solutions

NaOH (g/L) Corrosion Rate (mpy [mm/y])

0 2 (0.051)0.001 2 (0.051)0.1 2 (0.051)1.0 0.7 (0.018)10 0100 0.1 (0.003)540 0

source says that the corrosion rate in 50% will be 2.5 mpy (0.06 mm/y) or less at70�F to 100�F (21�C–38�C).13 Some of the discrepancy between various stated safelimits is caused by other factors, such as purity of the caustic soda, velocity, oxygen,concentration effects leading to caustic SCC, and so on. In other strengths of caustic,there is less discrepancy, and typically carbon steel is said to be satisfactory up to70% at up to 80�C (176�F)15 and in 75% at up to 100�C (212�F), assuming that ironcontamination is acceptable.16

These limits of operating conditions are based on reasonable rates of metal lossand assume that iron contamination of the solution is acceptable. If this is not thecase, then a more resistant material must be employed or the vessel must be lined.Storage tanks are often lined with neoprene, phenolic-epoxy, or other resistant coat-ing or lining. In the absence of concerns about iron contamination, bare steel tanksare used effectively to store caustic solutions up to about 50% concentration and upto about 65�C (150�F).6

If steel is under tensile stress, as from welding or cold work (e.g. field flaring ofpipe flanges) and is exposed to caustic soda, caustic SCC is possible. Under theseconditions, a safe operating limit might be up to 50% caustic at up to 65�C, althoughcracking has occurred at temperatures as low as 48�C (118�F).17 A survey of fieldexperience was used to produce a curve that indicated temperatures and causticstrengths above which cracking was possible. These data were thought to be con-servative but were based on many years of practical experience.18 Another curvedepicting the parameters of caustic concentration and service temperature abovewhich caustic cracking may be a problem was produced from short-term (up to 62days) laboratory tests (see Figure 13.3).19 This curve is also thought to be conser-vative but is widely used and reproduced in various forms. Failures that occasion-ally occur in the safe zone are likely to be associated with other factors, such aslocalized overheating or contaminants present in the caustic.Figure 13.4 shows the suggested regions, as defined by temperature and concen-

tration, in which postfabrication thermal stress relief (or an alternative alloy) is rec-ommended to avoid SCC in steels.20

An iron alloy with 3% nickel was found to be immune to caustic cracking, andthe addition of 0.5% molybdenum had little effect on that SCC resistance. However,segregation of phosphorus at grain boundaries significantly reduced resistance. Sim-


Figure 13.3 Temperature and Concentration of Caustic Soda that Can Cause SCC ofCarbon Steels

Figure 13.4 Temperature and Concentrations of Caustic Soda that Require StressRelief to Prevent SCC of Carbon Steel


ilarly, the presence of carbon or carbides increases the risk of this type of intergran-ular SCC. These conclusions are probably applicable to other low-alloy steels.21

The use of low-alloy steels is not advised under dynamic loading. Slow-strain-rate laboratory tests have indicated that physical cracks in the passive layer do notheal sufficiently quickly and that hydrogen absorbed into the freshly exposed sur-face can cause embrittlement.22

Stainless Steels

All the standard stainless steels are resistant to general corrosion by all concentra-tions of caustic soda up to about 65�C. If low levels of iron are required in theproduct, then more resistant alloys or coatings may be needed. The resistance togeneral corrosion in caustic is almost directly proportional to nickel content. Stain-less steels, however, are subject to SCC at certain concentrations and temperatures.All stainless steels can be classified into three groups according to metallurgical

structure and response to heat treatment. These are the martensitic, ferritic, andaustenitic groups. Further subdivisions include duplex alloys with austenitic/fer-ritic microstructures and precipitation-hardening (PH) grades strengthened by anage-hardening treatment. Of the conventional groups of stainless steels, the standardmartensitic and ordinary ferritic grades find little application in caustic soda service.However, certain austenitic, duplex, and superferritic grades fill an economic nicheat certain intermediate parameters of concentration and temperature.

Ferritic Grades

Ferritic stainless steels show a transition from ductile to brittle behavior over a nar-row temperature band. This transition can occur above room temperature in steelswith high levels of carbon, nitrogen, or chromium. This effect, combined with atendency to sensitize from heating, limited the usefulness of the early ferritic stain-less steels and restricted them to thin sections. Themodern low-carbon, low-nitrogengrades (with or without stabilizing additions) have limited toughness and are stillusually restricted to thin sections. The ferritic stainless steels are resistant and some-times immune to chloride SCC. This type of alloy can be subject to 475�C (887�F)embrittlement caused by precipitation of �� chromium–rich phase. The ferriticgrades are also particularly prone to r-phase precipitation because of their highchromium and molybdenum contents. Both types of embrittlement can be removedby heating and rapid cooling.The standard ferritic grades of stainless steel such as 409-430 (S40900-S40300),

434, and 444 (S43400 and S44400) contain 11% to 17% Cr with carbon and nitrogenlevels kept low to avoid embrittlement. These standard ferritic grades are not gen-erally as resistant in caustic as the standard austenitic stainless steels. For example,in 60% NaOH at 212�F (100�C), grade 444 corroded at 0.61 mm/y (24 mpy), while


Table 13.5 Corrosion of Stainless Steels in NaOH Solutions

Type

NaOHConcentration

(%)Temperature(�C [�F])

TestDuration(Days)

Corrosion Rate(mm/y [mpy])

302 20 50–60 (120–140) 134 �0.0025 (0.1)304 22 50–60 (120–140) 133 �0.0025 (0.1)309 20 50–60 (120–140) 134 �0.0025 (0.1)310 20 50–60 (120–140) 134 �0.0025 (0.1)410 20 50–60 (120–140) 134 0.0025 (0.1)430 20 50–60 (120–140) 134 0.0025 (0.1)304 72 (a) 120–125 (245–255) 119 0.09 (3.7)316 72 (a) 120–125 (245–255) 119 0.08 (3.1)329 72 (a) 120–125 (245–255) 119 0.0025 (0.1)21% Cr, 4% Ni, 0.5% Cu 72 (a) 120–125 (245–255) 119 0.15 (6)410 72 (a) 120–125 (245–255) 119 0.8 (32)302 73 (b) 100–120 (212–245) 88 0.97 (38)304 73 (b) 100–120 (212–245) 88 1.1 (45)

(a) Solution moderately agitated(b) No aeration

type 304 austenitic had a corrosion rate of 0.07 mm/y (3.0 mpy).23 Some of thestandard ferritic grades are compared with the standard austenitic stainless steelsin Table 13.5.10

There are now much more highly alloyed and corrosion-resistant ferritic stainlesssteels, commonly known as superferritics. The first of these superferritic stainlesssteels was based on 26% Cr, 1% Mo (S44627) and the columbium-stabilized XM-27(S44627). These were developed to provide better resistance to chloride SCC thanthe austenitic 300 series. These ferritic steels have low interstitial content with highchromium and very low carbon levels. Other superferritic stainless steels are basedon a 29% Cr, 4% Mo alloy, and they need low C � N levels (i.e. less than 0.025%)to avoid intergranular corrosion caused by chromium depletion from precipitationof carbides and nitrides. Some of the current ferritics contain higher levels of C �N and have additions of titanium or niobium as carbon/nitrogen stabilizers. Su-perferritic steels include AL 29-4C� (S44735), AL 29-4-2� (S44800), Sea-Cure�(S44660), E-Brite� (S44627), and Monit� (S44635).While standard austenitic stainless steels show high rates in boiling 50% caustic,

the higher alloy 20 and the superferritic grades tend to be resistant, as shown inTables 13.624 and 13.7.25,26,27,28 The corrosion rates in all cases are unacceptably high,except for alloy 20 (N08020) and the superferritic stainless steels. In the standard300 series stainless steels, the weld is less resistant than the base metal and the low-carbon L grade more resistant than the normal carbon grades.A high-purity 30% Cr, 2% Mo (Shomac� 30-2) alloy was studied in hot, concen-

trated caustic and was found to be resistant to caustic strengths up to 50% at tem-


Table 13.6 Corrosion Rates of Stainless Steels in 50% NaOH at 140�C (290�F)

Type mpy (mm/y)

304 (S30400) �180 (�4.57)316 (S31600) �120 (�3.05)439 (S43035) �300 (�7.62)444 (S44400) �290 (�7.37)XM-27 (S44627) �1 (�0.025)29-4C (S44735) �1 (�0.025)29-4-2 (S44800) �1 (�0.025)

Table 13.7 Corrosion Rates (mpy [mm/y]) in Boiling 50% NaOH Solution

Alloy Base Metal Weld

304 (S30400) 118 (3.0) 130 (3.3)304L (S30403) 71 (1.8) 87 (2.2)316 (S31600) 123.6 (3.1) 136.8 (3.5)316L (S31603) 77.6 (1.97) 85.4 (2.17)317L (S31703) 32.8 (0.83) 31.9 (0.81)20 (N08020) 7.2 (0.18) 6.0 (015)AL 29-4C (S44735) 0.4 (0.01) —AL 29-4-2 (S44800) 0.1 (0) —E-Brite� (S44627) 0.11 (0.003) —

peratures below 120�C (248�F) under isothermal conditions. In more concentratedsolutions at higher temperatures, most specimens suffered from intergranular attack(IGA). Under heat transfer conditions, corrosion was more severe, and IGA wasseen even at lower caustic concentrations. The cause of IGA was found to be carbideor nitride precipitation at grain boundaries leading to chromium depletion locally.This mechanism was analogous to that occurring in austenitic stainless steels, so thesame test methods for susceptibility can be applied. Galvanic coupling to moreactive metals prevented IGA, but potential must be strictly controlled to avoid activedissolution.29

The ferritic alloys E-Brite� (S44627); 21/4%Cr, 1%Mo (K21950); type 446 (S44600);and 26-1S (S44626) were included in tests to determine corrosion and caustic SCCin high-temperature NaOH solutions.30 It was found that in dearated 50% NaOH attemperatures in the range 284�C to 332�C (543�F–630�F), these ferritic alloys wereseverely corroded, and their susceptibility to caustic SCC was dependent on theirheat treatment.Superferritic alloys are sometimes used for heat exchanger tubing. They have also

been successfully used in caustic evaporators but require a sustained supply of ox-idizing contaminants, such as chlorates, to maintain passivity at these higher con-


centrations and temperatures. The high-purity 26-1 grade (S44626) containing nio-bium and molybdenum is useful up to about 175�C (350�F) in caustic evaporators,depending on the caustic, chloride, and other contaminant concentration.6 This typeof ferritic stainless steel has been generally successful in evaporators, but failureshave occurred from either localized or general corrosion. These failures have beenassociated with one or more of the following factors:12

• Increase in carbon levels in the alloy due to pickup from oil, grease, or otherhydrocarbon contamination during tube preparation

• Temperature in the first-effect evaporator �150�C (300�F)• High local tube temperatures caused by blockages with insoluble salts

It has been shown that environmentally assisted cracking (EAC) in caustic isrelated to hydrogen formation so that conditions that encourage hydrogen perme-ation also encourage caustic cracking. Type 410 martensitic steel was found to beimmune to cracking in concentrated, deaerated caustic soda solution up to 90�C(194�F).31

Precipitation-Hardening Grades

The precipitation-hardening (PH) stainless steels see limited service in caustic en-vironments. These alloys exhibit very high strengths combined with good notchtoughness and corrosion resistance. For this reason, they are often the preferredmaterial for such parts as valve stems and certain critical fasteners. Some of thesealloys are susceptible to hydrogen embrittlement in corrosive or hydrogen-rich en-vironments.Corrosion rates are acceptable in concentrated caustic at moderate temperatures

as shown in Table 13.8.32 At the boiling point in 50% caustic, this martensitic alloy17-4PH (S17400) in condition H 1075 was severely corroded. Tests were carried outfor five 48-hour periods.

Table 13.8 Corrosion Rates of 17-4PH (S17400) in Hot, Concentrated Caustic

Concentration (%) Temperature (�C [�F]) Corrosion Rate (mm/y [mpy])

30 80 (176) 0.18 (7.1)50 80 (176) 0.10 (3.9)30 Boiling 0.28 (11.0)50 Boiling 14.2 (559)

Duplex Grades

Duplex stainless steels have a controlled balance between austenite- and ferrite-bearing constituents. The “duplex” structure contains approximately 50/50 austen-


ite and ferrite phases, resulting in higher strength as compared to the 18-8 grades,as well as improved corrosion resistance in some environments (e.g. chloride-bearing aqueous solutions). Duplex stainless steels are not commonly used in causticsoda applications except where their resistance to chloride SCC is useful from thewater side of heat exchangers. The modern grades, such as alloy 2205 (S31803),typically also contain molybdenum to achieve a composition of about 22% Cr, 5.5%Ni, 3% Mo, 0.03% C max., strengthened and stabilized by nitrogen additions. Themixed austenite-ferrite structure imparts strength and resistance (but not immunity)to chloride pitting and SCC. They have a higher strength than the lower austeniticgrades, such as type 304 (S30400), but are subject to temper embrittlement at about475�C (885�F).Corrosion testing in a range of caustic solutions at the boiling point showed that

duplex alloys can be used in boiling solutions up to at least 30% with negligiblecorrosion (see Figure 13.5).33 These tests also showed that these duplex alloys werenot susceptible to SCC in boiling caustic solutions from 20% to 70%.Corrosion rates of duplex stainless steels (21% Cr, 6.6% Ni, 2.3% Mo; Uranus� 50

and 26% Cr, 5.6% Ni, 3.2% Mo; Ferralium� 255) in boiling caustic soda up to 30mass % was less than 0.01 mm/y (0.39 mpy) with uniform corrosion rates. The rateincreased with caustic concentration reaching a maximum at 60mass %, and in thesemore concentrated solutions, the austenitic phase was slightly more attacked thanthe ferritic phase. Small flow rates (e.g. 0.08 m/s rotating disc) increased the cor-rosion rate by a factor of six compared with stagnant conditions. Further increasesin flow rate up to 1 m/s had no further effect on corrosion rate.34

Welding tends to lead to variations (20%–80%) in the austenite/ferrite ration inthe as-cast weld bead and fusion zone. To minimize this effect, they are weldedwitha nickel-rich, overmatching rod whose higher nickel content is beneficial in causticservice.E-Brite� and 7-Mo� stainless steels showed good resistance to SCC and corrosion

in 50% caustic at 135�C (275�F).6

Recent corrosion testing of various duplex stainless steels in 30% to 70% NaOHsolutions found that their resistance was in the order of

S32304 � S32205 � S32750 � S32906 � alloy X (a proprietary duplex alloy)

Alloy 200 (N02200) had a lower corrosion rate than any of the duplex alloys inpure caustic solutions at any test temperature. Statistical analysis of the resultsshowed that the most important alloying elements were chromium, nitrogen, andnickel to produce resistance to caustic corrosion in the duplex alloys. The effect ofchromium in the austenite phase was investigated in a set of experimental duplexalloys. It was found that corrosion resistance was directly proportional to the chro-mium content. The range of chromium contents was from 26.54% to 29.04% withcorrosion rates in boiling 60% NaOH of 0.39 mm/y to 0.05 mm/y, respectively. SCCtests in 50% NaOH at 137�C (279�F) showed that S32906 was immune to crackingin this environment.35


Figure 13.5 Corrosion Resistance of Duplex Stainless Steels in Boiling NaOHSolutions

Because of the uncertainties about the amount of chlorides and oxidizing salts,such as hypochlorites and chlorates, extensive field corrosion tests should be madebefore selecting duplex stainless steels for caustic soda service.

Austenitic Grades

The austenitic stainless steels constitute a large, diverse body of alloys developedfrom the original 18% Cr, 8% Ni stainless steel, type 302 (S30200). The standardcommercial grade is type 304 (S30400), approximately 18% Cr, 8% Ni, �0.08% C.Additions of titanium or columbium in amounts equal to 5 or 10 times the carboncontent, respectively, protect against sensitization and chromium carbide precipi-tation by precipitating carbon as titanium or columbium carbides, thus preventingchromium depletion. Types 321 (S32100) and 347 (S34700) are the common stabilizedgrades. A low-carbon (�0.03% Cmax.) version is available and is widely used. Type304L (S30403) overcomes the problems associated with chromium carbide precipi-tation and chromium depletion. A molybdenum-containing variant is type 316L(S31603), containing about 17% Cr, 12% Ni, and 2% to 3% Mo. The molybdenumaddition improves corrosion resistance in many environments, such as in chloride-containing solutions.There is also a stabilized version of the molybdenum-containing grade that is

commonly used in Europe and is becoming more common in North America. This


grade, 316Ti (S31635), is a high-carbon stainless steel with the carbon stabilized bythe addition of a titanium addition equal to at least five times the carbon content.Its main application is in situations where it is exposed to temperatures between550�C and 800�C (1022�F and 1471�F) for prolonged periods. It is used in Europe forpipelines or heated tanks handling caustic soda solutions.36

Modern steelmaking practice has made it possible to produce L grade stainlesssteels with controlled nitrogen additions that have the mechanical properties of theequivalent high-carbon, non–L grade. Dual grade stainless steels, 304/304L and316/316L, are now available and permitted for use in many applications. ASME hasruled that a dual grade steel may use the straight non–L grade allowable stressesfor all forms up to 540�C. If this trend continues, the incidence and likelihood ofintergranular attack from carbide precipitation will diminish, and demand for 321/347 will probably decrease further. Improved steelmaking control also means that316 can be made more cheaply with the molybdenum content at the low end of thepermitted range of 2% to 3%. This can have a serious effect on the corrosion resis-tance of this type of steel. In applications where molybdenum is a key factor incorrosion resistance, a minimum level should be specified, or type 317 or 317L, with3% to 4% Mo, should be used.37

There is little difference in the corrosion resistance of types 304L (S30403) and316L (S31603) in 50% or 73% caustic solutions. No difference was found betweenthe corrosivity of diaphragm cell caustic and mercury cell caustic in an extensivesurvey of caustic soda producers.38

Corrosion of 18-8 steels in hot, concentrated caustic solutions was found to in-crease under heat transfer conditions, possibly due to localized evaporation anderosion of the passive film by gas bubbles. Themolybdenum-containing 316 (S31600)was found to be less resistant than type 304 (S30400). Annealed and sensitized spec-imens were subject to IGA when held in the passive region. When in the activeregion, attack was general and transgranular.39

All stainless steels resist general corrosion by all concentrations of caustic sodaup to about 65�C (150�F). Other authorities give higher temperature limits for, forexample, 50% caustic, but these are probably influenced by other factors, such asthe presence of impurities in the caustic. Types 304 and 316 show low corrosion ratesin boiling caustic at concentrations up to nearly 20%. Type 316 has a better resistanceto pitting than type 304 in caustic solutions, especially if chlorides are present. Thelow-carbon grades perform marginally better than the high-carbon grades becauseof their resistance to sensitization. This means that 316L is a good choice for causticsolutions as long as operating conditions are such that caustic SCC is not a problem.6

SCC of 304 and 316 grades occurs at around 100�C (212�F). An isocorrosion curvein mpy for these grades is shown in Figure 13.6.40 This figure also shows the tem-perature and concentration region where caustic SCC of these steels is likely.The resistance to caustic SCC in 30% NaOH at 200�C increased in the order

N08904 (least resistant)� S31603� F138� S31803 (most resistant). (F138 is a specialgrade of S31603 with lower levels of inclusions.) In this environment, the additionof sulfide (20 g/L Na2S • 9H2O) dramatically increased the susceptibility of all thesealloys to SCC.41 Slow-strain-rate tests in the same environment determined that theorder of resistance for the alloys tested was as follows:


Figure 13.6 Isocorrosion Curves (in mpy) for 304 and 316 Stainless Steels in CausticSoda also Showing Limits of SCC

N08904 � N08825 � N08028 � R20033

Resistance increased as the chromium content of the alloy increased.When sulfideions were added, R20033 showed excellent corrosion resistance to pure caustic sodabut was very prone to SCC even at very low strain rates.42 The zone of concentrationand temperatures in which SCC is possible for 904L (N80904) is shown in Figure13.7.43 This figure also shows the isocorrosion curve at a 0.1-mm/y corrosion ratefor this and other stainless steels and for titanium.Caustic cracking can also be caused by potassium hydroxide. A type 304L

(S30403) bypass line in a steam methane reforming unit failed by SCC in KOH thatwas formed from a potassium-promoted catalyst. Cracking at relatively low tem-peratures was possible only in the presence of hydrogen and/or carbon monoxide.This study found that NaOH caused cracking at lower temperatures than didKOH.44

Cast Stainless Steels

Cast stainless steels perform well in caustic soda, and they are frequently used inpumps and valves. While corrosion rates are similar to those of their wrought equiv-


Figure 13.7 Isocorrosion Curve for a Corrosion Rate of 0.1 mm/y for 904 L(N80904), other Stainless Steels, and Titanium

alents, the presence of small amounts of ferrite in the cast “austenitic” stainless steelsimproves their resistance to caustic SCC compared with the fully austenitic wroughtalloys.45 Cast stainless steels are often used at operating conditions well in excess ofthose of the wrought version.6

The equivalent cast version of types 304 (S30400) and 304L (S30403) are CF8(J92600) and CF3 (J92700), respectively, and they exhibit approximately the samecorrosion response as the wrought alloys. Isocorrosion curves for CF 8 (J92600) pro-duced in tests at atmospheric pressure in open containers and at equilibrium pres-sure in closed containers are shown in Figure 13.8.46

Castings can, however, have surface layers containing more than the maximumallowable carbon content of 0.08%, which can significantly reduce corrosion resis-tance of the surface. In the cast form, the difference between CF 8 (J92600) or CF 3and the corresponding molybdenum-bearing grades CF 8M (J92900) and CF 3M(J92800) is insignificant. CF 3 or CF 8 grades are not particularly common, andmanufacturers of cast pumps and valves tend to standardize on CF 8M, which hasa broader range of applications. The molybdenum grade castings are often moreavailable and cheaper than the 304 or 304L grade equivalents. Since the cast versionof these alloys is unlikely to be welded, there is rarely a justification for specifyingthe low-carbon grades in this case. This assumes that the valves or pumps, if weldrepaired by the manufacturer, are properly reheat treated (solution annealed) torestore optimum corrosion resistance. Availability and price are likely to favor thenon–L grade, and a properly heat-treated casting in CF 8 or CF 8M is likely to be ascorrosion resistant as its low-carbon cast or wrought equivalents.


Figure 13.8 Isocorrosion Curves for CF 8 (J92600) in NaOH at Equilibrium Pressure(left) and Atmospheric Pressure (right)

High-Performance Austenitic Alloys

There is a group of high-performance austenitic alloys, sometimes called corrosion-resistant alloys (CRAs) or superaustenitic alloys, that consists of both stainless steels(i.e. containing �50% iron) with high chromium and nickel content and alloys withnickel as the predominant alloying element. The latter group are not true nickel-based alloys because they contain �50% nickel. The original high-performance aus-tenitic alloys were the molybdenum-free alloy 800 (20% Cr, 30% Ni; N08800) andthe molybdenum-containing alloy 20Cb3� (20% Cr, 33% Ni, 2.5%Mo; N08020). Mol-ybdenum is now a common alloying element in this group of superaustenitic alloys,which started with the stainless steel alloy 254SMO (20% Cr, 18% Ni, 6% Mo;S31254). The molybdenum in this type of alloy gives resistance to chloride pitting,crevice corrosion, and chloride SCC.Alloy 800 (N08800) has been used in up to 73% caustic at 120�C (250�F) but is

susceptible to caustic SCC above 150�C (300�F). Alloy 825 (N08825), which has 3%Mo, is slightly more resistant than alloy 800 (N08800). These nickel-chromium-ironalloys with and without molybdenum (e.g. N08800, N08825, N08020) have usefulresistance up to 73% caustic at temperatures up to about 120�C (250�F). The beneficialeffects of nickel and molybdenum in nickel-containing and nickel-based alloys isshown in Figure 13.9.47 These data show that both these alloying elements improvecorrosion resistance in this caustic solution. The effect of molybdenum is minimalabove about 3% to 4% Mo.


Figure 13.9 Effect of Molybdenum and Nickel in Various Alloys on Corrosion Ratein NaOH Solutions

In studies of austenitic and duplex alloys in 30 wt% NaOH at 150�C, it was foundthat under deaerated conditions, chromium and nickel passivate the alloys and thatiron is dissolved. Under very oxidizing conditions, chromium was dissolved, prob-ably as CrVI, and that nickel and iron form a thicker, less protective oxide film.Caustic SCC cracking occurred in alloy 800 (N08800) only at high anodic potentialswhere chromium is being dissolved from the oxide.48

In oxygenated 50% caustic at 300�C (572�F), an increase in nickel content between15% and 45% was found to improve the caustic SCC resistance of iron-nickel-chromium alloys containing 10% to 15% chromium but had little effect on alloyswith 20% to 25% chromium.49 Nickel also has a beneficial effect on the corrosionresistance of iron-based alloys in hot caustic solutions. The higher the nickel content,the lower the corrosion rate, as shown in Figure 13.10.50 The maximum benefit isreached at about 20% nickel with little further benefit from further increases in nickelinto the nickel-based alloys. These data also show that when the caustic was deaer-ated (with hydrogen), the corrosion rates for all these metals and alloys were re-duced.Comparative tests of various stainless steels in 50% NaOH are shown in Table

13.9.51 This shows the lowest temperature at which the corrosion rate exceeds 5 mpy(0.127 mm/y). These same data are plotted for the three alloying elements in Figure13.11. Nickel has a strong beneficial effect on corrosion resistance, while there is noapparent benefit of additional molybdenum or chromium in this environment,within the range of compositions represented by these steels.


Figure 13.10 Effect of Nickel Content on Corrosion in NaOH of Various Alloys Withand Without Oxygen in the Atmosphere

Table 13.9 Lowest Temperature at Which Corrosion Rate Exceeds 5 mpy(0.127 mm/y)

Alloy % Ni % Cr % Mo Temperature (�F [�C])

304 9 18.5 — 185 (85)316L 13 17.5 2.7 194 (90)2205 Code Plus Two 5 22 3.2 194 (90)SAF 2304 (S32304) 4.5 23 — 203 (95)SAF 2507 7 25 4.0 230 (110)254 SMO (S31254) 18 20 6.1 239 (115)654 SMO (S32654) 22 24 7.3 275 (135)904L (N08904) 25 20 4.5 Boiling (�142�C)

Data on the corrosion of the 6% Mo austenitic stainless steel alloy 6XN (N08367)and other steels and alloys in boiling 50% NaOH are shown in Table 13.10.52 Thesedata show that none of these materials is really acceptable for this duty.For pumps and valves, a cast version of the “20” type alloy CN7M (20% Cr, 29%

Ni, 3% Mo; N08007) is quite often employed. Figure 13.12 shows an isocorrosionchart for this alloy up to 73% NaOH and compares it with a cast duplex stainlesssteel (CD-4MCu).53,54 Other isocorrosion curves for this alloy in caustic soda areshown in Figure 13.13.55 The two sets of data in this figure represent results atatmospheric pressure and also in a closed container. The corrosion rates in the closed


Figure 13.11 Effect of Molybdenum, Nickel, and Chromium on the ThresholdTemperature at Which the Corrosion Rate Exceeds 5 mpy (0.127 mm/y) in 50% NaOH

Table 13.10 Corrosion Rates (mpy [mm/y]) for Various Alloys in Boiling 50% SodiumHydroxide

Type 316L Type 317L Alloy 904L Alloy AL-6XN Alloy 27677.69 (1.92) 32.78 (0.83) 9.61 (0.24) 11.4 (0.29) 17.77 (0.45)

container, that is, at temperatures above the boiling point, are substantially higherthan those at lower temperatures. Corrosion rates are below 0.13 mm/y (5 mpy) forthe complete range of NaOH concentration at temperatures up to the atmosphericboiling point.Alloy 28 (N08028) is another iron-nickel-chromium alloy that has useful resistance

to caustic solutions, particularly if contaminated. This alloy is compared with alloy800 (N08800) and alloy 904L (N08904) in NaOHwith and without chlorides in Table13.11.56 These data show that the presence of chlorides does not significantly increasecorrosion in these alloys; in fact, it decreases the corrosion of alloy 28 and alloy 800in the 43% caustic. Time of tests was a series of 1 � 3 � 3 days.An isocorrosion curve for alloy 28 (N08028) based on laboratory and plant tests

together with service experience is shown in Figure 13.14.56 This figure also includescomparative data for 304L (S30403) and 316L (S31603) and shows that this iron-chromium-nickel-molybdenum alloy is much more resistant in hot, concentratedcaustic than the standard stainless steels.


Figure 13.12 Isocorrosion Data for CN 7M (J92700) Compared with CD 4MCu(J93370) in NaOH Solutions

Figure 13.13 Isocorrosion Curves for CN 7M (J92700) in NaOH at EquilibriumPressure (left) and Atmospheric Pressure (right)


Table 13.11 Corrosion Rates (mm/y [mpy]) of Various Alloys in NaOH and NaOH/NaCl

Alloy28% NaOH(99�C [210�F])

28% NaOH/8% NaCl(99�C [210�F])

43% NaOH(135�C [275�F])

43% NaOH/6.7% NaCl(135�C [275�F])

Alloy 28 0.008 (0.31) 0.008 (0.31) 0.074 (2.91) 0.045 (1.77)Alloy 800 0.011 (0.43) 0.013 (0.51) 0.397 (15.63) 0.283 (11.14)904L 0.013 (0.51) 0.018 (0.71) 0.301 (11.85) 0.349 (13.74)

Figure 13.14 Isocorrosion Curves (in mm/y) for Alloy 28 (N08028) and StandardAustenitic Stainless Steels in NaOH Solutions

The effect of nickel on the corrosion resistance of iron-chromium-nickel alloys isshown in Figure 13.15.57 These results of tests in 28% NaOH with 8% NaCl at 99�C(210�F) show that nickel-free and high-nickel alloys are resistant with a maximumcorrosion rate at about 5% nickel. The filled dots in this figure relate to alloy 28(N08028), which has good resistance in this chloride-contaminated environment.This alloy has been used for the evaporation of diaphragm cell NaOH. In this ap-plication, erosion by sodium chloride crystals can be a problem, and this alloyshowed considerably better performance than pure nickel.Alloy 33� (R20033) is an austenitic alloy based on chromium with a nominal

analysis of 33% Cr, 32% Fe, 31% Ni, 1.6% Mo, 0.6% Cu, and 0.4% N. Testing incaustic soda has shown that it could be used to replace standard austenitic stainlesssteels in, for example, pipes or vessels for hot caustic solution where type 316Ti


Figure 13.15 The Effect of Nickel on the Corrosion of Iron-Nickel-Chromium Alloysin Caustic Soda

(S31635) is limited to 90�C (190�F). This alloy 33 could be acceptable in 25% and 50%NaOH up to the boiling point based on 28-day laboratory tests (see Table 13.12).36

At temperatures of the order of 300�C (575�F), even 25% caustic will cause SCCof the CRAs. It is very important, when both chlorides and caustic are present in anenvironment causing SCC, to know which is the active species. If it is the chloride,a CRA will suffice to overcome the problem, whereas if caustic is the cause of thecracking, then a high-nickel alloy will be needed.

Nickel and Its Alloys

The nickel-based alloys are generally to be preferred for caustic soda service but arenot without their problems. They may be discussed under separate groupings thatdistinguish between the chromium-free alloys (resistant to reducing corrosive con-ditions) and the chromium-bearing alloys, which are more resistant under oxidizingconditions. Nickel and nickel alloys can suffer from caustic SCC. The range of con-ditions that can cause SCC in nickel and various nickel alloys and stainless steels isshown in Figure 13.16. These data show that nickel and the high-nickel alloys aresusceptible to SCC only in very hot, concentrated caustic and that alloy 600 is moreresistant than nickel.4 Nickel, alloy 400, alloy 600, and alloy 690 are all susceptibleto SCC over a wide range of caustic concentrations at temperatures above 290�C(550�C).58


Table 13.12 Corrosion Rates (mm/y [mpy]) of Stainless Steels in Sodium Hydroxideat Various Conditions

25% NaOH 50% NaOH

Alloy75�C(167�F)

100�C(212�F)

BP 104�C(189�F)

75�C(167�F)

100�C(212�F)

125�C(257�F)

BP 146�C(333�F)

316 Ti �0.01(�0.39)

0.12(4.72)

0.63(24.8)

0.08(3.15)

0.35(13.8)

1.60(63)

7.99(315)

X1CrNiMoN25-25-2

�0.01(�0.39)

0.03(1.18)

0.02(0.79)

�0.01(�0.39)

�0.01(�0.39)

0.26(10.2)

1.35(53)

33 �0.01(�0.39)

�0.01(�0.39)

�0.01(�0.39)

�0.01(�0.39)

�0.01(�0.39)

�0.01(�0.39)

�0.01(�0.39)

Figure 13.16 SCC Regions for Nickel and Other Alloys in NaOH

Chromium-Free Alloys

Commercially pure nickel, alloy 200 (N02200), is one of the best metals for resistingcorrosion while simultaneously avoiding unacceptable metal contamination. Alloy200 has a corrosion rate of �0.2 mpy (�0.005 mm/y) below 50% NaOH at up toboiling temperature. Above 315�C (600�F), the low-carbon variant, alloy 201(N02201), must be used to avoid embrittlement by graphitization and attendant


Figure 13.17 Isocorrosion Curve (in mpy) for Alloy 200 (N02200) and Alloy 201(N02201) in Caustic Soda

intergranular attack.59 A chart showing corrosion regions for alloy 200 and 201 isshown in Figure 13.17.7 The mechanical strength of nickel also diminishes at ele-vated temperatures, encouraging the use of chromium-bearing nickel alloys.Nickel has corrosion rates of�0.1 mpy (0.0025 mm/y) up to 73% NaOH at 115�C

(240�F) and only 1 mpy (0.025 mm/y) at 130�C (265�F). Caustic soda is usually pro-duced at 11% to 15% solution and concentrated up to 50% or higher by evaporationin nickel 200 vessels. It has outstanding corrosion resistance to caustic soda at allconcentrations up to anhydrous at boiling or molten temperatures.6

Nickel is more severely attacked in hot, concentrated caustic under heat transferconditions and in the presence of chlorates in solution.60 The presence of oxidizablesulfur compounds also increases the corrosion of nickel in caustic. The effect is morepronounced with sulfides, such as hydrogen sulfide, mercaptans, or sodium sulfide,and, to a lesser extent, with partly oxidized compounds, such as thiosulfates andsulfites.59

Alloy 201 (N02201) and alloy 33 (R20033) were exposed in caustic evaporators invarious Russian plants and to laboratory evaluation. This study concluded the fol-lowing:61

• Alloy 201 and its welded joints were resistant to corrosion in evaporation of elec-trolytic caustic soda solutions (from both mercury and diaphragm cells) up to60% NaOH at temperatures up to 170�C; increased grain growth (index 2–3) was


Table 13.13 Static Corrosion Rates (mpy [mm/y]) in Molten Caustic Soda

Alloy (UNS No.) 400�C (750�F) 500�C (932�F) 580�C (1076�F) 680�C (1256�F)

Alloy 201 (N02201) 0.9 (0.022) 1.3 (0.033) 2.5 (0.064) 37.8 (0.96)Alloy 400 (N04400) 1.8 (0.046) 5.1 (0.13) 17.6 (0.45) —Alloy 600 (N06600) 1.1 (0.028) 2.4 (0.061) 5.1 (0.13) 66.4 (1.69)

seen in the HAZ of the welded joints of alloy 201, which may be a contributingfactor to the failure of the welded joint of the alloy 201 in diaphragm caustic sodain more severe conditions: 65% to 70% NaOH, 180�C to 195�C (365�F–383�F). In-tercrystalline and knife-line (IGA) corrosion was seen in the welded joint, and theweld metal suffered selective corrosion. The overall corrosion rate of the weldedjoint of alloy 201 was 0.8 mm/y.

• The base metal and welded joints of alloy 201 were fairly corrosion resistantwhenconcentrating to more than 60% NaOH of mercury cell caustic soda; alloy 33 andits welded joints possess good resistance under the conditions of evaporationconcentration of diaphragm cell caustic soda (up to 65% NaOH), but, as the con-centration increased to 70%, the corrosion rate of the welded joint increased to0.3 mm/y with general corrosion and marked etching of the weldmetal; at higherconcentrations of the diaphragm cell caustic soda, the alloy suffered pitting cor-rosion.

• Under the conditions of concentration from 45% to 60% of mercury cell causticsoda (free from chlorides), alloy 33 was resistant, but not as resistant as alloy 201(0.019 and 0.0065 mm/y, respectively). At caustic soda concentrations rising from60% to 97.5%, the corrosion rate of alloy 33 was about one-third that of alloy 201(0.125 and 0.336 mm/y, respectively). On further concentration of the caustic sodato 99.5%, the corrosion rate of alloy 201 was about 20 times higher than that ofalloy 33 (0.0022 and 0.0485 mm/y, respectively).

When even nickel ion contaminationmust beminimized, cathodic protectionmaybe applied in concentrators and storage tanks, with effective current densities aslow as 0.11 A/m2 (0.01 A/ft2).14 The corrosion rate increases linearly with chloratecontamination and other oxidizing contaminants.The behavior of alloy 201 (N02201) is compared with other nickel alloys in molten

caustic in Table 13.13.62 It can be seen that nickel is more resistant to corrosion inmolten caustic than the more highly alloyed materials even at elevated temperature.In molten caustic containing 0.5% sodium chloride, 0.5% sodium carbonate, and

0.0.03% sodium sulfate at 510�C (950�F), the corrosion rate of wrought nickel was 9mpy (0.23 mm/y). This high rate was still much lower than rates measured for otheralloys tested.10 Rates of over 50 mpy (�1.27 mm/y) were determined for nickel inlaboratory tests in 75% caustic concentrated to the anhydrous product at 480�C(900�F). This rate was also much lower than the other alloys tested, apart from silver,which corroded at a rate of 5.3 mpy (0.135 mm/y).63 In plant exposures at 540�C


Table 13.14 Corrosion Rates (mpy [mm/y]) of Nickel and Other Materials in CausticEvaporation Plants

14% NaOHFirst-EffectMultieffectEvaporator

23% NaOHin Tank

Receiving Liquorfrom Evaporator

Single-EffectEvaporator—NaOH from30% to 50%

EvaporatorConcentratingNaOH to 50%

AlloyAve. temp. 190�F

(88�C)Ave. temp. 220�F

(104�C)Ave. temp. 179�F

(81.7�C)—

Alloy 200 0.02 (�0.001) 0.16 (0.004) 0.10 (0.003) 0.1 (0.003)Alloy 400 0.05 (0.001) 0.20 (0.005) 0.19 (0.005) —Alloy 600 0.03 (�0.001) 0.17 (0.004) — 0.3 (0.008)Mild steel 8.20 (0.21) — 3.70 (0.09) —Cast iron 8.20 (0.21) — 7.00 (0.18) 22 (0.56)NiResist� type 1 2.90 (0.07) — — 4 (0.10)NiResist� type 2 — — — 2.6 (0.07)3% Ni cast iron — — — 9 (0.23)Copper — — 2.30 (0.06) —75% Cu, 20% Ni,5% Zn

— — 0.50 (0.01) —

14% Cr steel — — 33.0 (0.84) —

(1000�F), the corrosion rate of nickel soared to 260 mpy (6.60 mm/y) because ofsulfur and nitrate contamination.64

The least expensive chromium-free, nickel-based alloy is alloy 400 (N04400), andit is stronger than nickel itself. It is used in caustic soda service up to about 73%but, having a 25% to 30% copper content, may be objectionable from the contami-nation standpoint in some applications where it might be suitable from the corrosionpoint of view. It is less resistant than nickel at concentrations above 73% at theatmospheric boiling point and can suffer SCC in hot caustic. It is subject to SCC athigher temperatures, such as in 0.2 to 0.27 MPa (300–400 psi) steam contaminatedwith alkaline boiler-treating chemicals. Also, liquid metal embrittlement (LME) hasoccurred with alloy 400 (N04400) components handling mercury cell caustic. AlloyK-500 (N05500), a tougher and more abrasion-resistant variant that undergoes pre-cipitation hardening, has substantially the same corrosion characteristics as alloy400.Results of plant tests of various nickel-based alloys and other materials in caustic

evaporators are shown in Table 13.14.59 These data show the superiority of nickelover the other alloys but also show that alloy 400 and alloy 600 have acceptablecorrosion resistance.Nickel-clad steel plate is often used to provide simultaneously the corrosion re-

sistance of nickel and the strength of steel at moderately elevated temperatures. Ofcourse, to prevent corrosion of the substrate and iron contamination, special pre-cautions must be taken in fabricating and welding.


The molybdenum-rich alloy B2 (N10665) has good corrosion resistance in theabsence of oxidizing contaminants but finds no application because of the excellentresistance of nickel itself.Although electroless nickel plating (ENP) has a corrosion resistance superior to

pure nickel 200 (N02200), such coatings are used only to minimize iron contami-nation since they contain “holidays,” which allow the caustic solution to access thesteel surface. The resistance of ENP in caustic depends on its phosphorus content.The phosphorus is present since ENP is produced by the autocatalytic reduction ofnickel in the presence of sodium hypophosphite. In tests of ENP samples with vari-ous phosphorus contents (from 0 to 18.2 at%) in 50% NaOH at room temperature,the following was found:65

• All coatings had a corrosion rates of�2 lm/y (0.002 mm/y) in 50% NaOH at RT.• High-phosphorus EN (HPEN) was less resistant than low-phosphorus (LPEN) ormedium-phosphorus (MPEN) coatings in hot, concentrated NaOH. LPEN andMPEN had corrosion rates similar to pure nickel in this environment.

• This inverse effect of phosphorus on corrosion resistance was said to be due tothe formation of soluble nickel complexes in the presence of high phosphorusleading to the removal of Ni from the film instead of the formation of the protec-tive nickel hydroxide/nickel oxide film.

Chromium-Bearing Alloys

The nickel-iron-chromium alloys can be subject to caustic SCC in hot, concentratedcaustic. In deaerated 50% NaOH, the presence of nickel improves the resistance toSCC. In aerated 50% NaOH, both chromium and nickel need to be present to resistcaustic SCC. The presence of molybdenum in these alloys was found to have noeffect on the cracking resistance in dearated 50%NaOH. Similarly, no particular heattreatment improved the resistance of these alloys to caustic SCC. The effect of tem-perature in the range tested, 284�C to 332�C (543�F–630�F), was found to have onlya minor effect on the extent of cracking.30 The beneficial effect of nickel in weakerdearated caustic (10%) at 316�C (601�F) was found to be less pronounced with alloy800, alloy 600, and alloy 690, all showing similar cracking resistance.66

The most commonly used chromium-bearing nickel alloy is the basic nickel-chromium-iron alloy 600 (76% Ni, 16% Cr, 8% Fe; N06600). This alloy combinescorrosion and heat resistance with excellent mechanical strength and workability.The high nickel content provides resistance to caustic corrosion. It is also resistantto chloride-induced SCC and to corrosion by many organic and inorganic acids andwater should equipment be exposed to these environments external to the causticapplication.Alloy 600 (N06600) is the preferred alloy for NaOH at higher temperatures, es-

pecially if sulfur compounds are present. The high chromium content of alloy 600confers resistance to sulfur compounds and oxidizing environments in caustic. Alloy600 (N06600) has performed well in alkaline sulfur solutions, such as those encoun-tered in the manufacture of sulfate or kraft paper. However, sulfur contamination


Figure 13.18 Isocorrosion Curves for Alloy 600 (N06600) and Alloy 201 (N02201) inCaustic Soda

of nickel alloys can cause a form of liquid metal cracking or embrittlement (LME)at welding temperatures. This is not, in fact, true LME but a form of intergranularpenetration caused by the formation of a low-melting-point nickel/nickel sulfideeutectic. Alloy 600 (N06600) is more resistant to this type of attack than is alloy 201(N02201).12

Minimal corrosion rates for alloy 600 (N06600) are found in boiling NaOH up toabout 50% concentration, as shown in Figure 13.18, in comparison with alloy 201(N02201).67,68 However, this alloy is subject to caustic SCC above about 190�C (375�F)in strong caustic, although short-term U-bend tests showed no cracks after 1 monthin various solutions of caustic up to 70% at 184�C (363�F).The possibility of SCC can be lessened if the alloy is fully stress relieved for 1

hour at 900�C (1650�F) or for 4 hours at 790�C (1450�F). In tests to assess the effectsof heat treatment on SCC resistance, alloy 600 (N06600) and alloy 690 (N06690) werecompared in 40% NaOH at 315�C. It was found that sensitized alloy 600 was moreresistant to SCC than the fully annealed samples. The explanation given is that thechromium carbides present in the sensitized alloy have a more beneficial effect onSCC than the detrimental effect of chromium depletion. Effects of carbides and chro-mium depletion on IGA were not investigated in these tests. SCC resistance alsoincreased with increase in grain size and with chromium content.69

Alloy 600 (N06600) is used worldwide as a steam generator tube material inpressurized light water reactor nuclear plants where caustic SCC is also encoun-tered. In this case, the environment is typically 10%NaOH at around 300�C (572�F).70


A variant of alloy 600 (N06600) is alloy 601 (N06601). This solid-solution-strengthened higher-chromium (23%) alloy was developed primarily for high-temperature applications. However, it too shows excellent corrosion resistance inup to 98% caustic because of the high nickel content.The nickel-chromium-molybdenum alloys, such as alloy 625 (N06625) and alloy

C276 (N10276), are rarely employed in caustic service because the conventionalnickel alloys are usually suitable. The improvement in corrosion resistance affordedby the highmolybdenum content is minimal for all practical purposes in this service.Although not usually associated with alkaline service, these expensive alloys maybe usefully employed or encountered in some special applications. In 10% NaOHat 199�F (93�C), there was no measurable corrosion on alloy 625 (N06625). In boiling50%NaOH at 143�C (289�F), a corrosion rate of 0.061 mm/y (2.4 mpy) wasmeasuredon alloy 625, while on alloy C276 the corrosion rate was 0.452 mm/y (17.8 mpy).71

The other chromium-containing nickel-based alloy that is sometimes used incaustic is alloy 690 (N06690). The corrosion rate of alloy 6030 (N06690) in 70%NaOH at 170�C (338�F) was 0.03 mm/y (1.18 mpy). In the same tests, alloy 400(N04400) and alloy 33 (R20033) had the same corrosion rate (i.e. 0.03 mm/y), whilealloy 59 (N06059) corroded at 0.48 mm/y (18.9 mpy) and alloy C-22 (N06022) at0.51 mm/y (20.1 mpy).72

Clamps made from an alloy of nickel with 16% Mo, 15% Cr, and tungsten (C typealloy) holding internal steam coils in a reactor failed by caustic SCC after 3 yearsservice. The reactor environment was 20% organics, 8% NaCl, 4% NaOH, and bal-ance water, and the steam in the coils was at 180�C (356�F). It was postulated thatthe caustic must have become more concentrated under the clamps. An extensiveinvestigation found that while the 22% Cr and 25% Cr duplex stainless steels werenot immune to caustic SCC, they were much more resistant than the original nickel-based alloy. The failed clamps were replaced with a duplex stainless steel that isstill in service after more than 12 years.73

Failures of alloy C-276 (N10276) plate heat exchanger and other field failuresby dealloying have been reported.2 Laboratory investigation showed that otherhigh-performance nickel alloys, including the entire “C” family apart from alloy C-4 (N06455), are susceptible to this type of attack. The effect is to remove alloyingelements, such as chromium, tungsten, molybdenum, and to some extent iron, fromthe matrix. It was found that dealloying did not occur at test temperatures below90�C (194�F) or in caustic solutions with 5% or 0.5% NaOH. Tests in reagent grade50% caustic produced dealloying only at test temperatures of 140�C (284�F), andattack occurred at lower temperatures if oxidizers were present; in this, case 0.5%sodium chlorate was used. Potassium hydroxide was found to be less aggressive atcausing dealloying than sodium hydroxide.

Copper and Its Alloys

There is very little published corrosion data on copper alloys in caustic soda. Thisis because the major industries that use large amounts of caustic find copper con-


tamination intolerable. For example, copper causes rancidity in soap and discolor-ation in rayon.Copper, gunmetal (C90550), and bronze have limited resistance to caustic soda

and potash, as do copper-nickel alloys and high-tin, aluminum bronze.74 High-zincbrasses suffer dezincification and should be avoided. However, conventional bronzecastings, such as the 85% Cu, 5% Sn, 5% Zn, 5% Pb (C83600), can be used for valvesor pumps for 25% caustic solutions, for example. Bronzes solution annealed at 850�C(1,560�F) can show SCC at pH 12.3 (about 500 ppm NaOH).Corrosion resistance does increase with nickel content, and the 90-10 cupronickel

(C70600) and 70-30 grade (C71500) are sometimes used for caustic service. Becauseof improved heat transfer properties and strength, the 70-30 (C71500) grade has beenused in evaporators up to 50% concentration in applications in which copper con-tamination is acceptable.Copper piping has sometimes been used for caustic soda solutions in situations

in which thermal stress relief of steel piping was impractical.15 In the absence ofoxidizing agents (e.g. chlorites, chlorates), copper may be used up to 73% NaOH to100�C (212�F).In molten caustic, however, copper is much less resistant than iron or nickel, being

attacked at about 500 mpy (12.7 mm/y) as compared with 20 mpy (0.51 mm/y) and7 mpy (0.18 mm/y), respectively, for the alternative materials.

Titanium and Its Alloys

Titanium (e.g. R50400) is a reactive metal that forms a thin, tenacious, self-healingoxide film in oxygen-containing environments. It resists alkaline media, includingcaustic solutions at subboiling conditions, but is not recommended for boiling con-centrated solutions. Temperature and concentration limits for titanium are 80�C(175�F) and 50% NaOH. Titanium is at “high” risk for LME in caustic contaminatedby mercury. Titanium is also rapidly attacked by hot caustic containing powerfuloxidizing agents, such as permanganate in pulp/paper applications.Titanium and its alloys experience excessive pickup of hydrogen from corrosion

reactions, causing hydride formation and possible hydrogen embrittlement inNaOH above 80�C (175�F) at pH of 12 or more (�400 ppm NaOH).6 Corrosion oftitanium increases rapidly as temperature increases; for example, the corrosion rateis 0.18 mm/y in 73% NaOH at 130�C (266�F) but is �1 mm/y at 190�C (374�F).Titanium finds application (subject to the limitations noted) chiefly where the

surrounding media external to the caustic vessel or assembly contain impuritiessuch as chloride, chlorate, hypochlorite, and wet chlorine. For example, titaniumwas not corroded in 10% NaOH � 15% NaCl at 82�C (180�F) or in 60% NaOH �2% sodium hypochlorite � trace ammonia. In 50% NaOH containing free chlorineat 38�C (100�F), the corrosion rate was 0.023 mm/y. The corrosion behavior of com-mercially pure titanium in various caustic solutions is shown in Table 13.15.75,76

In a highly alkaline mixture of 52% NaOH and 16% NH3, titanium corroded�0.9mm/y (35.4 mpy) in long-term tests at about 140�C. In 73% NaOH without am-


Table 13.15 Corrosion Rates of Commercially Pure Titanium in Various Solutions ofNaOH

Concentration (%) Temperature (�C [�F]) Corrosion Rate (mm/y [mpy])

5–10 21 (70) 0.001 (0.04)10 Boiling 0.02 (0.79)28 Boiling 0.0025 (0.10)40 RT 0.13 (5.1)

93 (199) 0.064 (2.5)121 (250) 0.127 (5.0)

50 38–57 (100–135) 0.013 (0.5)66 (151) 0.018 (0.7)80 (176) 0.0025–0.013 (0.1–5.0)121 (250) 0.033 (1.3)

50–73 190 (374) �43 (�1.09)73 60 (140) 0.18 (7.1)

110 (230) 0.051 (2.0)116 (240) 0.127 (5.0)129 (264) 0.178 (7.0)

75 121 (250) 0.033 (1.3)Saturated RT Nil

monia, the corrosion rate of titanium was only about 0.18 mm/y (7.1 mpy) at about130�C.77

In tests to assess the resistance of titanium and other alloys to caustic SCC atelevated temperature, it was found that the corrosion rate was too high to detectdiscrete cracks in the specimens. These tests were carried out in 50% NaOH at tem-peratures in the range of 284�C to 332�C (543�F–630�F).30

Zirconium and Its Alloys

Zirconium (e.g. R60702) is a reactive metal, forming a very resilient oxide film (zir-conia) that provides resistance to almost all alkalis, fused or in solution. The oxidefilm is self-healing and is protective up to 300�C (570�F). Zirconium has outstandingresistance to sodium hydroxide, tending to mirror the performance of nickel. Zir-conium is considered useful in 73% NaOH at up to 138�C (280�F). Like titanium,zirconium is considered at high risk for LME in caustic contaminated with mer-cury. Zirconium is not attacked by oxidizing media unless chlorides are present.14

Zirconium is very resistant to molten sodium hydroxide at temperatures above1000�C (1830�F). It is resistant to SCC in boiling caustic solutions.6 Corrosion ratesof zirconium Zr702 (R60702) in pure and contaminated caustic soda show that it isresistant to all conditions except very hot, very strong solutions (see Table 13.16).78


Table 13.16 Corrosion Rates of Zirconium (R60702) in Sodium Hydroxide Solutionsand Mixtures

EnvironmentTemperature(�C [�F])

Corrosion Rate(mpy [mm/y])

5%–10% NaOH 21 (70) �1 (�0.025)28% NaOH Room �1 (�0.025)10%–25% NaOH Boiling �1 (�0.025)40% NaOH 100 (212) �1 (�0.025)50% NaOH 38–57 (100–135) �1 (�0.025)50%–73% NaOH 188 (370) 20–50 (0.508–1.27)73% NaOH 110–129 (230–1,000) �2 (�0.05)73% NaOH to anhydrous 212–538 (414–1,000) 20–50 (0.508–1.27)9%–11% NaOH, 15% NaCl 82 (180) �1 (�0.025)10% NaOH, 10% NaCl, wet CoCl2 10–32 (50–90) �1(�0.025)0.6% NaOH, 2% NaCl, trace of NH3 129 (264) �1 (�0.025)7% NaOH, 53% NaCl, 7% NaClO3,80–100 ppm NH3

191 (376) �1 (�0.025)

52% NaOH, 16% NH3 138 (280) �5 (�0.127)20% NaOH (suspended salt, violent boiling) 60 (140) 10–20 (0.254–0.508)50% NaOH, 750 free Cl2 38–57 (100–135) �1 (�0.025)

Cast pumps and valves utilize the benefits of zirconium where service conditionsare too severe for stainless steels, nickel alloys, or titanium.

Other Nonferrous Metals and Alloys

The common nonferrous metals, such as zinc, tin, and lead, are amphoteric and findno application in caustic service.In ambient aqueous alkaline solutions, niobium has corrosion rates of less than

0.025 mm/y (1 mpy). At higher temperatures, even though the corrosion rate doesnot seem excessive, niobium is embrittled even at low concentrations (5%) of sodiumhydroxide and potassium hydroxide. Like tantalum, niobium is embrittled in saltsthat hydrolyze to form alkaline solutions. These salts include sodium and potassiumcarbonates and phosphates.79

Silver is probably the most resistant metal in caustic soda and is not attacked bysodium hydroxides at temperatures below 500�C (930�F). At one time, it was usedin the evaporation of anhydrous caustic.Tantalum is destroyed in caustic solutions by the formation of successive surface

oxide layers. The rate of attack increases with increasing concentration or tempera-ture, and tantalum is dissolved in molten alkalis. Tantalum has, however, been usedin dilute alkaline solutions (pH 10) in a paper mill.80 Ta –10 wt% W alloy showed


passive behavior in 5%, 10%, and 15% NaOH at 50�C and in 5% NaOH at 75�C. Thistantalum-tungsten alloy was found to be slightly more corrosion resistant than tan-talum when both materials were in the passive state but less resistant under activecorrosion conditions. The tantalum-tungsten alloy can be used in caustic only overa very limited range of concentrations at low temperatures.81

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137

Table 14.1 Dry, Hot Air Temperature Limits for Various Elastomers

Elastomer Family Max. Temperature

Hard rubber (“Ebonite”) 80�C (175�F)Soft natural rubber 80�C (175�F)Neoprene* (polychloroprene) 116�C (240�F)Chlorinated rubber 115�C (238�F)Nitrile rubber 150�C (300�F)Chlorosulfonated polyethylene (CSPE) 120�C (250�F)Isobutylene isoprene (butyl) 150�C (300�F)Butadiene acrylonitrile (Buna-N�) 150�C (300�F)Ethylene propylene diene monomer (EPDM) 175�C (340�F)Silicone rubber 315�C (600�F)Fluorosilicone 204�C (400�F)Fluorocarbon elastomers 250�C (480�F)Perfluoroelastomers 327�C (620�F)

*Neoprene latix is in service in numerous transportation applications where low-temperaturecuring material was required. Neoprene latex has very limited resistance to hot water or to hot,dilute NaOH. It must be noted that many of these elastomers are not available as liners. In anyevent, the maximum temperature for immersion service is usually about 80�C (175�F).

14Resistance of NonmetallicMaterials

Elastomers

The following list (Table 14.1) represents the dry, hot air temperature limits for someelastomeric materials. Sources vary on “maximum recommended service tempera-tures,” and the user is advised to consult with the supplier regarding suitability forcaustic service.

Seals made from neoprene latex is in service in numerous transportation appli-cations where low-temperature curing material was required. Neoprene latex hasvery limited resistance to hot water or to hot, dilute NaOH. Some elastomers iden-tified here may not be available as liners.


Table 14.2 Estimated Elastomer’s Maximum Temperature (�C [�F]) in Various-Strength Solutions of NaOH

Elastomer 10% 30% 50% 70%

Butadiene 60 (140) 60 (140) 60 (140) 60 (140)Butyl, grade 1 (IR) 100 (212) 100 (212) 100 (212) 100 (212)Chlorsulfonated polyethylene (CSM,Hypalon�, etc.)

120 (248) 120 (248) 120 (248) 120 (248)

Fluoroelastomer (FKM, Viton� TBR-S,ETP-S, etc.)

100 (212) 100 (212) 70 (158) 50 (122)

Natural rubber, hard (NR) 100 (212) 100 (212) 100 (212) 100 (212)Natural rubber, soft (NR) 60 (140) 60 (140) 60 (140) 60 (140)Nitrile (NBR, Buna-N�, etc.) 90 (194) 90 (194) 90 (194) 70 (158)Perfluoroelastomer (FFKM, Kalrez� 6375,1050LF, etc.)

260 (500) 260 (500) 225 (437) 225 (437)

Polysulfide (T) 25 (77) 25 (77) 25 (77) 25 (77)Silicone (SI) 25 (77) 25 (77) 25 (77) 25 (77)Polytetrafluoroethylene (PTFE, Teflon�, etc.) 260 (500) 260 (500) 260 (500) 260 (500)

Some suggested temperature limits for elastomeric seals for immersion service incaustic soda solutions are given in Table 14.2. These data are a compilation fromvarious sources. Testing is recommended for applications involving elastomers inspecific caustic soda environments.

Plastics

There are areas of use in which both thermoplastic and thermoset resins find ap-plication in caustic service. They also find some application as immersion coatings,such as modified epoxy-phenolics.

From a practical viewpoint, the majority of polymeric materials can withstandcaustic soda solutions up to about 80�C (176�F). Careful attention needs to be givento the coating techniques or to fabrication involving polymeric materials. Factory-applied systems can usually be expected to be more reliable than those applied on-site. Preparation of vessels to be coated should be carried out by competent com-panies to a recognized standard, such as NACE RP0178, “Fabrication Details,Surface Finish Requirements, and Proper Design Considerations for Tanks and Ves-sels to Be Lined for Immersion Service.” Transport of coated components should bein accordance with the supplier’s directives.

Some plastics, such as hydrolyzable esters, are easily permeated by dilute causticand are therefore more resistant in more concentrated (40%–50% NaOH) solutions.

Thermoplastics

Polyolefins are inherently resistant to caustic within their temperature limits. High-density polyethylene (HDPE) and polypropylene (PP) generally show better chem-


Table 14.3 Thermoplastics Maximum Temperature (�C [�F]) in Various Strengths ofNaOH

Plastic 10% 30% 50% 70%

Acrylic 25 (77) 25 (77) 25 (77) 25 (77)Acrylonitrile-butadiene-styrene (ABS) 40 (104) 40 (104) 40 (104) 40 (104)Polyethylene (PE) 60 (140) 60 (140) 60 (140) 60 (140)Polypropylene (PP) 100 (212) 100 (212) 100 (212) 100 (212)Chlorinated polyether 60 (140) 120 (248) 120 (248) 100 (212)Polystyrene 60 (140) 60 (140) 60 (140) 60 (140)Polycarbonate 25 (77) 25 (77) NR NRPolyvinyl chloride (PVC) 60 (140) 40 (104) 40 (104) 40 (104)Chorinated polyvinyl chloride (CPVC) 80 (176) 80 (176) 80 (176) 80 (176)Polyvinylidene chloride (PVDC) 25 (77) 25 (77) 40 (104) 40 (104)Polyvinylidene fluoride (PVDF) 60 (140) 60 (140) 60 (140) 60 (140)Fluorinated ethylene-propylene (FEP) 200 (392) 200 (392) 200 (392) 200 (392)Polytetrafluorethylene (PTFE)* 220 (428) 220 (428) 220 (428) 220 (428)

Note: NR � not recommended*Limit to 150�C as lining

ical resistance than the low-density PE. HDPE resists all concentrations of NaOH to60�C (140�F). It should be noted that some copolymers are subject to environmentalstress cracking (ESC) and that others, such as PE/vinyl acetate (PVA) copolymers,are nonresistant because the PVA is subject to saponification. ASTM D 1693-70, 1980,provides a standard test method for evaluating ESC of ethylene plastics.

Generally, PVC, in rigid form, resists alkaline solutions to about 50�C (122�F).PVC-lined FRP piping is used for caustic service, but unsupported PVC is not safe.For safety’s sake, the chlorinated polymer CPVC, which can tolerate 10% to 70%caustic at up to about 80�C (175�F), is often preferred. This limit for CPVC is notuniversally accepted, and some suppliers suggest much lower limits. Similarly, thereare a range of limits suggested for PVDF and some other polymers. When usingany polymeric materials, it is best to consult the supplier and/or test the actualpolymer under service conditions.

Table 14.3 shows some suggested temperature limits for thermoplastic materialsin caustic service, as a consensus of several sources.

Use of many of these materials is confined to linings for pipe and vessels ratherthan as solid construction items because of the hazardous nature of caustic soda.Fluoroplastic materials are to be preferred, although PP-lined steel pipe has alsobeen used to handle 50% NaOH at 90�C (195�F). Recommended temperature limitsfor plastic lined steel pipes are given in Table 14.4.1

Thermoplastics are also used as the resistant liner in dual laminate constructionin which fiberglass-reinforced plastic (FRP) is used as the reinforcing, structuralelement. The corrosion resistance depends on the resistance of the thermoplasticliner, although resistant resins are often used in the construction of the FRP rein-


Table 14.4 Temperature Limits for Plastic-Lined Pipe in Caustic Soda

NaOHConcentration PVDC Saran� PP PVDF PTFE

�10% 150�F (65�C) 200�F (93�C) 175�F (79�C)* 450�F (230�C)10–50% 75�F (24�C) 200�F (93�C) 125�F (52�C)* 450�F (230�C)50% 75�F (24�C) 200�F (93�C) NR 450�F (230�C)�50% — 150�F (65�C) NR 450�F (230�C)

*If mercury amalgam is present, rating drops to not resistant (NR)

Table 14.5 Temperature Limits (�C [�F]) for Various Plastics in Dual Laminate Con-struction in Up to 10% Caustic Soda Solutions

Plastic Temperature Limit

PVC 40 (104), 60* (140)CPVC (chlorinated PVC) 80 (176)PE 60 (140)PP 100 (212)PVDF —ECTFE (ethylene chlorotrifluoroethylene) 100 (212)ETFE (ethylene trifluoroethylene) 100 (212)FEP (fluorinated ethylene propylene) 150 (302)PFA (perfluoro alkoxy) 150 (302)

*Conditional at this temperature, can attack or cause swelling. The medium can attack thematerial or cause swelling. Restrictions must be made in regard to pressure and/ortemperature, taking the expected service life into account. The service life of the installation canbe noticeably shortened.

forcement in case of permeation and leaks in the thermoplastic liner. Most commonthermoplastics are used in this type of construction, and many of them are suitablefor use in caustic applications (see Table 14.5).2

Thermoset Resin Materials

The thermoset resins are used almost entirely in reinforced construction, with theexception of modified epoxy coatings. FRP is used for piping and equipment incaustic service but not usually for high-concentration, high-temperature service.3,4

Such applications are contradictory to some extent in that the glass reinforcementis subject to attack by even quite weak alkalis. The use of FRP is effective becauseof the incorporation of special interior surface treatment, that is, the use of syntheticsurfacing veils (polyesters and acrylics are common). The selection and protection


Table 14.6 Thermosets as FRP Maximum Temperature (�C [�F]) in Various Strengthsof NaOH

Resin 10% 30% 50% 70%

Epoxy 60 (140) 60 (140) 80 (176) 120 (248)Furane 60 (140) 120 (248) 120 (248) 120Bisphenol-A fumarate 65 (149) 65 (149) 80 (176) NRHydrogenated Bis-A polyester 25 (77) NRChlorinated polyester 25 (77) NRVinyl ester 60 (140) 60 (140) 100 (212) NRPolyurethane 60 (140) NR

Note: NR � not recommended

of a highly resistant inner surface (the corrosion barrier) is critical for satisfactoryservice. The temperature limitations vary with the type of resin employed, and itshould be noted that lower NaOH concentration applications may be more aggres-sive to FRP than the stronger alkalis because of hydrolysis effects.

There are a great many resins used in conventional FRP construction. Conven-tional isophthalates and phenolic resins are not resistant to caustic and should notbe considered. Some special formulations, such as the bisphenol-A fumarate, arevery good up to 50% caustic at about 80�C (175�F). The hydrogenated Bis-A is limitedto dilute caustic at room temperature. The vinyl esters are also useful in the lowerconcentrations of caustic soda at moderate temperatures. It must be emphasizedthat the chlorinated polyester (chlorendic anhydride) should not be used in NaOH.

Polyurethane shows variable performance in caustic limited by the permeabilityof the material; the lower the permeability of the polyurethane, the better the per-formance. This material is limited to 10% to 15% NaOH concentration. Epoxy resinformulations are widely used up to about 90�C (195�F). Furane (furfural-furfurylalcohol) resins can be used both as reinforced materials (e.g. with graphite) and ascoatings for immersion service in intermediate concentrations. Furane is resistant toboiling 20% NaOH but not 5%. Cold-setting resins (as coatings for steel) have shownsuitable resistance for up to 20% concentration NaOH even at the boiling point.

Temperature limits for applicable thermoset resins in immersion service at vari-ous caustic concentrations are shown in Table 14.6 (data derived from varioussources).

The supplier should always be consulted in selecting thermoplastic or thermosetconstruction for caustic service, and adequate consideration must be given to themechanical and physical properties, particularly thermal expansion.

Carbon and Graphite

Corrosion resistance of carbon and graphite in caustic is good; impervious graphite(i.e. impregnated with carbon, not organic binders) is suitable for 80% NaOH up to


Figure 14.1 Wastage Rates (in mm/y) of Glass-Lined Steel in Caustic Soda. Vol:Surface Area � 20

80�C (175�F).5 The resistance of resin-impregnated graphite depends on the resinsused. Graphite impregnated with phenolic resin is marginally resistant to 10% to15% NaOH at up to 100�C (212�F). The furane-impregnated grade is rated at up to50% NaOH at 130�C, and the grade that is impregnated with fluorocarbon is claimedto be acceptable at up to 80% or even 100% at up to 230�C (446�F).6 Graphite im-pregnated with other resins are only slightly or not at all resistant to �10% NaOH.5

Impregnated raw graphite uses modified phenolic or furane resins under highheat and pressure. The phenolic binders and cements are attacked by caustic. Im-pervious graphite tube-and-shell heat exchangers contain cemented joints that aretraditionally made with phenolic resin for other services. It is essential that joints inequipment intended for caustic service be made with an epoxy or furane resin ce-ments.

Impervious graphite is used for seals and gaskets because it has dimensionalstability combined with self-lubricating, nonfatiguing, and noncontaminating prop-erties.

Ceramics

Ceramics find practically no application in caustic service. Rates of attack on glasslinings are shown in Figure 14.1.7 These data show that this type of lining is suitableonly for very low concentrations of caustic at very low temperatures, 1% or less ataround 60�C (140�F).


Table 14.7 Corrosion Rate in mm/y of Borosilicate Glass in NaOH at Various Tem-peratures

Temperature (�C [�F])NaOHConcentration (%) 41 (106) 51 (124) 65 (149) 75 (167) 100 (212)

5 0.007 0.015 0.055 0.126 0.8710 0.010 0.021 0.063 0.154 1.1815 0.012 0.024 0.071 0.178 1.4220 0.012 0.024 0.071 0.190 1.5825 0.010 0.020 0.071 0.197 1.7430 0.009 0.017 0.067 0.190 1.8535 0.008 0.014 0.063 0.166 1.9040 0.007 0.012 0.055 0.150 1.9045 0.006 0.011 0.051 0.142 1.8250 0.006 0.010 0.051 0.137 1.78

Rates of corrosion of borosilicate glass passes through a maximum at around 20%concentration at temperatures up to about 60�C (140�F). At higher temperatures, themaximum is at higher concentrations, and the corrosion rate at all concentrationsincreases dramatically. These effects are shown in the data in Table 14.7.8

The data for 50% NaOH are also presented graphically in Figure 14.2. These data

Figure 14.2 Effect of Temperature on the Corrosion of Borosilicate Glass in 50%NaOH


Table 14.8 Corrosive Weight Loss (mg/cm2/y) of Various Ceramics in 50% NaOH at100�C (212�F)

Hexoloy SA(No Free Si)

Hexoloy KT(12% Si)

Tungsten Carbide(6% Co)

Aluminum Oxide(99.0%)

2.5 �1000 5.0 75

show that glass is attacked at modest rates in 50% caustic at up to around 90�C(194�F) and above that temperature is rapidly corroded.

Certain grades of silicon carbide can be used in caustic soda service. The rate ofattack of silicon carbide (Hexoloy�) and other ceramics is shown in Table 14.8. Theweight loss rate is given in mg/cm2/y, and a material with a rate of 0.3 to 9.9 mg/cm2/y is considered suitable for long-term service.9

References

1. Anon, “Chemical Resistance Guide” (Bay City, MI: Dow Chemical, 1991), 20 pp.2. Anon, “Chemical Resistance of Thermoplastics Used in Dual Laminate Con-

structions,” DLFA (2002), 143 pp., http://www.dual-laminate.org/html/corrosion_guide.html.

3. P.J. Gegner, “Corrosion Resistance of Materials in Alkalies and Hypochlorites,”Process Industries Corrosion (Houston, TX: NACE International, 1975), pp. 296–305.

4. J.K. Nelson, “Corrosion by Alkalies and Hypochlorites,” in Metals Handbook—Corrosion, vol. 13, 9th ed., ed. J.R. Davis (Metals Park, OH: ASM International,1987), pp. 1174–1180.

5. Anon, “The Chemical Industry Build on Graphite,” brochure no. PE 200/07(Meitingen, Germany: SGL Carbon Group, 2001), 24 pp.

6. Anon, “Corrosion Chart for Grafilor,” brochure no. GC 5 FED 7821 (Moselle,France: Le Carbone-Lorraine, undated), 22 pp.

7. Anon, “Worldwide GLASTEEL 9100,” brochure no. SB95-910-5 (Rochester, NY:Pfaudler Reactor Systems, 2000), p. 5.

8. N. Tattam, ICI PLC, unpublished report (1978).9. Anon, “Hexoloy SA Corrosion Test in Liquids” (Niagara Falls, NY: Saint-Gobain

Advanced Ceramics, 2003), http://www.carbo.com/datasheets/corrosiontest.html.

145

15Corrosion in ContaminatedCaustic and Mixtures

There are various kinds and degrees of contamination encountered in caustic sodaproduction and usage, and these may profoundly affect the corrosion resistance ofthe materials of construction commonly used in this service. As mentioned in Chap-ter 11, there are some impurities inherent in the manufacturing process that aresubstantially removed during the refining process. In other instances, the causticsoda may be mixed with other process chemicals, either deliberately or inadver-tently. This chapter will discuss the effects of contamination of and by caustic andof admixtures of caustic with other chemicals.

Contaminants in Caustic Soda

Contaminants in caustic soda solutions derive from impurities in the feed brine,reactions occurring in the chlorine cell, or corrosion products formed during pro-duction, processing, shipping or storing. Since the source of the caustic influencesthe purity of the product, it should also affect the corrosiveness of the solution. Asurvey of producers, however, carried out by NACE found no substantial differencein the corrosion of steels or nickel alloys in either 50% or 73% caustic soda derivedfrom either mercury or diaphragm cells. The conclusion was that differences inconcentration and temperature were more important than the manufacturingmethod used.1 This does not mean that all caustic soda solutions are equal in termsof their corrosion behavior, and the presence of impurities does influence the attacksuffered by metals and alloys. Producers go to great lengths to either reduce ag-gressive impurities and/or use materials that aremore resistant to the type of causticthat they produce and concentrate.


Figure 15.1 Effect of Chlorates on Various Alloys in 50% Caustic at 150�C (302�F)

Chlorates

Chlorates are produced at the anodes of the electrolytic cells and are strong oxidants.They are usually present at�100 ppm and have a beneficial effect on stainless steelsand an adverse effect on nickel. It has been found that nickel pickup (from corrosionin the first-effect evaporator tubing) was directly proportional to the chlorate contentof the cell liquor, within the 120- to 200-ppm range encountered.2 Some plants adda continuous feed of 10% sucrose solution to react with this chlorate. No residuesare left from this reaction, and product quality is not compromised.3 More com-monly, the chlorate is removed before or during evaporation by proprietary treat-ments or by extraction with ammonia, which also reduces the dissolved salt.4

Chlorate ions can increase the rate of corrosion of carbon steel. For 48% NaOHwith 0.5% NaC1O3, carbon steel showed a 10-fold increase in corrosion rate com-pared with caustic, which does not contain chlorate ions.5 Chlorates also increasethe corrosion rate of nickel alloys as shown in Figure 15.1.4

Corrosion of nickel in the caustic plant is ascribed either to chlorate ions (ClO3)or to hypochlorites (OC1–). General corrosion rates can exceed 20 mpy (0.5 mm/y).Corrosion is aggravated by high velocity, as over welds on pump discharge piping.Velocity and increased temperature are thought to be the most probable causes ofattack of nickel in caustic service rather than chlorate or hypochlorite levels. Inlaboratory tests at 185�C (365�F) in actual first-effect caustic liquor (43% NaOH,0.15% C1O3) and in a synthetic mix (51% NaOH; C1O3 0.20%), corrosion resistanceand stress corrosion cracking (SCC) resistance of alloy 200 and 26-1 were both su-perior to any of the other alloys tested. The relative resistance of the other alloys instatic and circulating simulated and actual liquors was as follows:


Figure 15.2 Effect of Nickel Content on Corrosion of Various Materials in SimulatedFirst-Effect Liquor at 185�C (365�F)

alloy 600 �� alloy 825 � alloy 800 � alloy 28

Some of these data are summarized in Figure 15.2. This shows that chloratesincrease the corrosion of nickel and nickel alloys. This accelerating effort is not suf-ficient to cause failures of nickel in evaporators without the presence of additionalfactors, such as temperature or velocity. Increased nickel content in the alloys in-creases the corrosion resistance, particularly in caustic without chlorates. Alloy 26-1, which has no nickel, has good corrosion resistance but can suffer from SCC inboiling solutions of �30% NaOH and can also be subject to intergranular attack(IGA). The presence of hypochlorites was found to increase the corrosion rate of allthe alloys to a lesser or greater extent.6

The effect of sodium chloride and sodium chlorate additions to hot caustic sodasolutions on the corrosion of the ferritic stainless steel, E-Brite� (S44627), is shownin Table 15.1.7 The corrosion rate of this alloy is little affected by these contaminants,common in cell liquors, unlike that of nickel, in which the corrosion rate is accel-erated. This alloy is attacked in hotter, stronger caustic solutions as shown in thedata for 70% at 188�C (370�F) that is included in this table.8 Corrosion is decreasedin caustic by the addition of sodium chlorate and hypochlorite up to 1000 ppm, butIGA occurred when these additions were at 10,000 ppm.The effect of chorides and chlorates on the corrosion resistance of another ferritic

alloy, Sea-Cure� (S44660), in hot caustic soda solutions is shown in Table 15.2.9 These


Table 15.1 Resistance of E-Brite� Alloy to Caustic Solutions Containing NaCl andNaClO3

% NaOH % NaCl % NaClO3Temperature(�C [�F])

Corrosion Rate(mm/y [mpy])

20 10 — 104 (219) 0.015 (0.59)45 5 — 143 (289) 0.041 (1.61)50 — — 143 (289) 0.003 (0.12)50 5 — 152 (306) 0.076 (2.99)50 5 0.1 152 (306) 0.069 (2.72)50 5 0.2 152 (306) 0.028 (1.10)50 5 0.4 152 (306) 0.028 (1.10)70 5 0.15 188 (370) 0.110 (4.33)

Table 15.2 Corrosion Rates (mm/y) of Various Alloys in Hot Caustic Solutions

Alloy

55% NaOH � 8% NaCl �0.3% NaClO3 at 99�C

(210�F)

50% NaOH at143�C (289�F)

55% NaOH � 8% NaCl �0.3% NaClO3 at 158�C

(316�F)

316 0.15 0.38 Very high26–1S �0.0025 0.015 0.02Nickel 200 �0.0025 0.023 0.07Sea-Cure �0.0025 0.025 IGA

data compare the behavior of this alloy with that of 316, another ferritic stainlesssteel, and pure nickel. These data show that this alloy is as resistant to these con-ditions as is alloy 200 but does have a tendency to IGA in this environment.The effects of the presence of chlorides and chlorates were investigated for a

duplex stainless steel (S32906) and alloy 200 (N02200). These data show that whenhigher levels of chloride and chlorate are present, simulating membrane and dia-phragm caustic, the duplex alloy is much more resistant than nickel (see Table15.3).10

Chlorides

Because NaOH is produced by electrolysis of brine, chlorides may be present fromabout 20 ppm to 5,000 ppm, the minimum concentration being typical of membranecell caustic. Chlorides in caustic soda solution do not cause chloride SCC of austen-itic stainless steels. A solution of 0.5 g/L NaOH at pH 12 is sufficiently alkaline toavoid this type of cracking that is common in neutral or acidic solutions.11 It hasalso been claimed that chlorides inhibit caustic SCC. Mercury cell caustic typically


Table 15.3 Corrosion Rates (mm/y) of S32906 and N02200 in Boiling NaOH SolutionsSimulating Membrane and Diaphragm Cell Liquors

50% NaOH �30 ppm Cl�,20 ppm ClO�

3

50% NaOH �7% NaCl, 800ppm ClO�

3

S32906 0.001 0.016N02200 0.001 0.15

contains 20 to 30 ppm chlorides, while diaphragm cell caustic is more likely tocontain up to 1% chlorides. While chloride-contaminated caustic soda solution maynot cause SCC of austenitic stainless steel if the caustic soda is consumed, for ex-ample, in a neutralizing process, the residual neutral chloride containing solutionmay then initiate cracking.4

See also Table 15.1 for the effect of chlorides with and without chlorates on theferritic E-Brite� alloy (S44627), Table 15.2 for Sea-Cure� (S44600), and Table 13.11for the effect of chlorides on alloy 28 (N08028).Zirconium is resistant to sodium hydroxide solutions even when high levels of

chlorides are present (see Table 13.16).Alloy 800 (N08800) was tested in a NaOH cooling tank at 70�C to 105�C (158�F–

221�F) for 119 days. The environment was 50% NaOH with 10% to 15% NaCl, andthe corrosion rate measured was negligible at �0.003 mm/y (�0.12 mpy).12

Chlorine/Hypochlorite

Hypochlorites are formed when chlorine is introduced into water or alkaline solu-tions:

C1 � 2NaOH r NaC1 � NaOC1 (1)2

Sodium hypochlorite (NaOC1) is liable to initiate pitting in otherwise passivemetalsbecause of the concentration cells between freely exposed and occluded surfaces,analogous to an oxygen concentration cell.The corrosion resistance of various alloys in caustic soda–containing chlorinewas

tested in field trials. The results of these tests showed that the chromium-nickel-ironalloy 33 (R20033) performed better than the other alloys tested (see Table 15.4).13 Allthe alloys showed acceptable corrosion rates in the liquid and the vapor, althoughpitting and crevice corrosion, severe in cases, was observed.

Mercury

Mercury can be entrained from the production source, while mercuric ions can bereduced to metallic mercury at local cathode sites. Metallic mercury contamination


Table 15.4 Corrosion Rates (mm/y) of Various Alloys in NaOH/NaOCl Exposed toLiquid and Vapor

50% NaOH20% NaOH � NaOCl(80–100 g Cl2/L)

12.5% NaOH � NaOCl(130 g Cl2/L)

110�C (230�F)—180 Days 30�C (86�F)—195 Days 30�C (86�F)—225 Days

Alloy Liquid Liquid Vapor Liquid Vapor

316 Ti � 0.01 0.01 pitting andsevere crevicecorrosion

0.02 pitting — � 0.01 severepitting

926 — � 0.01 pitting andsevere crevicecorrosion

� 0.01 � 0.01 � 0.01 someuniformcorrosion

654 SMO � 0.01 � 0.01 someuniform corrosion

— � 0.01 � 0.01

C-4 — � 0.01 pitting — �0.01 � 0.01 somecrevice anduniformcorrosion

33 � 0.01 � 0.01 � 0.01 somecrevicecorrosion

� 0.01 somecrevicecorrosion

� 0.01 somecrevicecorrosion

can lead to liquid metal embrittlement or liquid metal cracking in some alloy sys-tems. Titanium and zirconium and their alloys, copper and its alloys, aluminum andits alloys, alloy 400 (N04400), and alloy 200 (N02200) at elevated temperatures areknown to be at risk from this form of attack.14

Sulfur

Sulfides, sulfite, or sulfate can increase the rate of corrosion of carbon steel if thepassive oxide film on the steel is damaged, scratched, defective, and so on. In con-sidering low-alloy steels for service in the cellulose and paper industry, wherestrongly alkaline solutions often contain sulfide, it has been found that sodium sul-fide and sodium thiosulfate exacerbate damage, whereas sodium sulfite/sulfatehave little effect.15 See the section “Pulp and Paper” later in this chapter.Sulfur species can also accelerate the corrosion of nickel alloys, especially at ele-

vated temperatures.

Iron

Because much of the processing, storage, and transportation of caustic soda is af-fected in steel, iron contamination is often present, either as the soluble ferroite or


ferroate or as fines of colloidal ferric oxide. The resultant discoloration is objection-able for many applications, notably the soap/detergent, pulp/paper, and rayon/textile industries. More resistant materials of construction are often selected just toprevent this undesirable contaminant.

Sodium Hydroxide Treatments

There are a number of processes that utilize caustic soda as a reactant, as a scrubbingmedium, or simply as a neutralizing agent.

Petroleum Refining

Both sodium and potassium hydroxides are used to remove organosulfur com-pounds, such as mercaptans, from petroleum components in oil refineries. Econom-ics require that the caustic must be regenerated and reused, which entails concen-trations up to 45% and temperatures up to 150�C (300�F), conditions that arecorrosive to steel. Stripping tower internals, tubular heaters, reboilers preheaters,and piping for handling hot caustic solutions are usually fabricated with alloy 400(N04400). These components may be solid or clad, depending on design. Pumpsand valves can be made from NiResist� alloy cast iron.16

For operations in which aminodiisopropanol is used to remove sulfur, causticsoda is used to recover the extraction chemical. Steel would be attacked by the sulfurcompounds present, but type 316L (S31603) is adequate.17

Severe caustic corrosion of the crude transfer line immediately downstream ofthe caustic injection point can occur when 40% caustic is injected into hot desaltedcrude oil to neutralize remaining HCl. This problem can be controlled by design ofthe injection point to ensure adequate mixing of the fluids and also is minimized ifcaustic is diluted to about 3% before injecting. Traces of caustic can become concen-trated in boiler feedwater in boiler tubes that alternate between wet and dry con-ditions because of overheating. This concentrated caustic can cause corrosion, andcracking occurs under the deposits.18

Bauxite Refining

In the production of aluminum from bauxite ore, alumina (Al2O3) is digested at hightemperature in caustic, leaving iron oxides and silicates behind as a waste products(red mud). The aluminum oxide is then recrystallized by cooling the mother liquor.Carbon steel is used at appropriate concentrations and temperatures, with the

usual provisions for stress relief. Alloy 400 (N04400) tubes are used for digesterpreheaters, which operate at temperatures that are too high for steel. The solution


is conveyed from the heaters to the reactors in alloy 200 (N02200) or nickel-linedsteel piping.17

A study aimed at finding suitable alloys to replace nickel-coated steel found thatthe duplex stainless steels 23% Cr, 5% Ni (S32304), and 22% Cr, 5% Ni, 3% Mo(S31803) and the austenitic alloy 28 (N08028) were much more resistant than thestandard 300 series stainless steels. Any of these alloys would be suitable replace-ment in cases where the nickel-coated steel failed from caustic SCC.19 In a relatedstudy in typical solutions encountered in this service, 180 to 400 g/L NaOH, 10 to20 g/L NaCl at temperatures between 180�C and 250�C (356�F and 482�F), the goodresistance of duplex stainless steels was confirmed. The presence of some crackingin weld areas indicated the importance of using appropriate welding procedures.The nickel-based alloys, such as N06600 and N06625, were also resistant to generalcorrosion and SCC under these conditions. The 13% chromiummartensitic stainlesssteel had insufficient resistance to general corrosion.20

In lower temperature parts of the Bayer process for alumina production, SG (sphe-roidal graphite) cast iron is used. Failures in this material have been identified asbeing due to SCC. Examination of sections that had failed in this service suggesteda mechanism that involves grain boundary embrittlement ahead of SCC cracks inthis material.21

Soap Manufacture

Caustic is a major reactant in the saponification of fatty acids for the production ofsoap. Both iron and copper contamination cause rancidity in the product, and alloy200 (N02200) is the preferred material for vessels replacing the cast iron and steelonce employed. Alloy 400 (N04400) is also used andmay yield a satisfactory productfor some purposes despite the 30% copper component in the alloy. Corrosion ratesin the top of a soap boiling kettle showed that alloy 200, alloy 400, and alloy 600were all effectively unattacked (�0.1 mpy), while mild steel corroded at 3.2 mpy(0.08 mm/y) and cast iron at 11 mpy (0.28 mm/y).16

Sodium Hydrosulfide Production

Sodium hydrosulfide (NaSH) is produced by the reaction of hydrogen sulfide (H2S)with 45% to 40% caustic. At reaction temperatures of about 110�C (230�F), alloy20Cb3� (N08020) is a suitable material of construction as compared with 316L(S31603), which is used by some producers, although it tends to suffer general cor-rosion. However, field corrosion tests suggest that alloy 600 (N06600) is the prefer-able material of construction.4

Caustic Fusion Reactions

These manufacturing processes involve reaction between organic chemicals andmolten caustic. In the absence of sulfur compounds, these fusions are carried out in


alloy 201 (N02201) vessels at about 315�C (600�F). Should sulfur compounds be pres-ent, as in caustic fusion of benzene metasulfonic acid to produce resorcinol (dihy-droxybenzene), alloy 600 (N06600) is required because the nickel sulfide eutecticcauses intergranular corrosion of nickel.

Metal Finishing

Molten soda ash is used in some high-temperature baths for finishing metal partsby removing metal oxides (e.g. annealing scale, mill scale). Once the reducing opera-tion stops, there is no further reaction with the metal. Operating at about 700�C(1250�F–1325�F), the molten caustic acts as a carrier for 1.5% to –2% sodium hydride,a powerful reducing agent. Formed in place by reacting sodium with hydrogen, thesodium hydride (NaH) is used to descale metals and alloys by direct reduction ofsurface oxides. Surface oxides are removed from carbon steels, stainless steels, cop-per, silver, and nickel alloys by the general reaction

MO � NaH r M � NaOH (2)

and, since the reaction takes place in molten caustic, no contamination is involved.A typical treating cycle consists of the sodium hydride treatment, water quench,and water rinse, usually followed by an acid rinse and second water rinse to removetraces of caustic. The short duration of treatment effectively precludes any significantcorrosion for metals and alloys that react only slowly with molten caustic.16

Solutions of sodium hydroxide are used instead of molten salts to color steel.Immersion in a caustic soda bath of 6 to 8 lb/gal at 285�F (140�C) produces a blackiron magnetite coating in 10 to 20 minutes. The item is then rinsed in water, dried,and immersed in a light oil or dry-to-touch sealant to displace water and seal outatmospheric humidity to prevent rusting. This is an attractive finish with good re-sistance to atmospheric corrosion. The duration of treatment is too short to causecaustic cracking, which is also inhibited by oxidants present in the caustic solution.22

Pulp and Paper

Pulp and paper operations encounter a wide range of caustic solutions that can causemajor corrosion and cracking problems, depending on the process operated and theconditions encountered.Austenitic stainless steels are susceptible to caustic SCC above about 121�C (250�F)

in pure NaOH solutions. When sulfides are present, SCC of austenitic stainless steelscan occur at lower temperatures. Hot sulfide-containing caustic solution (white li-quor) is used in the kraft pulping process. In different process streams of pulp mills,there are different concentrations of sulfide and hydroxide concentrations, alongwith other chemicals. In some areas, composite tubes with an external layer of 304L(S30403) on an inner shell of carbon steel are used, such as in the floor and lower


furnace waterwalls in kraft recovery boilers. SCC occurs during boiler shutdownfrom exposure to the stagnant floor water that is rich in sulfides and hydroxides. Asystematic study of this phenomenon found the following:23

• SCC did not occur in 304L in pure NaOH �100�C (�212�F) in the range of solu-tions tested (up to saturation; �30 M).

• 304L stainless steels were not susceptible to SCC inNa2S solutions at temperaturesup to 100�C (212�F).

• In the presence of sodium sulfide in sodium hydroxide solutions, 304L stainlesssteel is susceptible to SCC at temperatures as low as 50�C (122�F).

• Cracking susceptibility and crack velocities increased with an increase in tem-perature.

• In recovery boilers, the solution composition possible at the floor surface cancause SCC during boiler startup at temperatures as low as 75�C (167�F).

Another example of corrosion failures of stainless steels in alkaline sulfide envi-ronments in pulp mills is in the green liquor quench system. Here 316L (S31603)and 304L (S30403) stainless steels fail by corrosion in the caustic environment at220�C (428�F) and 30 atm pressure.24

Caustic cracking of the impregnation zones of carbon-steel continuous digestershas occurred if the seam welds were not fully PWHT. Caustic solutions are used toperiodically clean paper machines to remove accumulated organic matter. The so-lution used for this caustic boil out is at pH around 13 at 50�C (122�F). Some poly-meric materials are badly attacked, high-strength steels can be corroded or subjectto SCC, and copper alloys can be corroded and suffer from dealloying during thistreatment.25

Caustic Contamination

Caustic can be introduced accidentally into both aqueous and organic streams.

Contamination of Steam

A common problem in petrochemical plants is the carryover of alkaline boilerwater-treating compounds in high-pressure steam. Above 300�C (570�F), the danger ofcaustic SCC is great.Type 321 (S32100) bellows-type piping expansion joints in 300 to 400 lb of steam

are prone to rapid SCC when there is entrainment of alkaline chemicals from boilerwater treatment. When high-temperature caustic SCC of stainless is encountered,there is a characteristic gunmetal blueing of the metal surface. The presence of chlo-rides is of no significance, as indicated by failure of alloy 800 (N08800) and 825(N08825) replacements. Russian investigators found that chlorides do not aggravate


SCC and may, in fact, act to inhibit such attack. Alloy 600 (N06600) is satisfactoryunder these conditions, but alloy 625 (N06625) is currently used almost exclusivelyfor such bellows.26

A carbon-steel steam line from the heat recovery steam generator (HRSG) to asteam host failed by caustic SCC after 6 years in service. The steam in the 24-inch(0.6-m) diameter pipe was maintained at a pressure of about 350 psi (2.4 MPa) anda temperature of 700�F (371�C). The steam temperature is reduced to about 20�F(11�C) of superheat by direct injection of boiler feedwater that contained about 40ppb sodium hydroxide, used for pH control. The feedwater spray caused causticsoda to concentrate on the wall of a steam line elbow and produced the environmentnecessary for SCC. Residual stresses from the circumferential weld provided thenecessary tensile stress.27

Contamination of Organic Media

When caustic is inadvertently introduced into organic media, its corrosion behaviordepends largely on whether the organic compound is an effective diluent. Causticin alcohol, for example, behaves verymuch like aqueous sodium hydroxide. In otherenvironments, there may be no effective dilution.A 0.2% NaOH solution in heavy glycolate tails at about 150�C (300�F) caused

severe corrosion in the bottom of a steel vessel. Apparently, the dilution effect wasminimal, themetal “sensing” amore concentrated caustic concentration. It was dem-onstrated that neutralization with concentrated sulfuric acid in stoichiometricamounts to neutralize the caustic prevented attack on steel, but there seemed nopractical way to do this in a quantitative manner in plant. Replacement with a type304 (S30400) vessel was a practical solution, providing total corrosion resistance,which seems to indicate that the effective concentration of caustic was less thanperhaps 20%. Such a concentration would corrode steel but not type 304 (S30400)at the process temperature involved.

Contamination of Molten Sodium

Molten sodium is used as a carrier in certain high-pressure hydrogenation processesfor organic compounds at 325�C (615�F). Traces of water contained in the organiccompounds immediately react to form NaOH, creating an environment of smallamounts of 100% caustic dissolved in molten sodium.4

There have been incidents of SCC with type 347 (S34700) heavy-walled pipingwhen such contamination occurred. The reactor proper had been fabricated fromalloy 600 (N06600)–clad steel in anticipation of possible sulfurous contaminants.Some austenitic stainless steel bolts, occasionally installed inadvertently as mechan-ical fasteners for the agitators, would fail by SCC within 24 hours. The original alloy600–clad hydrogenation sphere lasted about 2 years before suffering SCC. A replace-ment vessel, given a thermal stress relief, lasted about twice as long.


References

1. NACE Task Group T5-A report, Materials Protection and Performance 10, 7(1971): p. 39.

2. B.M. Barkel, “Accelerated Corrosion of Nickel Tubes in Caustic EvaporationService,” CORROSION/79, paper no. 13 (Houston TX: NACE International,1979), 9 pp.

3. M.P. Sukumaran Nair, “Stress Corrosion Cracking—A Caustic Experience,”Chemical Engineering, January (2003): pp. 1–3.


5. K. Hauffe (1986), in Anon, NACE Network, reported in MP 42, 2 (2003): pp.82–83.

6. J.R. Crum, W.G. Lipscomb, “Correlation between Laboratory Tests and FieldExperience for Nickel 200 and 26–1 Stainless Steel in Caustic Service,” COR-ROSION/83, paper no. 23 (Houston, TX: NACE International, 1983), 18 pp.

7. Anon, “E-Brite Alloy,” technical data sheet no. B-150-Ed1-10M-181P (Pittsburgh,PA: Allegheny Ludlum Corp., 1980), p. 7.

8. J.R. Kearns, M.J. Johnson, I.A. Franson, “The Corrosion of Stainless Steels andNickel Alloys in Caustic Solutions,” CORROSION/84, paper no. 146 (Houston,TX: NACE International, 1984), 18 pp.

9. Anon, “Trent SEA-CURE Stainless Steel for Power Generation and ChemicalProcessing,” brochure no. A18-7/00-5000 (East Troy, WI: Trent Tube, 2000),20 pp.

10. D. Leander, “Corrosion Characteristics of Different Stainless Steels, Austeniticand Duplex, in NaOH Environment,” Stainless Steel World Conference, Maas-tricht, Netherlands (2003), 9 pp.


12. Anon, “Incoloy Alloys 800, 800H and 800HT,” brochure no. 1A1 172/7M (Hunt-ington, WV: Inco Alloys International, 1997), 28 pp.

13. M. Kohler, U. Heubner, K.W. Eichenhofer, M. Renner, “Progress with Alloy 33(UNS R20033), a New Corrosion Resistant Chromium Based Austenitic Mate-rial,” CORROSION/96, paper no. 428 (Houston, TX: NACE International, 1996),18 pp.

14. J.R. Davis, ed., Corrosion—Understanding the Basics (Materials Park, OH: ASMInternational, 2000), p. 191.

15. D.A. Wensley, R.S. Charlton (1980), in “Sodium Hydroxide Advisor,” ChemCor6 (MTI/NACE/ NiDI/NIST, 1992).

16. Anon, “Corrosion Resistance of Nickel and Nickel-Containing Alloys in CausticSoda and Other Alkalies,” CEB-2 (New York, NY: International Nickel CompanyInc., 1973), 40 pp.


18. J. Gutzeit, R.E. Merrick, L.R. Scharfstein, “Corrosion in Petroleum Refining andPetrochemical Operations,” in Metals Handbook—Corrosion, vol. 13, 9th ed.,ed. J.R. Davis (Metals Park, OH: ASM International, 1987), p. 1269.


19. A. Cigada, G. Rondelli, B. Vincentini, M.F. Brunella, “Caustic Stress CorrosionBehavior of Some Duplex and Austenitic Stainless Steels,” Proc. 3rd Ibero-American Congress of Corrosion and Protection, Brazilian Corrosion Congress,vol. 1 (1989), pp. 223–233.

20. A. Cigada, M.F. Brunella, M. Cabrini, G. Rondelli, B. Vincentini, S. Ventura,“Stress Corrosion Behaviour of Duplex Stainless Steels in Caustic Environments:Laboratory and Field Experiences,” Proc. 14th ICC, paper no. 32.0 (1999), 7 pp.

21. R.K. Singh Raman, B.C. Muddle, R.M. Tomlins, “Stress Corrosion Cracking ofSteels and Their Weldments in Strong Caustic Environments,” paper no. 38-065,Corrosion and Prevention 1998 Conference (1998), 7 pp.

22. M. Ruhland, “In-House Blackening of FerrousMetals” (Eden Prairie,MN: Birch-wood Laboratories Inc., 2003), http://www.birchwoodcasey.com/presto/presto-9359.html.

23. P.M. Singh, O. Ige, J. Mahmood, “Stress Corrosion Cracking of 304L StainlessSteel in Sodium Sulfide-Containing Caustic Solutions,” CORROSION/2003, pa-per no. 03518 (Houston, TX: NACE International), 14 pp.

24. J.R. Keiser, R.A. Peascoe, C.R. Hubbard, J.P. Gorog, “Corrosion Issues in BlackLiquor Gasifiers,” CORROSION/2003, paper no. 03354 (Houston, TX: NACEInternational), 19 pp.

25. A.H. Tuthill, ed., “Stainless Steels and Specialty Alloys for Modern Pulp andPaper Mills” NiDI reference book no. 11 025 (Toronto, ON, Canada: NiDI, 2002),152 pp.

26. C.P. Dillon, “Corrosion of Stainless Steel by Hot Caustic,” MP 37, 1 (1998): pp.64–65.

27. F.C. Anderson, P.S. Jackson, D.S. Moelling, F.M. Glasgow, “HRSG Tube Failures:Prediction, Diagnosis and Corrective Actions,” CORROSION/2003, paper no.03495 (Houston, TX: NACE International), 12 pp.

159

16Related Chemicals

There are a number of chemicals related to caustic soda that are really outside thescope of this monograph but some of which should at least be introduced here forcompleteness. The first is chlorine, whose production is intimately tied to causticsoda production. Materials of construction for chlorine have, however, been fullydescribed in another MTI monograph1 in this series and will not be discussed here.

Soda Ash

Soda ash is anhydrous sodium carbonate (Na2CO3), CAS number 497-19-8. It existsas a naturally occurring mineral in parts of Africa and in the United States, notablyin Wyoming. Native soda ash is found in the form of Trona, which is sodium car-bonate and bicarbonate, or as brines that are mixtures of sodium carbonate, sulfate,and sulfite.2 Soda ash can be produced by the reaction of caustic soda and carbondioxide or in the Solvay (ammonia soda) process that reacts limestone, ammoniatedbrine, and coke.

Soda ash is used in household cleaners (washing soda), glass making, water treat-ment, chemical processing, and so on. In some pulp and paper applications, sodaash is reacted with limestone to produce caustic soda, used in neutralizing and otherprocesses.

Pure soda ash is a hygroscopic white powder whose molecular weight is 105.99and specific gravity 2.533. It has a melting point of 851�C (1564�F) and generatesheat of solution in water. It has a strong degree of alkalinity.

The sodium carbonate decahydrate (Na2CO3 •10 H2O) melts at 34�C (93�F) to forma solution of about 37% concentration. Corrosion problems may develop because itcan either lose carbon dioxide to release free caustic or further absorb CO2 to formthe bicarbonate (NaHCO3).

Sodium carbonate at elevated temperature will cause caustic corrosion in a man-ner similar to caustic soda corrosion. The propensity to caustic stress corrosion crack-ing (SCC) is less with soda ash, and carbon steel is routinely used to handle boiling


solutions. The alkalinity of soda ash is less than that of NaOH, and the carbonateions have an inhibiting effect on many metals. Soda ash can form the bicarbonatethat may corrode steel because of its lower pH.3

A typical application is the use of sodium carbonate solutions for the absorptionand release of carbon dioxide from hydrocarbon streams, in a manner analogous toalkanolamine acid gas scrubbing systems. The carbonate reacts selectively with CO2

to form the bicarbonate, which is reheated in the separation column to release theacid gas, returning the regenerated lean Na2CO3 solution to the absorber. This pro-cess is inherently corrosive to steel, particularly in the regeneration step. Additionof oxidizing inhibitors such as sodium vanadate is used to control corrosion in thestripping still, and type 304 (S30400) reboilers are employed to resist corrosion inthe reboiler.

Potassium Hydroxide

Potassium hydroxide (KOH), CAS number 1310-58-3, is very similar to sodium hy-droxide in its chemical and corrosion characteristics, but it is much less common.Caustic potash is produced in a similar manner to caustic soda by the electrolysisof potassium chloride. The equipment for evaporation is based largely on nickel andnickel alloys with cathodic protection being employed for �50% KOH solutions,which have a higher boiling point than NaOH.

Corrosion data is limited, but KOH corrosion behavior is similar to that of NaOH,so we can usually extrapolate from the NaOH data and experience. Caustic SCCcan occur in caustic potash solutions at elevated temperatures, but cracking is notso severe in the potassium salt for alloy 400 (N04400) and alloy 600 (N06600). Afailure of a low-carbon pressure vessel after 10 years of service in a hydrogen sulfideabsorber was diagnosed as due to caustic SCC from KOH. The environment was20% aqueous KOH, potassium carbonate, and arsenic at 33�C (91�F). Although theoperating temperature is low for SCC, the vessel had been exposed to a fire thatmay have taken the temperature above the induction temperature. It may also haveincreased the local concentration of KOH and added stress to the existing residualand operating stresses.4

In a problem analogous to that which occurs with caustic carryover in steam, SCCof type 347 (S34700) by potassium hydroxide has been reported. In one plant, acatalyst containing potassium oxide was to be used in a hydrogenator. When theplant was started upwith 300�C (570�F) steam after a catalyst change, sufficient KOHwas formed and entrained to cause failure of the stainless equipment downstream.

Some data are available for alloy 200 in caustic potash (KOH) under velocityconditions and when mixed with potassium chloride (KCl) and potassium chlorate(KClO3) (see Table 16.1).5

Data for various metals and alloys in KOH are shown in Table 16.2.6 This tableshows that KOH at these temperatures and concentrations is not very corrosive evento low-carbon steel.


Table 16.1 Laboratory Corrosion Tests in Potassium Hydroxide

EnvironmentTemperature

(�C [�F])Corrosion Rate(mm/y [mpy])

30% NaOH, saturated, with KCl� 0.05% KClO3

LiquidVapor

Boiling 0.005 (0.20)0.008 (0.31)

47% NaOH saturated, with KCl� 0.078% KClO3

LiquidVapor

Boiling 0.002 (0.08)0.008 (0.031)

50% KOH 6.6 m/min106 m/min

150 (302) Weight gainWeight gain

70% KOH 6.6 m/min106 m/min

150 (302) 0.01 (0.39)0.04 (1.57)

Table 16.2 Corrosion Rate (mm/y [mpy]) of Metals and Alloys in KOH Solutions

Alloy13% KOH at 30�C (85�F)

13% KCl added 50% KOH at 25�C (80�F)

Titanium 0.023 (0.9) 0.01 (0.4)Zirconium 0.005 (0.2) 0.0015 (0.06)Nickel Nil 0.00008 (0.003)Monel Nil 0.00005 (0.002)Inconel Nil NilLow C steel 0.013 (0.5)* 0.0013 (0.05)

*Slight attack under spacer

Zirconium has a corrosion rate of �0.025 mm/y (1 mpy) in 0% to 50% KOH attemperatures up to boiling.7 Zirconium in 50% KOH at 27�C (81�F) corroded 0.06mpy (0.0015 mm/y), and in short-term tests in 50% to anhydrous KOH at 241�C to377�C ( 466�F–711�F), the corrosion rate was �0.03 mpy (�0.001 mm/y).8

References

1. C.P. Dillon, W.I. Pollock, eds., Materials Selector for Hazardous Chemicals: Hy-drochloric Acid, Hydrogen Chloride and Chlorine, vol. MS-3 (St. Louis, MO:MTI, 1995), 200 pp.

2. Anon, “Lake Natron Soda Ash Project” (Dar es Salaam, Tanzania: National De-velopment Corp., 1997), http://www.ndctz.com/sodaash.htm.

3. J.K. Nelson, “Corrosion by Alkalies and Hypochlorites,” in Metals Handbook—Corrosion, vol. 13, 9th ed., ed. J.R. Davis (Metals Park, OH: ASM International,1987), pp. 1174–1180.


4. G.W. Powell, S.A. Mahmoud, eds., Metals Handbbook—Failure Analysis andPrevention, vol. 11 (Metals Park, OH: ASM International, 1986), p. 658.

5. Anon, “Wiggin Corrosion Resisting Alloys” (Hereford, UK: Wiggin Alloys Ltd,1983), p. 18.

6. P.J. Gegner, W.L. Wilson (1959), in J.K. Nelson, “Corrosion by Alkalies and Hy-pochlorites,” in TheMetals Handbook—Corrosion, vol. 13, 9th ed., ed. J.R. Davis(Metals Park, OH: ASM International, 1987), pp. 1174–1180.

7. K. Bird, “The Caustic Truth about Zircadyne�,” Outlook 15, 3 (1994): pp. 6–7.8. D.R. Knittel, R.T. Webster, “Corrosion Resistance of Zirconium and Zirconium

Alloys in Inorganic Acids and Alkalies,” ASTM Symposium on Industrial Ap-plications of Zirconium and Titanium (1979).

163

17Summary of Corrosion ofMaterials in Caustic Soda

The detailed behavior of materials in caustic soda has been discussed so far. Insummary, mild steel is acceptable for most applications of low-temperature, low-concentration caustic soda except where the avoidance of iron contamination is criti-cal. The 300 stainless steels can be used at higher temperatures and concentrationsor to reduce iron contamination. At higher concentrations and temperatures, morehighly alloyed materials are used, including nickel-based alloys and austenitic, fer-ritic, and duplex stainless steels. For the hottest, strongest solutions, nickel is pre-ferred unless oxygen or oxidizing agents are present. In that case, super stainlesssteels of nickel alloys are specified.

The two factors that mainly control the limits of applications of materials areresistance to metal loss (general corrosion) and resistance to caustic stress corrosioncracking. These factors are summarized for common alloys in Figures 17.1 and 17.2.The curves for stainless steels (304/316) and nickel in Figure 17.1 and for nickel-richaustenitic alloys in Figure 17.2 are speculative and not based on rigorous statisticaltesting. These curves are included with the “standard” curves as a suggested com-parison. The range of applications for various nickel alloys is shown in Figure 17.3.Above 320�C (608�F), alloy 201 (N02201) should be used.

There have been attempts to summarize applicable limits of concentration andtemperature. One such summary is shown in Table 17.1.1

This is a useful summary given the proviso that the actual behavior of any ofthese alloys does depend on other factors such as velocity, the presence or absenceof contaminants, and so on. Selection of a materials also depends on the application.For example, the material recommended for a heat exchanger cooling recirculatingcaustic might be different from the material used in a once-through pipeline han-dling the same strength and temperature of solution.

Reference

1. Anon, “Alloys for Caustic Soda Service by Corrosion Resistance Category”(Houston, TX: The Hendrix Group Inc., 2003), http://www.hghouston.com/naoh.html.


Figure 17.1 Isocorrosion Curve (at 1 mpy) for Stainless Steel and Nickel in CausticSoda

Figure 17.2 Summary Curves for SCC Regions for Carbon Steel, Stainless Steel,Nickel-Rich, and Nickel-Based Alloys

Figure 17.3 Range of Use of Various Nickel Alloys in Caustic Soda

Table 17.1 Alloys for Caustic Soda Service by Corrosion Resistance Category

Category A alloys are useful at all concentrations and temperatures.Alloy 200 (N02200)

Category B alloys are useful up to 350�F (176�C) and 50% concentration.May showusefulresistance up to 600�F (315�C) but should be tested first.

Alloy 690 (N06690)Alloy 600 (N06600)Alloy 400 (N04400)Alloy Ebrite 26–1� (S44627) tubing and piping only

Category C alloys are useful up to 50% caustic at atmospheric boiling (300�F [149�C]).May exhibit similar resistance to category B alloys but based on limited data should betested first.

Alloy 625 (N06625)Alloy C-276 (N10276)Alloy B-2 (N106650)

Category D alloys are useful up to 250�F (121�C) and 50% concentration.Alloy 800 (N08800)Alloy 20 (N08020)

Category E alloys are useful up to 200�F (93�C) and 50% concentration.Type 304 (S30400)Type 316SS (S31600)


167

18Specific Production Equipment

This chapter covers details relating to specific items of equipment as well as rec-ommendations for such equipment as determined by environment and service con-ditions. It provides materials recommendations for specific equipment in sodiumhydroxide service. This chapter also covers aspects of design and operation that arerelevant to particular types of equipment. Production plant equipment includes re-actors, heat exchangers, condensers, liquefiers, evaporators, dryers, fractionatingstills/columns, boilers, heaters, and crystallizers. A corrosion engineer should beconsulted when contamination is a potential problem or where the caustic is mixedwith other chemicals.

Production Equipment

Caustic soda is normally coproduced with chlorine using a variety of different sys-tems. The materials for the construction of mercury, diaphragm, or membrane chlo-rine cells are discussed in the MTI Monograph 3. This monograph also describesthe process and materials used to prepare the brine prior to hydrolysis, and thisstage of caustic production will not be dealt with here.1

The weak, impure caustic soda solution from the diaphragm cell is treated toremove some of the impurities and then concentrated using multiple-effect evapo-rators. A summary of some of the areas in caustic production in which nickel alloysare being successfully used is shown in Table 18.1.2

Other alloys and materials are used for some of these items of equipment, andthese will be discussed here.

Pressure Vessels

Pressure vessels must be designed and fabricated to meet the provisions of SectionVIII of the ASME Code. Carbon steel is used within the limiting parameters of


Table 18.1 Nickel Alloys Used in Caustic Soda Production

Equipment Material

Brine pumps Ni-Resist�, alloy 400Brine heaters Alloy 200, alloy 400Evaporator Bodies Ni-clad or alloy 200-sheet lined steel

Steam chests Alloy 200 or alloy 400 tube sheetsAlloy 200 or alloy 400 or Ni-clad downtakesAlloy 200 or alloy 400 tubes

Anhydrous Alloy 201 or alloy 600 tubesHeat exchangers Alloy 200 or alloy 400Pumps Bodies Alloy 200 or alloy 400 or Ni-Resist�

Shafts Alloy 200 or alloy 400Impellers Alloy 200 or alloy 400 or Ni-Resist�

Valves and fittings Alloy 200 or alloy 400 or Ni-Resist�Pipelines Alloy 200 or alloy 400 or Ni-Resist�Filters Bodies and drums Alloy 200 or Ni-clad steel

Filter cloth, backing,and winding wire

Alloy 200 or alloy 400

Piping, valves,and fittings

Alloy 200 or alloy 400 or Ni-Resist�

Settling tanks Ni-clad steel or lined with alloy 200 sheetCrystallizers Bodies Ni-clad steel or lined with alloy 200 sheet

Shafts and agitators Alloy 200 or alloy 400Centrifuges Alloy 400 baskets and wire cloth linersTanks cars Bodies Ni-clad steel

Coils Alloy 200Transfer piping Alloy 200 or alloy 400

concentration and temperature, with nickel, nickel alloys, and stainless steels beingused at higher NaOH concentrations and temperatures.

For carbon steel, the ASME Boiler and Pressure Vessel Code allows stress reliefat temperatures as low as 482�C (900�F). However, this is not reliable for caustic sodaservice. A typical stress relief for a carbon-steel vessel in caustic service would beto hold at 593�C (1100�F) for 1 hour per inch thickness, with a minimum of half anhour.

There are no fabrication requirements for stainless steel that specifically relate tocaustic service. Stainless steel vessels need not be stress relieved because chloridestress corrosion cracking (SCC) does not occur in caustic service.

Nickel and its alloys (although ductile and malleable in the annealed conditionand readily fabricated by normal techniques) require stress relief for some appli-cations. Highly stressed components should be stress relieved at 480�C to 870�C(900�F–1600�F). Alloy 400 (N04400) can experience embrittlement in the range650�C to 870�C (1200�F–1600�F), so the alloy is normally hot worked at about1040�C (1900�F).


Recommendations for effective stress relief are important especially in hot con-centrated solutions of caustic where SCC is possible even for some of the strongernickel-bases alloys, such as alloy 625 (N06625).

All high-nickel alloys are extremely sensitive to even traces of sulfur compoundsthat are absorbed by the metal at the welding temperature, causing serious embrit-tlement. Welding must be undertaken with extremely clean conditions.

Brine Circulation Piping

Brine circulation piping has been made from various grades of stainless steels,nickel, or nickel alloys or NiResist� cast iron. FRP and dual laminate piping is alsobeing successfully used in this application, as is rubber-lined carbon steel.

Evaporators and Crystallizers

The primary concern in caustic production is the multiple-effect evaporator. In thepast, caustic evaporators and crystallizers were made from gray cast iron, but nowthey are usually made of nickel alloy 200 (N02200) in the form of steam-heatedtubular exchangers. Alloy 201 (N02201) is specified when operating above 315�C(600�F). Solid nickel, nickel clad, or nickel lined have all been used in this application.For production of concentrations above 73%, the heating medium is a high-temperature heat transfer fluid or a molten salt. For this duty, operating at tem-peratures above 315�C (600�F), alloy 201 (N02201) is used since alloy 200 (N02200)can be subject to graphite formation and embrittlement. In some cases, stress-relieved alloy 600 (N06600) has been used instead of alloy 200 (N02200), particularlywhen sulfur compounds are known or suspected to be present in the liquor to beconcentrated.3

Where chlorate contamination is a problem (causing accelerated corrosion ofnickel), some operators have used the ELI 26-1 (S44626), depending on the chlorates,to maintain passivity. The ferritic alloy E-Brite� has often been used successfully,but there have been some failures. Failures of 26-1 ferritic alloys in this service havebeen due to intergranular attack (IGA). Short-term, low-temperature testing doesnot cause IGA, but such attack becomes more probable as time and temperatureincrease. Contributory factors include crevices, corrosion products, and perhaps el-emental sulfur. This type of alloy is also susceptible to SCC in hot caustic solutionsat �30% concentration. The most probable cause of alloy 200 failures in causticevaporators is increased temperature and velocity.4

Another superferritic stainless steel, alloy 29-4 (S44700), has excellent resistanceto boiling 50% NaOH, even in the presence of chlorates, and has become a standardmaterial for caustic evaporator steam chests and other associated heat exchangers.5

While most evaporators are based on a shell and tube design, plate exchangersare also now being used to concentrate 32% membrane caustic up to 50%.6

The presence of sodium hypochlorite in diaphragm cell liquor has caused cor-rosion of nickel and 26-1 in first-effect evaporators. Sodium chlorate in the cell liquor


has caused accelerated attack of nickel. In the presence of chlorates, the ferritic steelsare better than nickel since they tend to be passivated by this oxidizing chemical.In some cases, the cell liquor is treated to reduce corrosion in the evaporators. Forexample, chlorates are often removed by ammonia extraction, proprietary treat-ments, or additions of sucrose. Sodium sulfite is added to reduce hypochlorite.7

Other inhibitor chemicals are added to reduce corrosion rate. One of these is areducing chemical, sodium borohydride, said to reduce corrosion due to high tem-peratures and turbulence. Treatment with this chemical can reduce nickel pickupacross evaporators by up to 50% to 80%. This same treatment also reduces othermetallic contaminants, such as iron.8

To produce solid caustic soda, one method is to use a falling film evaporator thatprovides heat and gradually removes water from the original 50% NaOH solution.One such evaporator is made from alloy 201 tubes and is heated with molten heattransfer salt. This unit failed after 18 months of service by a combination of pittingand longitudinal cracking, mainly in the bottom third of this vertical heater. Thefailure was diagnosed as being caused by a combination of intergranular SCC andaccelerated corrosion due to local overheating. The excessive stress derived fromconditions during shutdown and startup. Improved operating conditions to avoiddry, hot spots and control of chlorates have been instituted with the replacementunit. Once the fused caustic mass leaves the evaporator, it is broken up in a flakerdrum. In this particular plant, this unit also failed prematurely by cracking that wasdiagnosed as being due to residual fabrication stress and operating thermal stresses.Since carbon precipitation may have been a factor in the failure of this alloy 200(N02200) flaker drum, the replacement will be made from alloy 201 (N02201).9

Salt Separators

Salt settlers are usually fabricated from nickel alloy 200 (N02200) to resist chlorideconcentration cell effects, while slurry tanks are usually alloy 400 (N04400). Saltsettlers often incorporate cooling using alloy 200 for the heat transfer surfaces. Afailure of one of these nickel heat exchangers was caused when seawater was usedas the coolant. It was replaced with alloy 400 (N04400) welded with alloy 625(N06625).2

Austenitic stainless steels have been used in salt recovery systems and have failedfrom SCC.

Caustic Soda Handling

All manufacturers and transporters of solid caustic soda or caustic soda solutionsprovide information regarding safe handling and equipment needed to deal withthese products. This information is available in the form of MSDS (Materials SafetyData Sheets) and in specific brochures available from the suppliers. There is also a


comprehensive document prepared by the Chlorine Institute that gives guidanceand detailed information on the handling of caustic solutions.10 Some of the prin-cipal features of materials and equipment suitable for handling caustic are presentedin this section.

Heat Exchangers

Heat exchangers of various types and designs are used to add heat, such as toprevent freezing, or to remove heat, such as to make caustic soda solutions coolenough for storage.

Heaters Heaters are needed in storage tanks to maintain caustic solutions liquid.Internal coils, bayonets, or U-tubes are made from alloy 200 (N02200) or alloy 400(N04400). Alternatively, external shell and tube heat exchangers can be used to pro-vide the necessary heat to prevent freezing. These are normally made with nickeltubes and steam heating on the shell side. These external heaters have the advantageof ease of inspection and maintenance.

Coolers Heat exchangers used for cooling must be resistant not only to caustic butalso to the cooling water being used. For example, a 6% molybdenum austeniticstainless steel might be used for a high-chloride water if the process-side conditionsof caustic concentration and temperature permit. More often, alloy 400 (N04400) oralloy 625 (N06625) is chosen for brackish or seawater cooling. However, in onecaustic membrane heater/cooler handling 33% NaOH, failures were observed ascoming from the seawater inside the tube, probably because of marine fouling, mi-crobial corrosion, or the alternating exposure to steam and seawater.

Plate heat exchangers are more often used than shell and tube exchangers forremoving heat from caustic dilution operations. Type 304L (S30403) or 316L (S31603)are used for low temperatures and concentrations, with alloy 400 or nickel usedunder more aggressive conditions (�60�C [�140�F]). Elastomeric gaskets have beensuccessfully employed with safety screens to avoid caustic spray in case of a gasketfailure.

Storage Tanks

The basis of the design for metallic, vertical, cylindrical storage tanks for causticsolutions can be API 12F,11 API 620,12 or API 65013 standard, depending on thedesign parameters. An appropriate design standard for metallic, horizontal tanks isASME Section VIII,14 while for FRP tanks, ASME RTP-115 is appropriate.

Bare carbon steel is commonly selected for storage of �51% NaOH at up to 50�C(122�F) unless iron contamination is a problem. Such vessels should be stress re-lieved if operating within the parameters defined by Figure 13.4. Carbon steel doesnot need to be stress relieved when it is coated or when the vessel is operated


consistently outside the ranges in which stress relief is required. Some operators,however, choose to stress relieve carbon-steel tanks even when operating in the“safe” region. At higher concentrations and temperatures, accelerated corrosion andSCC can occur, and other materials should be selected.

The standard low-carbon 300 series stainless steels have been used successfully,but they must be protected from external chloride SCC. Ferritic or duplex stainlesshave also been used because of their superior resistance to chloride SCC. Nickel andnickel alloys are satisfactory for any combination of temperature and concentrationand are normally used in high-concentration or critical purity applications. For stor-age of 73% NaOH, nickel or nickel clad on carbon steel is commonly used. FRP isnot common for caustic storage but may be used if metal contamination must beavoided. FRP tanks need structural strength, must be able to be heat traced, andhave a resistant resin (e.g. vinyl or epoxy) and an appropriate, internal corrosionbarrier. Linings such as PVC, CPVC, PP, polysulfone, or fluorocarbons are used in�50%NaOH, usually in the form of dual laminates with FRP structural. Other liningmaterials that are used in appropriate applications include epoxy, vinyl, naturalrubber, neoprene, and chlorobutyl rubber.

Regardless of the material used for the tank, care must be taken over all aspectsof fabrication. Errors, shortcuts, mix-ups, and general lack of supervision or clearinstruction to fabricators and/inspectors and so on account for many unnecessaryfailures in process plant and equipment. Residual tensile stresses resulting from coldforming, such as cutting, bending, and twisting, or from welding can cause SCCunless stress relief is subsequently applied. Welding always produces localizedstresses above the yield point because of restraint under cooling and may be vul-nerable to SCC at the operating concentrations and temperatures. A postweld beattreatment (PWHT) for stress relief effectively reduces the stresses to below the criti-cal threshold stress level, where caustic embrittlement will not occur.

Carbon steel should be stress relieved at a minimum of 593�C (1100�F), holdingthe material at temperature for 1 hour per inch of thickness for at least 0.5 hours.Stress relief, typically in the range 550�C to 650�C (1020�F–1200�F), increases the safeworking temperature by at least 20�C (36�F). It must be noted, however, that on-siteprocedures may not permit fully effective stress relief. Compromise approaches re-quire full attention to temperature distribution and control, times and cooling, andso on. Methods include the following:

• Sectional stress relief, using a furnace on horizontal sections of fabricated equip-ment

• Field insulation with heat input to total vessel, which demands very careful con-trol and inspection

• Local section stress relief, using portable heating tapes or resistance heaters• Welding torch stress relief, where unevenness of temperature, temperature con-

trol, times, and cooling rates are more in question than expected for the othertechniques

Furthermore, if operating stresses are high, they can cause SCC, the time to crack-ing diminishing as the magnitude of the stresses increase.


Fabricated carbon steel, used for handling and storing NaOH at up to 50% con-centration, does not generally require thermal stress relief because the storage tem-perature is usually less than 60�C (140�F). Overheating can still occur because ofdirect exposure to the sun or if there is a loss of control of the heat applied to preventfreezing. Under the influence of this localized heating, an apparently nonaggressivecaustic solution can become considerably more corrosive and cause SCC in suscep-tible materials.

There have been cases of caustic embrittlement (SCC) when soil subsidenceplaced stress-relieved storage tanks in a bending moment, reintroducing tensilestresses.

In the design of both storage tanks and other process (“day”) tanks or vessels,certain detailed service requirements must be considered. These include the fol-lowing:

• The expected caustic concentration and possible concentration changes.• Possible changes in the quality of caustic from various sources and the type and

extent of impurities.• Exposure to air, condensation, and dilution effects as well as the potential for

evaporation or concentration.• The operating temperature and the appropriate heatingmethod to avoid freezing.• Tank holding (residence) time; frequency of use and duration of exposure of vol-

ume per unit area are related to the tolerance of caustic regarding iron contami-nation.

• It should be noted that high local concentrations of caustic can occur in certainundrained areas of a tank, in horizontal feed nozzles, or from localized heating.

Residues of caustic can remain in a vessel that has to be repaired after priorservice. Localized concentration effects can occur because of local overheating andwhen residual stresses are reimposed, which could cause unexpected SCC. Thor-ough cleaning prior to repair work is essential.

Stressed areas of carbon steel, exposed to hot strong caustic, rarely suffer fromSCC after a lining ultimately fails. It has been suggested that the steel effectivelyundergoes a low-temperature stress-relieving period during the life of the coating.16

Tanks require routine cleaning because sodium carbonate precipitates out ofstored caustic soda, forming a hard crust on the tank floor. Removing the precipi-tated carbonate is important to maintain product quality and to perform tank in-spections. Large caustic storage tanks (250,000 gallons and more) are typicallycleaned and/or inspected every 10 years, while smaller tanks are often cleaned ona more frequent basis. Carbonate buildup can be reduced by correct tank designand operating procedures.17

Piping

The most common and economically justified material for piping is carbon steelusually designed and fabricated to ANSI/ASME specification B31.3.18 Alternative


materials, both metallic and nonmetallic, are needed to comply with piping coderestrictions (conditions beyond the physical limits of carbon steel) or where contam-ination is a problem. Caustic SCC of steel occurs above threshold temperatureswhenstresses exceed the yield point. Steel pipe needs to be stress relieved not only whenwelded but also when cold formed, that is, when bent, field flared, and so on.

The stainless steel types 304L (S30403) and 316L (S31603) have been used incaustic piping systems; the low-carbon grade gives improved corrosion resistancein the weld zone. In applications where the austenitic steels may suffer from SCC,alloy 400 (N04400) has sometimes been used, but for high-temperature, high-concentration applications, nickel is usually specified.

There are many factors that influence the behavior of piping systems and theselection of suitable materials. Contributing factors include the following:

• Pressure/temperature limitations—especially for nonmetallic materials of con-struction.

• Support structures—nonmetallic systems will require additional support.• Tracing/insulation requirements—direct contact of steam tracing with the pipe

must be avoided. Steam tracing should match the pumping schedule for the caus-tic; overheating can result when static conditions obtain. Polypropylene- andPTFE-lined steel pipe have both been utilized as alternatives to steel to circumventthe overheating problem.

• Corrosion resistance of material(s), including concentration effects and impuritiesand flow effects (bends, joints, valves, and so on)—within normal design param-eters, velocity is not a major factor.

• Fabrication—permanent joints or dismountable, gaskets and sealants to be com-patible with caustic, and welds (residual stresses, compatibility, and so on).

• Physical effects, such as thermal expansion/contraction.

Special safety measures are often required for piping systems handling hazardouschemicals. Safeguards that are described in ANSI/ASME B31.3 specification includethe following:

• Plant layout to provide isolation or controlled access• Protective barricades or other systems for collecting or recovering spillage• Operating practice, including work permits, special training, and similar provi-

sions• Engineering design features, such as insulation, shock protection, armoring, and

double containment

This specification pertains only to piping and equipment and advises on safe-guarding by armoring or other means. However, special consideration should alsobe given to the following:

• Proper support• Provision for thermal expansion• Protection against mechanical damage, such as by water hammer


For commercial strengths of 50% or more, heat tracing is routine to prevent freez-ing. This can introduce problems because service experience shows that heat tracingcan produce a local increase in temperature (and/or increase in caustic concentra-tion) beyond the values of the bulk fluid. These effects need to be taken into accountwhen optimizing candidate material(s).

Self-regulating electric heating tape is recommended. Other heating tapes or ca-bles with thermostatic control may also be used. Steam tracing is not normally rec-ommended since the temperature of caustic soda can readily exceed 140�F (60�C)under static conditions, causing eventual SCC. Insulation and weatherproofing arealso required if the piping must be heated. If maintaining low iron concentration inthe caustic soda solution is important, use a flanged steel pipe with a polypropylenelining. Unsupported plastic pipe should not be used for caustic soda, and fiberglass-reinforced plastic pipe should be used cautiously only for specific applications.19

Pumps

Pumps for transferring caustic from storage to point of use are typically centrifugal,although positive displacement or other types are used for specific duties. Magnet-ically driven, sealless pumps are often recommended for caustic service.

Cast steel or ductile cast iron pumps may be used for up to 50% caustic to about65�C (150�F), provided that iron contamination is not of concern. Bronze pumps,sometimes with stainless steel impellers, may be used for intermediate strengths ofcaustic if copper contamination is not objectionable. To avoid such contamination,the cast austenitic stainless steels, such as CF8M (J92900), are the most convenientcommercially available materials. From 65�C (150�F) to 100�C (212�F), electrolessnickel plating (ENP) may be used simply to minimize iron contamination, or castalloy 20, such as CN7M (N08007), may be used. A cast duplex stainless steel suchas CD4MCu (J93370) is also used. However, cast nickel alloys are to be preferred,such as alloys CZ100 (equivalent N02200), M35 (equivalent N04400), and CY40(equivalent N06600). Above 100�C (212�F), only the nickel alloys or fluorocarbonsshould be specified.

Aluminum, magnesium, copper, brass, zinc, and tantalum must all be avoided incaustic pump construction. Heat tracing will need to be provided if the pump op-erates at �60�F (�15.6�C) in 50% caustic solutions.

Steel or cast iron pumps used for handling ambient 50% caustic solutions canneed frequent maintenance and have short lives. A more durable solution is to usecentrifugal pumps with mechanical seals and all wetted parts in alloy 20 (N08020).20

Valves

Types of valves used in caustic service include globe, gate, ball, plug, diaphragm,or butterfly. Materials used include resistant metals or polymers, often in combi-nation with each other, depending on design and service conditions. Carbon steel,


nickel or nickel alloys, stainless steel, fluorocarbons, EPDM, and chlorosulfonatedpolyethers have all been used successfully in different types of valves for caustic.

The major concern with valves, aside from mechanical integrity and innate cor-rosion resistance, is the difference between simple shutoff valves and throttlingvalves. In a shutoff valve, there are potential problems of crevice corrosion on theseat or closure. In caustic-containing oxidizing species, the oxidant will be consumedwithin the crevice, setting up a concentration cell effect. When a valve is left partiallyopen, the flow is made more turbulent and erosive and may require more resistantmaterials.

Gaskets, Seals, and O-Rings

Gaskets are used to seal the metallic or nonmetallic flange faces of pipes. Most ofthe gaskets used on caustic soda are based on PTFE. Some of these fluorocarbonplastic gaskets are prevented from flowing by mechanical means; other grades arefilled with graphite or other fillers to minimize cold flow. Another gasketing optionis an envelope gasket. This consists of a core of an elastomeric core sheathed in athin sheet of fluoroplastic to resist the caustic soda.

PTFE gaskets used with titanium or zirconium should be made from virgin PTFEand not include any recycled product. There have been cases of fluoride corrosionin gasket areas when recycled PTFE gaskets were used.

A newer development provides a soft, easily compressible chemically inert 100%PTFE material with a unique combination of chemical resistance and low torquerequirements. Most grades of Gylon� PTFE gaskets are suitable for caustic soda upto 60% at temperatures up to 121�C (250�F); other grades are appropriate for use incaustic up to 75% at temperatures up to 204�C (399�F).21

O-ring materials that are compatible with caustic soda include the following:22

• Ethylene/tetrafluoroethylene polymer (Viton�TBR-S, Aflas�)• Buna-N� (nitrile)• Butyl• Ethylene-propylene• Perfluoroelastomer (Kalrez�, Chemraz�)• Polychloroprene (neoprene)

These materials are suitable for static and dynamic seals, within their thermal limits.The following materials have found service as seals, as tapes and gaskets, or as

sealing pastes and packing materials in caustic environments:

• Flexible graphite—concentrated NaOH and for molten NaOH• Graphite and carbon• Natural rubber• Nitrile rubber—to 120�C (250�F)• Polychloroprene—to 105�C (220�F)• Chlorosulfonated PE (Hypalon�)


Table 18.2 DOT Classifications for Caustic Soda

Concentration DOT No. Classification

Dry, solid 1823 154 Non-flammable compressed gasSolution 1824 154 Corrosive material

• Fluorosilicone grease—to 50% NaOH• Fluoroplastics• Fluoroelastomers (FKM)• Butyl rubber cements—limited to 40% NaOH• Spiral-wound type 304/PTFE

Asbestos gaskets have been commonly used in the past but are now banned orrestricted in many locations.

Shipping of Caustic Soda

Shipping of caustic soda is regulated in most countries. In the United States, theDepartment of Transportation (DOT) regulates the transport of hazardousmaterials;these regulations are enforced by different agencies depending on the mode of ship-ping. Rail shipping is controlled by the Federal Railroad Administration, vesselsand water shipping by the U.S. Coast Guard, road shipping by the DOT, and pipe-line shipping by the DOT or state regulatory commissions.23 The DOT classificationof caustic soda solutions is shown in Table 18.2.

The relevant government regulations for both land and sea transport are detailedin Title 49 of Code of Federal Requirements. The DOT documents 111A60W or111A100W and 407 or 412 define the detail design requirements for the approvedshipping containers for rail and highway transport, respectively.

Different containers are used to transport caustic soda solutions, depending onthe grade of caustic and the temperatures involved. Containers include drums, bar-rels, tank trucks, cars, and barges. High-quality grades of caustic should not betransported or stored in unprotected steel containers.

Drums are usually either steel coated with selected polymeric organic linings orhave an inner pack of polyethylene or, for more critical applications, an austeniticstainless steel or solid nickel alloy 200 (N02200).

Railroad tank cars and over-the-road tank trucks may be of carbon steel, providedthat iron contamination is not a problem. Tank cars are usually coated with ENP ora baked epoxy coating to minimize such contamination. Tank trucks may be ofaustenitic stainless steel (standard for many commodities) or epoxy coated or rubberlined. 73% NaOH is best handled in alloy 200 (N02200).


For marine transport, barge tanks are either coated with organic coatings or ENPor of type 304 (S30400) or 316 (S31600) construction. The low-temperature curingneoprene latex coatings specified in the past are no longer available.

For all types of transport, labeling requirements and handling information areavailable from suppliers and from regulatory organizations, for example, in theUnited States, the Interstate Commerce Commission, the Coast Guard, and the In-ternational Air Transport Association.

References

1. C.P. Dillon, W.I. Pollock, eds. Materials Selector for Hazardous Chemicals: Hy-drochloric Acid, Hydrogen Chloride and Chlorine, vol. MS-3 (St. Louis, MO:MTI, 1995), 200 pp.



4. J.R. Crum, W.G. Lipscomb, “Correlation between Laboratory Tests and FieldExperience for Nickel 200 and 26-1 Stainless Steel in Caustic Service,” COR-ROSION/83, paper no. 23 (Houston, TX: NACE International, 1983), 18 pp.

5. A. Sabata, W.J. Schumacher, “Martensitic and Ferritic Stainless Steels,” in CASTIHandbook of Stainless Steels and Nickel Alloys, ed. S. Lamb (Edmonton, AB,Canada: CASTI Publishing Inc., 2000), p. 149.

6. B. Lundblad, “Akzo Nobel Steps Ahead with New Technology for Caustic Evap-oration,” The Alfa Laval International Customer Magazine 9, June (2003): p. 1.

7. F. Smith, Chemetics International (1981), unpublished report.8. Anon, “Improving Your Product Quality with VenPure Borohydride Products,”

Rohm and Haas (2003), http://www.rohmhaas.com/process/process_chem/app_causticsoda.html.

9. M.P. Sukumaran Nair, “Stress Corrosion Cracking—A Caustic Experience,”Chemical Engineering January (2003): pp. 1–3.

10. Anon, “Sodium Hydroxide Solution and Potassium Hydroxide Solution (Caus-tic) Storage Equipment and Piping Systems,” pamphlet no. 2, 2nd ed. (Wash-ington, DC: The Chlorine Institute Inc., 2001), 79 pp.

11. API 12F, “Shop Welded Tanks for Storage of Production Liquids” (Washington,DC: API, latest ed.).

12. API 620, “Design and Construction of Large, Welded, Low-Pressure StorageTanks” (Washington, DC: API, latest ed.).

13. API 650, “Welded Steel Tanks for Oil Storage” (Washington, DC: API, latest ed.).14. ASME Section V111, “Pressure Vessels” (New York, NY: ASME International,

latest ed.).15. ASME RTP-1, “Reinforced Thermoset Plastic Corrosion Resistant Equipment”

(Fiberglass construction) (New York, NY: ASME International, latest ed.).


16. J.K. Nelson, “Corrosion by Alkalies and Hypochlorites,” in The Metals Hand-book—Corrosion, vol. 13, 9th ed., ed. J.R. Davis (Metals Park, OH: ASM Inter-national, 1987), pp. 1174–1180.

17. Anon, “A Guideline for Cleaning & Inspection of Caustic Soda Storage Tanks”(Midland, MI: Dow Chemical Co., 1999), 4 pp.

18. ANSI B31-3, “Chemical Plant and Petroleum Refinery Piping,” ASME Code forPressure Piping, ASME/ANSI (1990).

19. Anon, “Caustic Soda Storage and Handling” (Midland, MI: Dow Chemical Co.,2003), pp. 19–20.

20. Anon, “General Guidelines for 50% Caustic Soda Storage” (Midland, MI: DowChemical Co., 2003), 1 p.

21. Anon, “Engineered Gasketing Products,” DPI-8/01 Rev. 0-5M (Palmyra, NY:Garlock Sealing Technologies, 2001), 56 pp.

22. Anon, “O-Ring Compatibilities,” Engineering Fundamentals, Efunda (2002),http://www.efunda.com/DesignStandards/oring.

23. Anon, “Caustic Soda” (Cleveland, TN: Olin Chlor Alkali Products), 48 pp.

181

Appendix A. NominalComposition of Alloys

Common Name UNS No. Nominal Composition (%)

Alloy Steels1.25 Cr, 0.5 Mo K11597 1.25 Cr, 0.5 Mo, 0.45 Mn, 0.75 Si, �0.15 C2.25 Cr, 1 Mo K21590 2.25 Cr, 1 Mo, 0.45 Mn, 0.5 Si, �0.15 C5 Cr, 0.5 Mo K41545 5 Cr, 0.5 Mo, 0.45 Mn, 0.5 Si, �0.15 C7 Cr, 0.5 Mo S50300 7 Cr, 0.5 Mo, �1 Mn, �1 Si, �0.15 C9 Cr, 1 Mo S50400 9 Cr, 1 Mo, �1 Mn, �1 Si, �0.15 C3.5 Ni steel K32025 3.5 Ni, 0.9 Mn, 0.28 Si, �0.2 C9 Ni steel K81340 9 Ni, �0.9 Mn, 0.23 Si, �0.13 C

NiResist� Alloy Cast IronsType 1 F41000 15.5 Ni, 2 Cr, 6.5 Cu, 1 Mn, 2 Si, �3 CType 2 F41002 20 Ni, 2 Cr, �0.5 Cu, 1 Mn, 2 Si, �3 CType 3 F41002 20 Ni, 4.5 Cr, �0.5 Cu, 0.8 Mn, �3 CType 5 F41006 35 Ni, �0.1 Cr, �0.5 Cu, 1 Mn, 1.5 Si, �2.4 CType D2 F43000 20 Ni, 2.25 Cr, 1 Mn, 2.25 Si, �3 CType D5 F43006 35 Ni, �0.1 Cr, �1 Mn, 2 Si, �2.4 C

Austenitic Stainless Steels302 S30200 18 Cr, 9 Ni, �0.15 C304 S30400 18 Cr, 8 Ni, �0.08 C304L S30403 19 Cr, 10 Ni, �0.03 C309 S30900 22 Cr, 12 Ni, �0.20 C309S S30908 22 Cr, 12 Ni, �0.08 C2RE10 S31002 24.5 Cr, 20.5 Ni, �0.015 C, �0.15 Si310 S31000 25 Cr, 20 Ni, �0.25 C310L S31050 25 Cr, 20 Ni, �0.02 C310S S31008 25 Cr, 20 Ni, �0.08 C316 S31600 17 Cr, 12 Ni, 2.7 Mo, �0.08 C316L S31603 17 Cr, 12 Ni, 2.7 Mo, �0.03 C316Ti S31635 17 Cr, 12 Ni, 2.7 Mo, �0.08 C, Ti �5 � C



317L S31703 19 Cr, 13.5 Ni, 3.5 Mo, �0.03 C321 S32100 18 Cr, 10.5 Ni, �0.08 C, Ti �5 � C347 S34700 18 Cr, 11 Ni, �0.08 C, Cb �10 � C

Heat-Resisting Alloys253 MA S30815 21 Cr, 11 Ni, 1.7 Si, �0.08 CHP N08705 26 Cr, 36 Ni, �2.5 Si, �2 Mn, �0.5 Mo, 0.55 CHK-40 J94204 25 Cr, 20 Ni, 1.0 Si, �1.5 Mn, 0.4 CHP-Mod — 26 Cr, 35 Ni, 1.5 SiXTM — 35 Cr, 48 Ni, 1.5 SiDS — 18 Cr, 11 Ni, 2.2 Si, 0.2 Al, 0.06 C

High-Performance Austenitic Alloys904L N08904 21 Cr, 25.5 Ni, 4.5 Mo, 1.5 Cu, �0.02 C2RK65 N08904 20 Cr, 25 Ni, 4.5 Mo, 1.5 Cu, �0.025 C254SMO S31254 20 Cr, 18 Ni, 6.2 Mo, 0.7 Cu, 0.2 N, �0.02 C25-6MO N08926 20 Cr, 25 Ni, 6.5 Mo, 0.9 Cu, 0.2 N1925hMo/926 N08926 20 Cr, 25 Ni, 6.2 Mo, 0.8 Cu, 0.2 N, �0.02 CAL-6X N08366 21 Cr, 24.5 Ni, 6.5 Mo, �0.035 CAL-6XN N08367 20.5 Cr, 24 Ni, 6.3 Mo, 0.2 Cu, 0.2 N, �0.02 C654SMO S32654 24 Cr, 22 Ni, 7.3 Mo, 0.5 Cu, 0.5 N, �0.01 C,3 Mn20Cb-3 N08020 20 Cr, 35 Ni, 2.5 Mo, 3.5 Cu, 0.07 Cb, �2 Mn, �1 Si,

�0.02 C20 Mod N08320 22 Cr, 26 Ni, 5 Mo, �2.5 Mn, �1 Si, �0.05 C, Ti800 N08800 20 Cr, 31 Ni, �0.08 C, 0.4 Si, 0.3 Al, 0.4 Ti825 N08825 21.5 Cr, 42 Ni, 3 Mo, 2.3 Cu, �0.05 C, 0.9 Ti, �0.2

AlAlloy 28 N08028 27 Cr, 32 Ni, 3.5 Mo, 1.0 Cu, �0.03 CAlloy 31 N08031 27 Cr, 31 Ni, 6.5 Mo, 1.2 Cu, 0.2 N, �0.02 CAlloy 33 R20033 33 Cr, Bal Ni, 32 Fe, 1.6 Mo, 0.6 Cu, 0.4 NG N06007 22.5 Cr, Bal Ni, 6.5 Mo, 2 Cu, �0.03 C, 19.5 Fe, 2

Cb,G-3 N06985 22.5 Cr, Bal Ni, 7 Mo, 2 Cu, �0.015 C, 19.5 Fe, �0.5

Cb � TaG-30 N06030 30 Cr, Bal Ni, 5 Mo, 1.7 Cu, �0.03 C, 15 Fe, 2.7

W,�5 Co, 0.9 Cb � Ta

Duplex Stainless Steels2205 S31803 22 Cr, 5.5 Ni, 3.0 Mo, 0.14 N, �0.03 C2304 S32304 23 Cr, 4 Ni, 0.1 N, � 0.032507 S32750 25 Cr, 7 Ni, 4.0 Mo, 0.3 N, �0.033RE60 S31500 18.5 Cr, 5 Ni, 2.7 Mo, 0.1 N, �0.03 C7-Mo — 27.5 Cr, 4.5 Ni, 1.5 Mo, �2 Mn, �0.10 C7-Mo PLUS S32950 27.5 Cr, 4.5 Ni, 2 Mo, �2 Mn, �0.03 C— S32906 29 Cr, 6 Ni, 2 Mo, 0.4 N, �0.03 C



Ferritic Stainless Steels409 S40900 11 Cr, 0.5 Ni, �0.08 C, Ti430 S43000 17 Cr, �0.12 C434 S43400 17 Cr, 1 Mo, �0.12 C444 S44400 18 Cr, 2 Mo, �0.02 C, Cb/Ti446 S44600 25 Cr, �0.20 CE-Brite 26-1� S44627 26 Cr, 1 Mo, 0.002 C, CbXM-27 S44627 26 Cr, 1 Mo, 0.002 C, CbSea-Cure� S44660 27.5 Cr, 1.2 Ni, 3.5 Mo, 0.5 Ti, 0.3 SiMonit S44635 25 Cr, 4.0 Ni, 4.0 Mo, 0.5 Ti, 0.35 Si29-4C S44735 29 Cr, 0.3 Ni, 4.0 Mo, 0.5 Ti, 0.35 Si29-4-2 S44800 29 Cr, 2.1 Ni, 4.0 Mo, 0.1 Si

Precipitation-Hardening Steels15-5 PH S15500 5 Ni, 15 Cr, 3.5 Cu, �1 Mn, �1 Si, �0.07 C17-4 PH S17400 4 Ni, 17 Cr, 4 Cu, �1 Mn, �1 Si, �0.07 C

Nickel-Based AlloysBalance is Ni unless Ni content is stated200 N02200 �99 Ni, �0.25 Cu, �0.4 Fe, �0.35 Mn, �0.02 C201 N02201 �99 Ni, �0.25 Cu, �0.4 Fe, �0.35 Mn, �0.02 C230 N02230 �99 Ni, �0.1 Fe, �0.1 Cu, �0.15 Mn, �0.15 C400 N04400 66.5 Ni, Bal Cu, �0.3 C, �2.5 Fe, �2 Mn,�0.5 SiK500 N05500 66.5 Ni, Bal Cu, �2 Fe, 0.27 Al, �1.5 Mn, �0.5 Si,

0.6 Ti, �0.25 C600 N06600 16 Cr, �72 Ni, �0.5 Cu, �0.15 C, 8 Fe601 N06601 60 Ni, 22 Cr, �1 Cu, 1.5 Al, �0.1 C602CA N06025 25 Cr, 9 Fe, 2.2 Al, 0.2 Si, 0.018 C, � Y, Zr, Ti617 N06617 22 Cr, 12.5 Co, 9 Mo, 1.2 Al625 N06625 22 Cr, 61 Ni, 9 Mo, �0.10 C, �5 Fe, 3.6 Nb690 N06690 29 Cr, 9 Fe693 N06693 29 Cr, 4 Fe, 3 Al, 2 Nb, �1 Mn, � 1 Ti, �0.5 Si,

�0.5 Cu, �0.15 CB-2 N10665 �1.0 Cr, 68 Ni, 28 Mo, �0.02 C, �1 Co, 1.8 FeC-4 N06455 16 Cr, 54 Ni, 15.5 Mo, �0.015 C, �3 Fe, 0.7 Ti, �2

CoC-22 N06022 21 Cr, 13 Mo, 3 W, 4 Fe, 0.2 V, 1.7 Co, 0.003 CC-276 N10276 15.5 Cr, 54 Ni, 16 Mo, �0.02 C, �2.5 Co, 5.5 Fe, 4

WC-2000 N06200 23 Cr, 16 Mo, 1.6 Cu, �0.01 C, �0.08 SiAlloy 59 N06059 23 Cr, 16 Mo, 1 Fe

Copper AlloysETP copper C11000 �99.9 CuCartridge brass C26000 70 Cu, �0.05 Fe, �0.07 Pb, Bal Zn



Ounce metal C83600 85 Cu, �1 Ni, �0.3 Fe, 5 Sn, 5 Zn, 5 Pb, �0.25 SbGunmetal C90550 87.5 Cu, �1 Ni, �0.2 Fe, 10 Sn, 2 Zn, �0.3 Pb, �0.2

Sb

Titanium AlloysGrade 2 R50400 �0.3 Fe, Bal TiGrade 7 R52400 �0.3 Fe, Bal Ti, 0.15 PdGrade 12 R53400 �0.3 Fe, Bal Ti, 0.3 Mo, 0.8 Ni

Zirconium AlloysZirconium 702 R60702 �0.2 Fe � Cr, 99.2 Zr � Hf, �4.5 HfZirconium 704 R60704 0.3 Fe � Cr, 97.5 Zr � Hf, �4.5 Hf, 1.5 SnZirconium 705 R60705 0.2 Fe � Cr, 95.5 Zr � Hf, �4.5 Hf, 2.0–3.0 Nb

Cast AlloysCD 4MCu J93370 65 Fe, 25 Cr, 5 Ni, 2 Mo, 3 Cu, 0.03 CCF 3 J93500 69 Fe, 18 Cr, 12 Ni, 1 Si, 0.02 CCF 3M J92800 68 Fe, 19 Cr, 10 Ni, 2 Mo, 1 Si, 0.02 CCF 8 J92600 71 Fe, 19 Cr, 9 Ni, 1 Si, 0.05 CCF 8M J92900 68 Fe, 19 Cr, 10 Ni, 2 Mo, 1 Si, 0.05 CCN TM N08007 44 Fe, 20 Cr, 29 Ni, 2.2 Mo, 1 Si, 3.4 Cu, 0.05 C

185

Appendix B. ApproximateEquivalent Grade of Some Castand Wrought Alloys

Structure Alloy Name Cast (ACI) Cast UNS Wrought

Austenitic SS 304L CF3, CF3A J92500 S30403304 CF8 J92600 S30400316 CF8M J92900 S31600316L CF3M, CF3MA J92800 S31603310 CK20 J94202 S31000309 CH20 J93402 S30900

Alloy 20 CN7M N08007 N08020Duplex SS Alloy 2205 CD3MN J92205 S31803

S32205— CD4MCu J93370 —

Martensitic orferritic SS

Alloy 410 CA15 J91150 S41000

Alloy 420 CA40 J91153 S42000Nickel-basedalloys

Alloy 825 Cu5MCuC N08826 N08825

Alloy 600 CY40 N06040 N06600Alloy 625 CW6MC N26625 N06625Alloy 400 M35–2 N04020 N04400Alloy B-2 N7M

N12MVN30007N30012

N10665

Alloy C-4 CW2M N26455 N06455Alloy C-22 CX2MW N26022 N06022Alloy C-276 CW6M N30107 N10276

187

Appendix C. Glossary ofCorrosion and Materials Terms

These corrosion and materials terms have been selected from the “NACE Glossaryof Corrosion Related Terms” (Houston, TX: NACE International, 2002, 19 pp.), withpermission.

active—(1) The negative direction of electrode potential (2) A state of a metal thatis corroding without significant influence of reaction product

amphoteric—A metal that is susceptible to corrosion in both acid and alkaline en-vironments

anion—A negatively charged ion that migrates through the electrolyteanode—The electrode of an electrochemical cell at which oxidation occurs. Electrons

flow away from the anode in the external circuit. Corrosion usually occurs, andmetal ions enter the solution at the anode

anodic protection—Polarization to a more oxidizing potential to achieve a reducedcorrosion rate by the promotion of passivity

anodizing—Oxide coating formed on a metal surface (generally aluminum) by anelectrolytic process

austenite—The face-centered cubic structure of iron-based alloysaustenitic—A steel in which the predominant structure at room temperature is aus-

tenitebrittle fracture—Fracture with little or no plastic deformationcarbon steel—Alloy of carbon and iron containing up to carbon and up to manga-

nese and residual quantities of other elements, except those intentionally addedin specific quantities for deoxidation (usually silicon and/or aluminum)

cast iron—Iron-carbon alloy containing approximately 2% to 4% carboncasting (cast component)—Metal that is obtained at or near its finished shape by

the solidification of molten metal in a moldcathode—The electrode of an electrochemical cell at which reduction is the principal

reaction. Electrons flow toward the cathode in the external circuit


cathodic corrosion—Corrosion resulting from a cathodic condition of a structure,usually caused by the reaction of an amphoteric metal with the alkaline productsof electrolysis

cathodic protection—A technique to reduce the corrosion of a metal surface bymaking that surface the cathode of an electrochemical cell

cation—A positively charged ion that migrates through the electrolyte toward thecathode under the influence of a potential gradient

cavitation—The formation and rapid collapse of cavities or bubbles within a liquidthat often results in damage to a material at the solid/liquid interface underconditions of severe turbulent flow

corrosion—The deterioration of a material, usually a metal, that results from a re-action with its environment

corrosion fatigue—Fatigue-type cracking of metal caused by repeated or fluctuatingstresses in a corrosive environment characterized by shorter life than would beencountered as a result of either the repeated or fluctuating stress alone or thecorrosive environment alone

corrosion inhibitor—A chemical substance or combination of substances that, whenpresent in the environment, prevents or reduces corrosion

corrosion potential (Ecorr)—The potential of a corroding surface in an electrolyterelative to a reference electrode under open-circuit conditions (also known as restpotential, open-circuit potential, or freely corroding potential)

corrosion rate—The rate at which corrosion proceedscorrosion resistance—Ability of a material, usually a metal, to withstand corrosion

in a given systemcorrosion-resistant alloy (CRA)—Alloy intended to be resistant to general and lo-

calized corrosion of oilfield environments that are corrosive to carbon steelscorrosiveness—The tendency of an environment to cause corrosioncreep—Time-dependent strain occurring under stresscrevice corrosion—Localized attack of a metal at or near an area that is shielded

from the bulk environmentdealloying—The selective corrosion of one or more components of a solid solution

alloy (also known as parting or selective dissolution)dezincification—A corrosion phenomenon resulting in the selective removal of zinc

from copper-zinc alloys. (This phenomenon is one of the more common forms ofdealloying)

ductile (nodular) cast iron—Cast iron that has been treated wile molten with anelement (usually magnesium or cerium) that spheroidizes the graphite

electrochemical cell—A system consisting of an anode and a cathode immersed inan electrolyte so as to create an electrical circuit. The anode and cathode may bedifferent metals or dissimilar areas on the same metal surface

electrolyte—A chemical substance containing ions that migrate in an electric fieldembrittlement—Loss of ductility of a material resulting from a chemical or physical

changeenvironment—The surroundings or conditions (physical, chemical, mechanical) in

which a material exists


environmental cracking—Brittle fracture of a normally ductile material in whichthe corrosive effect of the environment is a causative factor. Environmental crack-ing is a general term that includes corrosion fatigue, hydrogen embrittlement,hydrogen-induced cracking (stepwise cracking), hydrogen stress cracking, liquidmetal cracking, stress corrosion cracking, and sulfide stress cracking

erosion—The progressive loss of material from a solid surface due to mechanicalinteraction between that surface and a fluid, a multicomponent fluid, or solidparticles carried with the fluid

erosion-corrosion—A conjoint action involving corrosion and erosion in the pres-ence of a moving corrosive fluid or a material moving through the fluid, leadingto accelerated loss of material

ferrite—Body-centered cubic crystalline phase of iron-based alloysferritic steel—Steel whose microstructure at room temperature consists predomi-

nantly of ferritefretting corrosion—Deterioration at the interface of two contacting surfaces under

load that is accelerated by their relative motiongalvanic corrosion—Accelerated corrosion of a metal because of an electrical contact

with a more noble metal or nonmetallic conductor in a corrosive electrolytegraphitic corrosion—Deterioration of gray cast iron in which the metallic constit-

uents are selectively leached or converted to corrosion products, leaving thegraphite intact

graphitization—The formation of graphite in iron or steel, usually from decompo-sition of iron carbide at elevated temperatures. (Should not be used as a term todescribe graphitic corrosion)

heat-affected zone (HAZ)—That portion of the base metal that is not melted duringbrazing, cutting, or welding but whose microstructure and properties are alteredby the heat of these processes

heat treatment—Heating and cooling a solid metal or alloy in such a way as toobtain desired properties. (Heating for the sole purpose of hot working is notconsidered heat treatment)

hydrogen blistering—The formation of subsurface planar cavities, called hydrogenblisters, in a metal resulting from excessive internal hydrogen pressure. Growthof near-surface blisters in low-strength metals usually results in surface bulges

hydrogen embrittlement—A loss of ductility of a metal resulting from absorptionof hydrogen

hydrogen-induced cracking—Stepwise internal cracks that connect adjacent hydro-gen blisters on different planes in the metal or to the metal surface (also knownas stepwise cracking)

inhibition—To inhibit means to retard or slow the rate of corrosion, usually by theaddition of other chemicals to the system

intergranular corrosion (IGC)—Preferential corrosion at or near the grain bound-aries of a metal

iron rot—Deterioration of wood in contact with iron-based alloysknife-line attack (KLA)—Local corrosion along a line adjacent to a weld after heat-

ing into the sensitization temperature range


liquid metal cracking (LMC)—Cracking of a metal caused by contact with a liquidmetal

low-alloy steel—Steel with a total alloying element content of less than about 10%but more than specified for carbon steel

metallizing—The coating of a surface with a thin metal layer by spraying, hot dip-ping, or vacuum deposition

oxidation—(1) Loss of electrons by a constituent of a chemical reaction. (2) Corrosionof a metal that is exposed to an oxidizing gas at elevated temperatures

passivation—a reduction in the anodic reaction rate of an electrode involved in acorrosion process

passive—(1) The positive direction of electrode potential. (2) A state of a metal inwhich a surface reaction product causes a marked decrease in the corrosion raterelative to that in the absence of the product

pH—The negative logarithm of the hydrogen ion activity written as: pH ��log10(aH�), where aH� � hydrogen ion activity � the molar concentrationof hydrogen ions multiplied by the mean ion-activity coefficient

pitting—Localized corrosion of a metal surface that is confined to a small area andtakes the form of cavities called pits

pitting factor—The ratio of the depth of the deepest pit resulting from corrosiondivided by the average penetration as calculated from mass loss

rust—Corrosion product consisting of various iron oxides and hydrated iron oxides.(This term properly applies only to iron and ferrous alloys)

sensitization—Precipitation of constituents (usually carbides) in a structure as aresult of heating and cooling through a certain temperature range. Can lead tointergranular corrosion

stress corrosion cracking (SCC)—Cracking of metal involving anodic processes oflocalized corrosion and tensile stress (residual and/or applied)

sulfidation—The reaction of a metal or alloy with a sulfur-containing species toproduce a sulfur compound that forms on or beneath the surface of the metal oralloy

transpassive—The noble region of potential where an electrode exhibits a higher-than-passive current density

weld (verb)—To join two or more pieces of metal by applying heat and/or pressurewith or without filler metal; to produce a union through localized fusion of thesubstrates and solidification across the interfaces

weld decay—Intergranular corrosion associated with sensitization due to weldingweldment—That portion of a component on which welding has been performed,

including the weld metal, the heat-affected zone (HAZ), and the base metalweld metal—That portion of a weldment that has been molten during weldingwrought metal—Metal in the solid condition that is formed to a desired shape by

working (rolling, extruding, forging, and so on), usually at an elevated tempera-ture

yield strength—Stress at which a material exhibits a specified deviation from theproportionality of stress to strain

191

Appendix D. Glossary ofAcronyms and Abbreviations

AAR—Association of American RailroadsACGIH—American Conference of Governmental Industrial HygienistsANSI—American National Standards InstituteAOD—Argon Oxygen DecarburizationAP—Anodic ProtectionAPI—American Petroleum InstituteASME—American Society of Mechanical EngineersASTM—American Society for Testing and MaterialsBAT—Best Available TechniquesBP—Boiling PointBSI—British Standards InstitutionCAF—Compressed Asbestos FiberCAS—Chemical Abstracting ServiceCFR—Code of Federal RegulationsCPVC—Chlorinated Polyvinyl ChlorideCR—Corrosion Rate�Be—Degree BaumeDBT—Ductile-Brittle TransitionDO—Dissolved OxygenDOT—Department of Transportation (US)ECTFE—Ethylene ChlorotrifluoroethyleneEEC—European Economic CommunityEFMA—European Fertilizer Manufacturers AssociationENP—Electroless Nickel PlatingESC—Environmental Stress Cracking (plastics)ESR—Electroslag RemeltingEU—European Union (formerly, European Community, EC)FEP—Fluorinated Ethylene PropyleneFKM—Fluorohydrocarbon Elastomers


FP—Freezing PointFRP—Fiber-Reinforced PlasticGRP—Glass Fiber–Reinforced PlasticsHAC—Hydrogen-Assisted CrackingHAZ—Heat-Affected ZoneHBN—Hardness Brinell NumberHDPE—High-Density PolyethyleneHRC—Hardness Rockwell CIDLH —Immediately Dangerous to Life or HealthIFA—International Fertilizer Industry AssociationIGA—Intergranular AttackIGC—Intergranular CorrosionKLA—Knifeline AttackLMC—Liquid Metal CrackingLME—Liquid Metal EmbrittlementMSDS—Material Safety Data SheetNDE—Nondestructive ExaminationNDTT—Nil Ductility Transition TemperatureNIOSH—National Institute for Occupational Safety and HealthOEL—Occupational Exposure LimitOSHA—Occupational Safety and Health AdministrationPE—PolyethylenePEL—Permissible Exposure LimitPFA—PerfluoroalkoxyPP—PolypropylenePTFE—PolytetrafluorethylenePVC—PolyvinylchloridePVDC—Polyvinylidene ChloridePVDF—Polyvinylidene FluoridePVF—Polyvinyl FluoridePWHT—Postweld Heat TreatmentREL—Recommended Exposure LimitSBR—Styrene Butyl RubberSCC—Stress Corrosion CrackingSp.Gr.—Specific GravitySS—Stainless SteelSTEL—Short-Term Exposure Limit.TLV—Threshold Limit ValueTWA—Time-Weighted Average.

193

Index

Locators in italics refer to tables or figures.

ABS (acrylonitrile-butadiene-styrene), 48, 139Acrylic, 139Acrylonitrile-butadiene-styrene (ABS), 48, 139Acrylonitrile rubber (Buna-N�), 47, 48, 69,

137, 138, 176Advanced membrane gap cells (MGPs), 88Aflas�, 176AL 29-4-2� (S44800), 31, 107, 108AL 29-4C� (S44735), 31, 107, 108Alloy 3RE60 (S31500), 32, 56, 65–66Alloy 6XN (N08367), 117Alloy 7-Mo�, 110Alloy 17-4PH, 109Alloy 20 (N08020), 107, 108, 165, 175Alloy 20Cb3� (N08020), 115, 152Alloy 21/4% Cr, 1%Mo (K21950), 108Alloy 25-25-2, 122Alloy 26-1, 146, 147, 148, 165Alloy 26-1S (S44626), 108Alloy 28 (N08028), 118, 120, 120, 149, 152Alloy 29-4 (S44700), 169Alloy 33� (R20033), 120, 122, 123, 149, 150Alloy 200 (N02200): in ammonia service, 39,

39; in caustic soda service, 110, 122, 123,123–24, 125, 126, 146, 148, 148, 149, 150,152, 164, 165, 168, 169, 170, 171, 177; inKOH service, 160

Alloy 201 (N02201), 122, 123, 123, 124, 127,153, 163, 164, 169, 170

Alloy 214, 36, 38Alloy 230 (N02230), 38Alloy 254 SMO (S31254), 115, 117Alloy 400 (N04400): in ammonia service, 40;

in caustic soda service, 96, 121, 124, 125,125, 128, 150, 151, 152, 165, 168, 168, 170,171, 174; in KOH service, 160

Alloy 446 stainless steel, 38Alloy 556, 38Alloy 600 (N06600): in ammonia service, 38,

65; in caustic soda service, 121, 124, 125,125, 126, 127, 128, 152, 153, 155, 165, 168,169; chromium equivalents, 36; isocorro-sion curves, 127

Alloy 601 (N06601), 35, 36, 36, 38, 128Alloy 602CA (N06025), 36Alloy 617 (N06617), 36, 38Alloy 620, 36Alloy 625 (N06625): in ammonia service, 35,

39; in caustic soda service, 128, 152, 155,165, 169, 171

Alloy 654 SMO (S32654), 117, 150Alloy 690 (N06690), 36, 121, 126, 127, 165Alloy 693 (N06693), 36Alloy 800 (N08800): in ammonia service, 36,

36, 63; in caustic soda service, 115, 116,118, 120, 126, 149, 154, 165

Alloy 800/800HT, 37, 38Alloy 800H (N08800), 36, 36, 38, 62Alloy 825 (N08825), 115, 154Alloy 904 (N80904), 114Alloy 904L (N08904), 117, 118, 120Alloy 926 (N06696), 150Alloy 2205 (S31803), 32, 110Alloy 2205 Code Plus 2, 117Alloy 6030 (N06690), 128Alloy 6375, 48, 138Alloy APM, 36Alloy B-2 (N106650), 126, 165Alloy C-4 (N06455), 128, 150Alloy C-276 (N10276), 39, 128, 165Alloy CF3 (J92700) cast alloy, 33–34, 114Alloy CF3M (J92800), 34, 114

194 Index

Alloy CF8 (J92600) cast alloy, 33–34, 114, 115Alloy CF8M (J92900), 34, 68, 69, 114, 175Alloy CN7M (J92700), 119, 175Alloy CN7M (N08007), 117Alloy CY40 (equivalent N06600), 175Alloy CZ100 (equivalent N02200), 175Alloy DS, 36Alloy ELI 21-1 (S44626), 169Alloy HK 40 (J94204), 33, 34, 36, 60, 62Alloy HK 45Nb, 62Alloy HP (N08705), 34, 60, 62Alloy HP 45Nb, 34Alloy HP-Mod, 36, 62, 63Alloy HT, 62Alloy HU, 62Alloy K-500 (N05500), 125Alloy M35 (equivalent N04400), 175Alloy S, 38Alloys, chromium equivalents, 36Alloy S32906, 148, 149Alloy SAF 2304 (S32304), 117Alloy SAF 2507 (S32507), 117Alloy X, 38Alloy X1CrNiMoN, 122Alumina (Al2O3), 151Aluminum (Al): bauxite refining and, 151–52;

caustic soda service, 99; corrosion, 20,23–24; hydrogen ion liberation, 19; LMErisk, 96, 150

Aluminum alloys, 23–24, 96, 99, 150Aluminum oxide, 144Aminodiisopropanol, 151Ammonia, 30%, 9Ammonia (CAS 7664-41-7): aluminum and,

23–24; carbon steels and, 25–30; cast ironand, 24–25; chemical properties, 9; com-mercial grades, 7; corrosion by, 19–22; dis-sociation, 27; health and safety, 9–11; igni-tion, 11; industrial uses, 4; nitrogen fixationand, 3; physical properties, 8; physiologicalresponses to, 10; production, 13–17, 14,59–73, 61; properties, 7–12; SCC mitigation,28; synthesis, 64–65

Ammonia converters, 38Ammonia plant, 60Ammonia solutions, 8Ammonium bicarbonate, 55Ammonium carbonates, 55Ammonium chloride, 55Ammonium hydroxide (CAS 1336-21-6), 3,

7–8, 24, 39, 40, 56–57Aqua ammonia, 9

Asbestos, compressed fibers, 69Asbestos gaskets, 177ASTM A213, 64, 65ASTM A312, 64ASTM A331, 64ASTM A335, 64ASTM materials, 64Austenitic alloys, high-performance, 115–21Austenitic nickel cast irons, 101Austenitic stainless steels, 21, 32–33, 106,

111–13, 153. See also specific alloys; specifictypes

Baffles, 72Basch, Karl, 13Bauxite refining, 151–52Bayer process, 152Bisphenol-A fumarate, 50, 141, 141Boilers, 168Bolting, 70Borosilicate glass, 143, 143, 144Brine circulation piping, 169Brine pumps, 168Budatiene-styrene rubber (Buna-S�), 47Buna-N� (acrylonitrile rubber), 47, 48, 69, 137,

138, 176Buna-S� (budatiene-styrene rubber), 47Butadiene, 138Butadiene acrylonitrile (Buna-N�), 47, 48, 69,

137, 138, 176Butyl, 176Butyl, grade 1, 138Butyl rubber, 69

Carbon, 52, 141–42, 176Carbon dioxide, 23Carbon dioxide, carbamates, 55–57Carbon dioxide removal system, 64Carbon steels: in ammonia service, 25–30, 40,

167; in caustic soda service, 101–6, 164Carburization, 35, 37, 62Cast alloys, 36. See also specific alloysCast duplex steels, 117Cast irons: in ammonia service, 24–25, 25; in

caustic soda service, 100–101, 102, 103, 125;gray, 95, 100, 102, 103; gray corrosion, 103;high-silicon, 101; SG (spheroidal graphite),152; white, 24

Cast stainless steels, 33–34, 113–14Cathodic protection, 26Caustic cracking, 113Caustic dilution, 85–86

Index 195

Caustic fusion reactions, 152–53Caustic potash. See Potassium hydroxide

(KOH)Caustic soda (CAS 1310-73-2, NaOH): boiling

points, 82; chemical properties, 82–83; con-taminants, 145–51; contamination by, 154;corrosion, 93–97; DOT classifications, 177;handling, 170–77; health and safety consid-erations, 83–84; impurities, 90–91; physicalproperties, 81–82, 82, 83; production,87–92, 88–90, 91, 167–79; protective equip-ment, 84; SCC and, 21; shipping, 177–78;solutions, 84; treatments, 151–54; uses, 78

CD-4MCu (J93370), 117, 119, 175Centrifugal pumps, 95Centrifuges, 168Ceramic materials, 52, 142–43CGA pamphlet G-2.1, 71Chemraz�, 176Chlor-Alkali plant, 77Chlorates, 146, 146–48Chlorides, 20, 21, 33, 57, 148–49Chlorinated polyester, 141Chlorinated polyether, 139Chlorinated polyvinyl chloride (CPVC), 48,

50, 139, 139, 140Chlorinated rubber, 137Chlorine/hypochlorite, 149Chlorine production, 87, 167Chloroprene (CR, neoprene), 48Chlorosulfonated polyethylenes (CSPE), 48,

137, 138, 176Chromium, 113, 118Chromium-bearing alloys, 126–28Chromium carbide precipitation, 21Chromium-molybdenum steels, 28–29, 30Coal feedstock plant, 60Code of Federal Regulations (CFR), 71Cold work, 104Compressed asbestos fiber (CAF), 69Compressors, 68–69Condensers, 66Contamination: caustic, 154; caustic soda ser-

vice, 145–57; chlorides, 57; sodium hydrox-ide, 90–91; of steam, 154–55

Coolers, 66, 171Copper, 129Copper alloys, 21, 40, 41, 66, 96, 150Copper (Cu), 96, 125, 128–29, 150Corrosion: aluminum, 23–24; by ammonia,

19–22; cast iron, 102; by caustic soda,93–97; forms of, 20–22, 94–96; of metalsand alloys, 99–136

Corrosion-resistant alloys (CRAs), 115, 121CPR, 69CPVC (chlorinated polyvinyl chloride), 50,

139, 139Creep resistance, 35Crevice corrosion, 21Crystallizers, 168, 169–70CSM, 138CSPE (chlorosulfonated polyethylenes), 137Cupronickel 70-30 (C71500), 129Cupronickel 90-10 (C70600), 129Cuprous oxide, 41

Dealloying, 22, 95Decarburization, 27, 29Department of Transportation, 70–71, 71,

177–78Desulfurization section, 59Diabon F100�, 52Diaphragm cell process, 88–89Disposal: ammonia, 11; caustic soda, 85Distillation columns, 65Drums, 177Ductile cast iron (DI), 101Duplex alloys, 106, 109–11, 111, 148, 152. See

also specific alloysDuplex stainless steels, 31–32

Ebonite (hard rubber), 137E-Brite� (S44627), 107, 108, 108, 110, 147, 148,

149, 165, 169ECTFE (Halar�, ethylene chlorofluoroethy-

lene), 48, 50, 140EFTE (ethylene trifluoroethylene), 50Elastomers, 47, 48, 137–38, 138. See also specific

elastomersElectroless nickel plating (ENP), 88, 126, 175,

178Electrolytic cells, 91ELI 21-1 (S44626), 169Environmental stress cracking (ESC), 139EPDM (ethylene propylene diene monomer

rubber), 47, 48, 69, 137Epoxy, 50, 51, 88, 141Erosion-corrosion, 21, 95ETFE (Tefzel�, ethylene trifluoroethylene), 48,

50, 140, 176Ethylene chlorofluoroethylene (Halar�,

ECTFE), 48, 50, 140Ethylene propylene, 48, 176Ethylene propylene diene monomer rubber

(EPDM), 47, 48, 69, 137

196 Index

Ethylene trifluoroethylene (ETFE, Tefzel�), 48,50, 140, 176

ETP-S, 48, 69, 138Evaporators, 92, 168, 169–70Explosions, 84Eye protection, 10–11

F138, 112Federal Railroad Administration, 177FEP (fluorinated ethylene propylene), 48, 50,

139, 140Ferritic stainless steels, 31, 106–9Ferrous alloys, 100. See also specific alloysFertilizers, ammonia in, 3FFKM, 48, 138Fiber-reinforced plastic (FRP), 49, 50, 50, 51,

140–41, 169Filters, 168Fire, 84First aid, 85Fittings, 168FKM, 47, 138Flexible graphite, 69, 176Fluorinated ethylene-propylene (FEP), 50, 139,

140Fluorocarbon elastomers, 137Fluoroelastomers, 48, 138Fluorohydrocarbon elastomers, 47Fluorosilicone, 13749 CFR 173.314, 71FPA (perfluoro alkoxy), 50FRP (fiber-reinforced plastic), 49, 50, 50, 51,

69, 140–41Furane (furfural-furfuryl alcohol), 50, 141, 141

Galvanic corrosion, 95Gaskets, 69, 176–77General corrosion, 94–95Glass, 52, 52, 69, 142, 142–43Glycolate, 155Grain boundary embrittlement, 152Grain size, corrosion and, 42Graphite, 52, 69, 141–42, 176Graphitization, 122Gray cast iron, 95, 100, 102Gunmetal (C90550), 129Gylon� PTFE gaskets, 69, 176

Haber, Fritz, 13Haber-Bosch process, 13Halar� (ECTFE, ethylene chlorofluoroethy-

lene), 48, 50, 140

Hard rubber (Ebonite), 69, 137Heaters, 65–66, 168, 171Heat exchangers, 65–66, 168, 171Heat recovery steam generators (HRSGs), 155Heat transfer, 96Hexoloy� (silicon carbide), 143, 144High-density polyethylene (HDPE), 138, 139High-phosphorus EN (HPEN), 126High-silicon cast irons, 101High-temperature converter, 64High-temperature corrosion, 22, 96Hoses, 69Hydrogenated bis-A-polyester, 141Hydrogen attack, 27–29Hydrogen chloride (HCl), 55Hydrogen embrittlement, 64Hydrogen service, 29Hydrogen sulfide, 23, 29–30, 35, 152Hypalon�, 48, 138, 176Hypochlorites, 146, 149

Iconel, 161Ingestion, ammonia, 10Intergranular attack (IGA), 21, 95, 108, 123,

127, 169Iron, 24, 100, 150–51Isobutylene isoprene, 137Isocorrosion curves: 904L, 114; alloy 28, 120;

alloy 201, 127; alloy 600, 127; CF8, 115; CN7M, 119; type 304, 113; type 316, 113

Isophthalates, 141Isoprene, 47, 69

Kalrez�, 48, 138, 176KFM, 48KHR 35 CT, 63Kynar� (polyvinylidene fluoride, PVDF), 48,

139

Lead, 43Leaks, 11, 85L grade stainless steels, 112Linings, 172Liquid metal embrittlement (LME), 96, 125,

127, 129, 130, 150Localized corrosion, 95Low-alloy steels, 101–6Low-C steel, 161Low-phosphorus EN (LPEN), 126

Magnesium, 43Magnetite, 100

Index 197

Martensitic stainless steels, 106Material Safety Data Sheets (MSDA), 71Medium-phosphorus EN (MPEN), 126Membrane cell process, 89–90, 90Mercury, 149–50Mercury cell process, 89Metal dusting, 35–37, 63–64Metal finishing, 153Metal wastage rates, 37Methyl diethanolamine (MDEA), 64Mid American Pipeline System (MAPCO), 72Mild steel, 125Molybdenum, 116, 118Monel, 161Monit� (S44635), 31, 107

N08904, 112Natural rubber (NR), 47, 48, 69, 137, 138NBR, 48, 138Nelson curves, 27Neoprene (chloroprene CR), 47, 48, 137, 176Neoprene latex, 137, 178NHT (naphtha hydrotreater), 55Nickel: caustic soda service, 121–28; corrosion

and, 102, 116, 161; effects of, 116, 117, 118,121

Nickel alloys: in ammonia service, 30, 37–39;in caustic soda service, 121–28, 165, 168; ef-fects of, 164; metal wastage rates, 37; ni-triding depth and, 38; nitrogen absorption,38. See also specific alloys

Nil Ductility Transition Temperature (NDTT),33

Niobium, 43NIOSH/MSHA acid-gas respirators, 10–11NiResist�, 25, 102, 103, 151, 168, 169; type 1

(F41000), 101, 125; type 2 (F41002), 101,125; type D2 (F3000), 101

Nitriding, steel, 27Nitriding depth, 38Nitriding tests, 38Nitrile rubbers, 48, 137, 138Nitrogen absorption, 38Nitrogen fixation, 3Nonmetallic materials: in ammonia service,

47–53; in caustic soda service, 137–44Nordel�, 48

1050LF, 48, 138O-rings, 69, 70, 176–77Oxide layers, 19–20, 94

Paper operations, 153–54

Partial oxidation process, 15Passivity, 19–20, 93–94Perfluoro alkoxy (PFA), 50, 140Perfluoroelastomers, 47, 48, 137, 138, 176Petroleum refining, 151PE/vinyl acetate (PVA), 139PFA (perfluoro alkoxy), 48, 140PH, 19, 24, 82Phenolic resins, 141Phosphorus, 126Pipelines, 72, 168Piping, 68, 129Pitting corrosion, 20Plastic-lined pipe, 49Plastics, 47–52, 138–41Polybutylene (PB), 48Polycarbonate, 139Polychloroprene (neoprene), 137, 176Polychloroproprene rubber, 47Polyesters, 48Polyethylene (PE), 48, 50, 139, 140Polyisobutylene, 48Polypropylene (PP), 48, 50, 138, 139, 140Polystyrene, 48, 139Polysulfide, 138Polytetrafluoroethylene (PTFE, Teflon�), 48,

138, 139, 140, 176Polyurethane, 141, 141Polyvinyl chloride (PVC), 48, 49, 50, 139, 139,

140Polyvinylidene chloride (PVDC), 139Polyvinylidene fluoride (PVDF), 48, 50, 139,

139, 140Postweld heat treatment (PWHT), 29, 67, 154,

172Potassium chlorate (KClO3), 160, 161Potassium chloride (KCl), 160Potassium hydroxide (KOH, CAS 1310-58-3),

78, 113, 128, 160–61Precipitation-hardening (PH), 30, 31, 106, 109Pressure vessels, 167–69Pressurized pressure vessels, 67–68Primary reformer, 60–63Production equipment: ammonia, 59–73; caus-

tic soda, 167–79Protective equipment: ammonia, 10–11; caus-

tic soda, 84PTFE (Teflon�, polytetrafluoroethylene), 48,

48, 69, 139, 176Pulp operations, 153–54Pumps, 68, 168, 175PVC (polyvinyl chloride), 48, 49, 50

198 Index

PVDF (Kynar�, polyvinylidene fluoride), 48,48, 139

PWHT (postweld heat treatment), 29, 67, 154,172

Railcar transport (tank cars), 71–72Refrigerants, 3Refrigerated storage tanks, 66–67Rubber, 47, 69, 137, 138Ruwais Fertilizer Industries (FERTIL), 63

Salt separators, 170SCM, 48Sea-Cure� (S44660), 31, 107, 147, 148, 149Seals, 69, 176–77Secondary reformers, 63–64Settling tanks, 168SG (spheroidal graphite) cast iron, 152Shomac� 30-2, 107Showa Denko, 16–17S-phase precipitation, 31Silica, 69Silicon carbide (Hexoloy�), 143Silicon cast irons (F47003), 101Silicone, 138Silicone rubber, 137Silver, 43, 131Skin, 10–11Soap manufacture, 152Soda ash, 153, 159–60Sodium, molten, 155Sodium bicarbonate (NaHCO3), 155, 156Sodium carbonate (Na2CO3, CAS 497-19-8).

See Soda ashSodium chlorate, 147Sodium chloride, 147Sodium hydrosulfide (NaSH), 152Sodium hydroxide (NaOH). See Caustic sodaSodium hypochlorite, 169Sol-gel procedures, 34–35Solvay (ammonia soda), 155Spills: ammonia, 11; caustic soda, 85Stainless steels: in ammonia service, 30–34;

cast, 113–14; in caustic soda service, 106,163, 164; corrosion, 20, 107, 108, 122; IGAin, 21; isocorrosion curves, 113, 114, 120; Lgrade, 112. See also specific types

Static corrosion rates, 124Steam, contamination of, 154–55Steam/air reforming process, 13, 14

Storage tanks, 66–68, 171–73Stress corrosion cracking (SCC), 96, 127; aus-

tenitic stainless steels, 153–54; in causticsoda service, 100, 104, 105, 122; chloride,31, 33, 172; chromium-bearing alloys,126–28; description, 21; isocorrosioncurves, 113, 114; mitigation measures, 28;nickel alloys and, 121; soda ash and, 155;soil subsidence and, 173; steam contamina-tion and, 154; steel, 26–27

Sulfides, 153Sulfur, 150Surface passivity, 93–94

T22 material, 65Tank cars, 71–72, 168, 177Tanks, 28Tank trucks, 72, 177Tantalum, 43, 131–32Tantalum-tungsten alloy, 132TBR-S, 48, 69, 138, 176Teflon� (polytetrafluoroethylene, PTFE), 48,

48, 69, 138, 139, 140Tefzel� (ETFE, ethylene trifluoroethylene), 48,

50, 140Temperature: alloy use and, 34–37; caustic

soda service and, 100–101, 105; corrosionand, 107, 108, 117; limits, 48, 49; nitridingdepth and, 38; SCC and, 105

Temper embrittlement, 29Thermoplastics, 48–49, 138–40, 139Thermoset resins, 49–51, 140–41Tin, 43T-I steel (A-517), 72Titanium alloys, 42, 129–30Titanium (R50400): in ammonia service, 42,

69; in caustic soda service, 96, 129–30, 130,150, 176; in KOH, 161

Title 29 (Labor), 71Title 33 (Navigation and Navigable Waters),

71Title 40 (Protection of the Environment), 71Title 46 (Shipping), 71Title 49 (Transportation), 71Transfer piping, 168Transportation equipment, 70–72, 177–78Triple-Effect Caustic Soda Evaporator, 92Trona, 155Tungsten carbide, 144Type 302 (S30200), 32, 111Type 304 (S30400), 107, 107; in ammonia ser-

Index 199

vice, 32, 33, 34, 38, 56, 60, 63, 64, 65, 67, 68,69, 71; in caustic soda service, 108, 110,111, 112, 114, 117, 155, 156, 164, 165, 178;chromium equivalents, 36; desulfurization,59; isocorrosion curves, 113. See also AlloyCF8 (J92600) cast alloy

Type 304L (S30403), 32, 68, 111, 112, 113, 114,118, 153, 154, 171, 174. See also Alloy CF3(J92700) cast alloy; Isocorrosion curves

Type 309 (S30900), 38Type 310 (S31000), 34, 36, 37, 38, 60Type 310S (S31008), 33Type 316 (S31600): in ammonia service, 33, 56,

64; in caustic soda service, 108, 122, 148,148, 164, 165, 178; desulfurization, 59; iso-corrosion curves, 113

Type 316L (S31603), 32, 108, 111, 112, 117, 118,118, 120, 151, 152, 154, 174

Type 316Ti (S31635), 32, 112, 120–21, 150Type 317 (S31700), 33, 112Type 317L (S31703), 33, 108, 112, 118Type 321 (S32100), 32, 33, 59, 65, 112, 154Type 347 (S34700), 32, 34, 60, 112, 155, 160Type 410 (S41000), 56, 68, 69, 109Type 439 (S43035), 108Type 444 (S44400), 108

Type 446 (S44600), 108Type XM-27 (S44627), 31, 107, 108

Valves, 69, 168, 175–76Vapor-phase attack, 21–22Vinyl ester, 50, 51, 141Viton�, 47, 48, 138, 176Viton� A, 69Viton� Extreme, 69

Waste heat recovery system, 64Welding, 32, 104, 172Weld seams, 27

XTM, 36

Yellow brass (C26800), 41, 42

Zinc, 43, 95, 125Zinc spraying, 26Zirconia, 130Zirconium alloys, 43Zirconium (R60702): in ammonia service, 43,

69; in caustic soda service, 96, 130–31, 131,149, 150, 176; in KOH, 161, 161

Zirconium Zr702 (R60702), 43, 130

foren material selector for ammonia and caustic soda- mti ms-6

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