1.PrefaceCorrosion is both costly and dangerous. Billions of dollars are spent annuallyfor the replacement of corroded structures, machinery, and components,including metal roong, condenser tubes, pipelines, and many other items.In addition to replacement costs are those associated with preventivemaintenance to prevent corrosion, inspections, and the upkeep ofcathodically protected structures and pipelines. Indirect costs of corrosionresult from shutdown, loss of efciency, and product contamination or loss. Although the actual replacement cost of an item may not be high, the lossof production resulting from the need to shut down an operation to permit thereplacement may amount to hundreds of dollars per hour. When a tankor pipeline develops a leak, product is lost. If the leak goes undetected fora period of time, the value of the lost product could be considerable. In addition,contamination can result from the leaking material, requiring cleanup, and thiscan be quite expensive. When corrosion takes place, corrosion products buildup, resulting in reduced ow in pipelines and reduced efciency of heattransfer in heat exchangers. Both conditions increase operating costs. Corrosionproducts may also be detrimental to the quality of the product being handled,making it necessary to discard valuable materials. Premature failure of bridges or structures because of corrosion can alsoresult in human injury or even loss of life. Failures of operating equipmentresulting from corrosion can have the same disastrous results. When all of these factors are considered, it becomes obvious why thepotential problem of corrosion should be considered during the early designstages of any project, and why it is necessary to constantly monitor theintegrity of structures, bridges, machinery, and equipment to preventpremature failures. To cope with the potential problems of corrosion, it is necessary tounderstand1. Mechanisms of corrosion2. Corrosion resistant properties of various materials3. Proper fabrication and installation techniques4. Methods to prevent or control corrosion5. Corrosion testing techniques6. Corrosion monitoring techniquesCorrosion is not only limited to metallic materials but also to all materialsof construction. Consequently, this handbook covers not only metallicmaterials but also all materials of construction.

2. Chapter 1 and Chapter 2 cover the mechanisms of corrosion and the effectsof atmospheric corrosion. Chapter 3 through Chapter 27 cover metallic materials and their alloys.Corrosions potential is discussed for each metal or alloy. Charts are providedfor the compatibility of each metal or alloy with selected corrodents.References are provided for additional compatibility data. It is the intention of this book that regardless of what is being built,whether it be a bridge, tower, pipeline, storage tank, or processing vessel,information for the designer/engineer/maintenance personnel/or whoeveris responsible for the selection of construction material, this book will enablethem to avoid unnecessary loss of material through corrosion. Philip A. Schweitzer 3. AuthorPhilip A. Schweitzer is a consultant in corrosion prevention, materials ofconstruction, and chemical engineering based in York, Pennsylvania. Aformer contract manager and material specialist for Chem-Pro Corporation,Faireld, New Jersey, he is the editor of the Corrosion Engineering Handbookand the Corrosion and Corrosion Protection Handbook, Second Edition; and theauthor of Corrosion Resistance Tables, Fifth Edition; Encyclopedia of CorrosionTechnology, Second Edition; Metallic Materials; Corrosion Resistant Linings andCoatings; Atmospheric Degradation and Corrosion Control; What Every EngineerShould Know About Corrosion; Corrosion Resistance of Elastomers; CorrosionResistant Piping Systems; Mechanical and Corrosion Resistant Properties ofPlastics and Elastomers (all titles Marcel Dekker, Inc.); and Paint and Coatings,Applications and Corrosion Resistance (Taylor & Francis). Schweitzer receivedthe BChE degree (1950) from Polytechnic University (formerly PolytechnicInstitute of Brooklyn), Brooklyn, New York. 4. ContentsChapter 1 Fundamentals of Metallic Corrosion ......................................... 11.1 Forms of Corrosion...................................................................................... 2 1.1.1 Uniform Corrosion ......................................................................... 3 1.1.1.1 Passive Film on Iron......................................................... 3 1.1.1.2 Passive Film on Nickel..................................................... 4 1.1.1.3 Passive Film on Austenitic Stainless Steel.................... 4 1.1.1.4 Passive Film on Copper................................................... 4 1.1.1.5 Passive Film on Aluminum ............................................ 5 1.1.1.6 Passive Film on Titanium ................................................ 5 1.1.1.7 Passive Film on Tantalum ............................................... 5 1.1.1.8 Uniform Corrosion Rates................................................. 5 1.1.2 Intergranular Corrosion ................................................................. 7 1.1.3 Galvanic Corrosion ......................................................................... 8 1.1.4 Crevice Corrosion ......................................................................... 10 1.1.5 Pitting Corrosion........................................................................... 12 1.1.6 Erosion Corrosion ......................................................................... 15 1.1.7 Stress Corrosion Cracking (SCC) ............................................... 16 1.1.8 Biological Corrosion ..................................................................... 18 1.1.8.1 Corrosion of Specic Materials..................................... 21 1.1.9 Selective Leaching......................................................................... 231.2 Corrosion Mechanisms ............................................................................. 241.3 Measuring Polarization............................................................................. 31 1.3.1 Anodic Polarization...................................................................... 341.4 Other Factors Affecting Corrosion .......................................................... 35Reference .............................................................................................................. 37Chapter 2 Atmospheric Corrosion............................................................... 392.1 Atmospheric Types .................................................................................... 402.2 Factors Affecting Atmospheric Corrosion .............................................412.2.1 Time of Wetness ............................................................................ 42 2.2.1.1 Adsorption Layers ..........................................................43 2.2.1.2 Phase Layers .................................................................... 43 2.2.1.3 Dew ................................................................................... 43 2.2.1.4 Rain ...................................................................................43 2.2.1.5 Fog ..................................................................................... 44 2.2.1.6 Dust ...................................................................................44 2.2.1.7 Measurement of Time of Wetness ................................ 442.2.2 Composition of Surface Electrolyte ........................................... 45 2.2.2.1 Oxygen..............................................................................45 5. 2.2.2.2 SOX ....................................................................................45 2.2.2.3 NOX ................................................................................... 45 2.2.2.4 Chlorides ..........................................................................45 2.2.2.5 CO2 ....................................................................................46 2.2.2.6 Concentrations of Different Species.............................46 2.2.3 Temperature...................................................................................46 2.2.4 Initial Exposure .............................................................................47 2.2.5 Sheltering........................................................................................47 2.2.6 Wind Velocity ................................................................................47 2.2.7 Nature of Corrosion Products .................................................... 47 2.2.8 Pollutants Present .........................................................................482.3 Mechanisms of Atmospheric Corrosion of Metals............................... 49 2.3.1 Damp Atmospheric Corrosion (Adsorption Layers) ..................................................................... 52 2.3.2 Wet Atmospheric Corrosion (Phase Layers) ............................ 54 2.3.2.1 Dew ................................................................................... 54 2.3.2.2 Rain ...................................................................................54 2.3.2.3 Fog ..................................................................................... 55 2.3.3 Deposit of Pollutants ....................................................................552.4 Corrosion Products.................................................................................... 562.5 Specic Atmospheric Corrodents ........................................................... 58 2.5.1 Sulfur-Containing Compounds ..................................................59 2.5.2 Nitrogen-Containing Compounds ............................................. 61 2.5.3 Chlorine-Containing Compounds.............................................. 62 2.5.4 Carbon Dioxide (CO2)..................................................................62 2.5.5 Oxygen (O2) ................................................................................... 62 2.5.6 Indoor Atmospheric Compounds .............................................. 632.6 Summary .....................................................................................................632.7 Effects on Metals Used for Outdoor Applications ...............................63 2.7.1 Carbon Steel................................................................................... 63 2.7.2 Weathering Steels.......................................................................... 64 2.7.3 Zinc..................................................................................................65 2.7.4 Aluminum......................................................................................65 2.7.5 Copper ............................................................................................ 65 2.7.6 Nickel 200....................................................................................... 66 2.7.7 Monel Alloy 400 ............................................................................66 2.7.8 Inconel Alloy 600 ..........................................................................66Reference .............................................................................................................. 66Chapter 3 Corrosion of Carbon and Low-Alloy Steels...........................673.1 Corrosion Data ........................................................................................... 673.2 Stress Corrosion Cracking ........................................................................ 783.3 Sulde Stress Cracking ............................................................................. 783.4 Pitting........................................................................................................... 793.5 Hydrogen Damage .................................................................................... 79 6. 3.5.1 Hydrogen Blistering .....................................................................80 3.5.2 Hydrogen Embrittlement ............................................................80 3.5.3 Decarburization............................................................................. 80 3.5.4 Hydrogen Attack .......................................................................... 803.6 Corrosion Fatigue ......................................................................................813.7 Microbiologically Inuenced Corrosion ................................................81Reference ..............................................................................................................82Chapter 4 Corrosion of Cast Iron and Cast Steel.....................................834.1 Cast Irons .................................................................................................... 86 4.1.1 Gray Iron ........................................................................................ 86 4.1.2 Compacted Graphite Iron............................................................87 4.1.3 Ductile (Nodular) Iron ................................................................. 87 4.1.4 White Iron ......................................................................................88 4.1.5 Malleable Iron................................................................................ 884.2 High Alloy Cast Irons ............................................................................... 88 4.2.1 Austenitic Gray Cast Irons ..........................................................88 4.2.2 Austenitic Ductile Cast Irons ...................................................... 89 4.2.3 High-Silicon Cast Irons ................................................................ 894.3 Carbon and Low-Alloy Carbon Steels ...................................................96References ............................................................................................................ 97Chapter 5 Introduction to Stainless Steel.................................................. 995.1 Stainless Steel Classication..................................................................... 995.1.1 Ferritic Family ............................................................................. 1005.1.2 Martensitic Family ...................................................................... 1025.1.3 Austenitic Family........................................................................ 1025.1.4 Precipitation-Hardenable Stainless Steels............................... 1035.1.5 Superferritic Stainless Steels ..................................................... 1045.1.6 Duplex Stainless Steels............................................................... 1045.1.7 Superaustenitic Stainless Steels ................................................ 1055.2 Passivation ................................................................................................ 1055.3 Sanitizing................................................................................................... 1065.4 Preparing for Service............................................................................... 1065.4.1 Iron Contamination .................................................................... 1065.4.2 Organic Contamination.............................................................. 1075.4.3 Welding Contamination............................................................. 107Chapter 6 Corrosion of Stainless Steels................................................... 1096.1 Pitting......................................................................................................... 1116.2 Crevice Corrosion .................................................................................... 1126.3 Stress Corrosion Cracking ...................................................................... 1126.4 Intergranular Corrosion .......................................................................... 1146.5 High-Temperature Corrosion.................................................................116 7. 6.6 Corrosion Fatigue .................................................................................... 1226.7 Uniform Corrosion .................................................................................. 122Chapter 7 Ferritic Stainless Steel Family................................................. 1237.1 Type 405 (S40500)..................................................................................... 1267.2 Type 409 (S40900)..................................................................................... 1277.3 Type 430 (S43000)..................................................................................... 1277.4 Type 439L (S43035) ..................................................................................1287.5 Type 444 (S44400)..................................................................................... 1287.6 Type 446 (S44600)..................................................................................... 132Reference ............................................................................................................ 132Chapter 8 Superferritic Stainless Steel Family ......................................1338.1 Type XM-27 (S44627) ...............................................................................1348.2 Alloy S44660 (Sea-Cure) .........................................................................1348.3 Alloy S44735 (29-4C) ............................................................................... 1368.4 Alloy S44800 (29-4-2)............................................................................... 1368.5 Alloy S44700 (29-4) .................................................................................. 137Reference ............................................................................................................ 137Chapter 9 Martensitic Stainless Steel Family.........................................1399.1 Type 410 (S41000)..................................................................................... 1399.2 Type 414 (S41400)..................................................................................... 1449.3 Type 416 (S41600)..................................................................................... 1449.4 Type 420 (S42000)..................................................................................... 1459.5 Type 422 (S42200)..................................................................................... 1469.6 Type 431 (S43100)..................................................................................... 1479.7 Type 440A (S44002).................................................................................. 1479.8 Type 440B (S44003) ..................................................................................1489.9 Type 440C (S44004).................................................................................. 1489.10 Alloy 440-XH ............................................................................................ 1499.11 13Cr-4N (F6NM) ...................................................................................... 149Reference ............................................................................................................ 149Chapter 10 Austenitic Stainless Steel Family.........................................15110.1 Type 201 (S20100)...................................................................................15510.2 Type 202 (S20200)...................................................................................15610.3 Type 22-13-5 (S20910) ............................................................................15610.4 Type 216L (S21603) ................................................................................ 15710.5 Type 301 (S30100)...................................................................................15810.6 Type 302 (S30200)...................................................................................15810.7 Type 303 (S30300)...................................................................................15810.8 Type 304 (S30400)...................................................................................15810.9 Type 305 (S30500)...................................................................................159 8. 10.10 Type 308 (S30800)................................................................................... 15910.11 Type 309 (S30900) ...................................................................................15910.12 Type 310 (S31000)................................................................................... 16410.13 Type 316 (S31600)................................................................................... 16410.14 Type 317 (S31700)................................................................................... 16910.15 Type 321 (S32100)................................................................................... 17210.16 Type 329 (S32900)................................................................................... 17410.17 Type 347 (S34700)................................................................................... 17510.18 Type 348 (S34800)................................................................................... 175Reference ............................................................................................................ 176Chapter 11 Superaustenitic Family of Stainless Steel ......................... 17711.1 Alloy 20Cb3 (N08020) ...........................................................................18011.2 Alloy 20Mo-4 (N08024) ......................................................................... 18511.3 Alloy 20Mo-6 (N08026) ......................................................................... 18511.4 Alloy 904L (N08904) ..............................................................................18611.5 Alloy 800 (N08800) ................................................................................ 18611.6 Alloy 825 (N08825) ................................................................................ 18711.7 Type 330 (N08330).................................................................................. 19011.8 Al-6XN (N08367) ....................................................................................19111.9 Alloy 254SMo (S31254).......................................................................... 19211.10 Alloy 25-6Mo (N08926)......................................................................... 19311.11 Alloy 31 (N08031) .................................................................................. 19411.12 Alloy 654SMo (S32654) .........................................................................19411.13 Inconel Alloy 686 (N06686).................................................................. 195Reference ............................................................................................................ 195Chapter 12 Duplex Stainless Steel Family ..............................................19712.1 Alloy 2205 (S31803)................................................................................ 20012.2 7-MoPlus (S32950).................................................................................. 20112.3 Zeron 100 (S32760)................................................................................. 20212.4 Ferralium 255 (S32550)..........................................................................203Chapter 13 Precipitation-Hardening Stainless Steel Family ...............20513.1 Alloy PH13-8Mo (S13800) ....................................................................20713.2 Alloy 15-5PH (S15500) ..........................................................................20713.3 Alloy 17-4PH (S17400) ..........................................................................20813.4 Alloy 17-7PH (S17700) ..........................................................................20913.5 Alloy 350 (S35000)..................................................................................21213.6 Alloy 355 (S35500)..................................................................................21213.7 Custom 450 (S45000) ............................................................................. 21313.8 Custom 455 (S45500) ............................................................................. 21413.9 Alloy 718 (N07718) ................................................................................ 21413.10 Alloy A286 (S66286) .............................................................................. 215 9. 13.11 Alloy X-750 (N07750) ............................................................................21513.12 Pyromet Alloy 31...................................................................................21613.13 Pyromet Alloy CTX-1............................................................................21713.14 Pyromet Alloy CTX-3............................................................................21813.15 Pyromet Alloy CTX-909........................................................................21813.16 Pyromet Alloy V-57...............................................................................21913.17 Thermospan Alloy.................................................................................220References ..........................................................................................................220Chapter 14 Cast Stainless Steel Alloys ....................................................22114.1 Martensitic Stainless Steels................................................................... 22414.2 Ferritic Stainless Steels ..........................................................................22514.3 Austenitic Stainless Steels ....................................................................22614.4 Superaustenitic Stainless Steels ...........................................................22914.5 Precipitation-Hardening Stainless Steels ...........................................23114.6 Duplex Stainless Steels.......................................................................... 231References ..........................................................................................................233Chapter 15 Nickel and High-Nickel Alloys ............................................23515.1 Nickel 200 and Nickel 201.................................................................... 23715.2 Monel Alloy 400 (N04400)....................................................................24315.3 Alloy B-2.................................................................................................. 24515.4 Alloy 625 (N06625) ................................................................................ 25215.5 Custom Age 625 Plus (N07716)........................................................... 25715.6 Alloy C-276 (N10276) ............................................................................ 26215.7 Alloy C-4 (N06455) ................................................................................ 26315.8 Alloy C-22 (N06022) ..............................................................................26415.9 Hastelloy Alloy C-2000 .........................................................................26515.10 Alloy X (N06002).................................................................................... 26715.11 Alloy 600 (N06600) ................................................................................26815.12 Alloy G (N06007) and Alloy G-3 (N06985) .......................................26915.13 Alloy G-30 (N06030) .............................................................................. 27015.14 Alloy H-9M ............................................................................................. 27215.15 Alloys for High-Temperature Corrosion............................................27215.15.1 Hastelloy Alloy S................................................................... 27315.15.2 Haynes Alloy 556 (R30556)..................................................27315.15.3 Alloy 214................................................................................. 27415.15.4 Alloy 230 (N06230)................................................................ 27515.15.5 Alloy RA333 (N06333) ..........................................................27615.15.6 Alloy 102 (N06102)................................................................ 277Reference ............................................................................................................ 277Chapter 16 Cast Nickel and Nickel-Based Alloys ................................. 27916.1 Commercially Pure Nickel ................................................................... 27916.2 NickelCopper........................................................................................ 279 10. 16.3 NickelChromium .................................................................................28116.4 NickelChromiumMolybdenum ....................................................... 28116.5 Other Nickel-Based Alloys................................................................... 282References .......................................................................................................... 282Chapter 17Comparative Corrosion Resistance of StainlessSteel and High-Nickel Alloys ............................................... 283Chapter 18 Copper and Copper Alloys ....................................................46918.1 Coppers ...................................................................................................47218.2 High-Copper Alloys ..............................................................................47518.3 CopperZinc Alloys (Brasses).............................................................. 47518.4 CopperTin Alloys................................................................................. 48318.5 CopperAluminum Alloys...................................................................48318.6 CopperNickel Alloys ........................................................................... 48518.7 CopperBeryllium Alloys..................................................................... 48818.8 Cast Copper Alloys ............................................................................... 48818.8.1 Corrosion Resistance...............................................................488References .......................................................................................................... 490Chapter 19 Aluminum and Aluminum Alloys.......................................49119.1 Classications and Designations......................................................... 49219.2 Temper Designations............................................................................. 49319.3 Strain-Hardened Subdivisions.............................................................49419.3.1 H1XStrain-Hardened Only ................................................49419.3.2 H2XStrain-Hardened and Partially Annealed................ 49419.3.3 H3XStrain-Hardened and Stabilized................................ 49419.4 Heat-Treated Subdivisions ...................................................................49419.5 Chemical Composition.......................................................................... 49519.6 General Corrosion Resistance ..............................................................49919.7 Pitting Corrosion....................................................................................50019.8 Intergranular Corrosion ........................................................................ 50619.8.1 Mechanism of Intergranular Corrosionin 2XXX Alloys ........................................................................ 50619.8.2 Mechanism of Intergranular Corrosionin 7XXX Alloys ........................................................................ 50819.9 Exfoliation Corrosion ............................................................................ 50919.10 Stress Corrosion Cracking ....................................................................50919.11 Filiform Corrosion..................................................................................51019.12 Crevice Corrosion ..................................................................................51019.13 Poultice Corrosion .................................................................................51119.14 Galvanic Relations .................................................................................51119.15 Reduction of Ions of Other Metals by Aluminum ........................... 51219.16 Weathering .............................................................................................. 514 11. 19.17 Waters (General)..................................................................................... 51419.18 Relative Resistance of Aluminum and Alloys .................................. 51419.19 Atmospheric Weathering......................................................................51519.19.1 Seacoast Atmosphere............................................................ 51519.19.2 Urban or Industrial Atmospheres ......................................51619.19.3 Rural Atmosphere .................................................................51719.19.4 Indoor Atmosphere...............................................................51719.20 Waters (Specic) .................................................................................... 51819.20.1 Freshwaters............................................................................ 51819.20.2 Seawater .................................................................................51919.20.3 Piping Applications.............................................................. 51919.21 Alclad Products ..................................................................................... 52019.22 Cast Aluminum ..................................................................................... 520References .......................................................................................................... 522Chapter 20 Titanium.....................................................................................52520.1 Alloys ....................................................................................................... 52620.2 Types of Corrosion.................................................................................52820.2.1 General Corrosion ...................................................................52920.2.2 Galvanic Corrosion ................................................................. 52920.2.3 Hydrogen Embrittlement.......................................................52920.2.4 Crevice Corrosion....................................................................53420.2.5 Stress Corrosion Cracking .....................................................53620.3 Corrosion Resistance ............................................................................. 536References .......................................................................................................... 538Chapter 21 Tantalum ....................................................................................53921.1 The Oxide FilmA Protective Barrier ...............................................54021.2 Effect of Specic Corrosive Agents.....................................................542 21.2.1 Water ......................................................................................... 542 21.2.2 Acids.......................................................................................... 542 21.2.2.1 Sulfuric Acid ............................................................545 21.2.2.2 Phosphoric Acid ......................................................545 21.2.2.3 Hydrochloric Acid ..................................................546 21.2.2.4 Nitric Acid................................................................ 547 21.2.2.5 Hydrouoric Acid ...................................................547 21.2.2.6 Acid Mixtures and Other Acids ........................... 547 21.2.3 Alkali Salts, Organics, and Other Media.............................548 21.2.4 Gases.......................................................................................... 549 21.2.4.1 Oxygen and Air .......................................................549 21.2.4.2 Nitrogen .................................................................... 550 21.2.4.3 Hydrogen..................................................................551 21.2.4.4 Halogens ...................................................................554 21.2.4.5 Carbon Monoxide and Carbon Dioxide .............. 554 21.2.4.6 Nitrogen Monoxide and Nitrous Oxide ..............554 21.2.4.7 Other Gases ..............................................................554 12. 21.2.5 Liquid Metals ...........................................................................55521.2.5.1 Aluminum ................................................................55621.2.5.2 Antimony.................................................................. 55621.2.5.3 Bismuth..................................................................... 55621.2.5.4 Calcium..................................................................... 55621.2.5.5 Cesium ......................................................................55621.2.5.6 Gallium .....................................................................55621.2.5.7 Lead ........................................................................... 55621.2.5.8 Lithium .....................................................................55621.2.5.9 Magnesium and Magnesium Alloys.................... 55721.2.5.10 Mercury ....................................................................55721.2.5.11 Potassium ................................................................. 55721.2.5.12 Silver .........................................................................55721.2.5.13 Sodium .....................................................................55721.2.5.14 Tellurium.................................................................. 55821.2.5.15 ThoriumMagnesium.............................................55821.2.5.16 Uranium and Plutonium Alloys .......................... 55821.2.5.17 Zinc ...........................................................................55821.2.6 General Corrosion Data.......................................................... 55821.3 Corrosion Resistance of Tantalum-Based Alloys..............................56121.3.1 TantalumTungsten Alloys ....................................................56321.3.2 TantalumMolybdenum Alloys ............................................56621.3.3 TantalumNiobium Alloys .................................................... 56621.3.4 TantalumTitanium Alloys ....................................................56721.3.5 Other Alloys ............................................................................. 568References .......................................................................................................... 568Chapter 22 Zirconium .................................................................................. 57122.1 Introduction ............................................................................................57122.2 General Characteristics .........................................................................573 22.2.1 Physical Properties.................................................................. 574 22.2.2 Mechanical Properties ............................................................574 22.2.3 Chemical and Corrosion Properties.....................................57722.2.3.1 Water and Steam..................................................... 58022.2.3.2 Salt Water ................................................................. 58122.2.3.3 Halogen Acids......................................................... 58222.2.3.4 Nitric Acid ...............................................................58622.2.3.5 Sulfuric Acid............................................................58822.2.3.6 Phosphoric Acid .....................................................59122.2.3.7 Other Acids..............................................................59422.2.3.8 Alkalies.....................................................................59422.2.3.9 Salt Solutions ........................................................... 59422.2.3.10 Organic Solutions ................................................... 59622.2.3.11 Gases.........................................................................59722.2.3.12 Molten Salts and Metals........................................ 598 13. 22.2.4Selected Corrosion Topics...................................................... 59822.2.4.1 Pitting .......................................................................59822.2.4.2 Stress Corrosion Cracking .....................................59922.2.4.3 Fretting Corrosion ..................................................60022.2.4.4 Galvanic Corrosion.................................................60022.2.4.5 Crevice Corrosion ...................................................60122.2.5 Corrosion Protection...............................................................60122.2.5.1 Oxide Film Formation............................................ 601 22.2.5.1.1 Anodizing............................................ 601 22.2.5.1.2 Autoclave Film Formation ...............602 22.2.5.1.3 Film Formation in Air or Oxygen ... 602 22.2.5.1.4 Film Formation in Molten Salts....... 60222.2.5.2 Electrochemical Protection ....................................60322.2.5.3 Others ....................................................................... 60422.3 Typical Applications.............................................................................. 60522.3.1 Nuclear Industry ..................................................................... 60522.3.2 Chemical Processing and Other Industries ........................60622.3.2.1 Urea...........................................................................60722.3.2.2 Acetic Acid .............................................................. 60822.3.2.3 Formic Acid .............................................................60822.3.2.4 Sulfuric Acid-Containing Processes ....................60922.3.2.5 Halide-Containing Processes ................................ 61222.3.2.6 Nitric Acid-Containing Processes ........................61322.3.2.7 Others ....................................................................... 61422.4 Zirconium Products...............................................................................61622.5 Health and Safety .................................................................................. 61622.6 Concluding Remarks.............................................................................617References .......................................................................................................... 617Chapter 23 Zinc and Zinc Alloys...............................................................62323.1 Corrosion of Zinc ...................................................................................623 23.1.1 White Rust (Wet-Storage Stain)............................................. 623 23.1.2 Bimetallic Corrosion ...............................................................624 23.1.3 Intergranular Corrosion ......................................................... 625 23.1.4 Corrosion Fatigue.................................................................... 625 23.1.5 Stress Corrosion....................................................................... 62523.2 Zinc Coatings..........................................................................................626 23.2.1 Principle of Protection ............................................................62623.3 Zinc Coatings..........................................................................................630 23.3.1 Hot Dipping ............................................................................. 630 23.3.2 Zinc Electroplating.................................................................. 631 23.3.3 Mechanical Coating ................................................................ 631 23.3.4 Sheradizing...............................................................................632 23.3.5 Thermally Sprayed Coatings................................................. 63223.4 Corrosion of Zinc Coatings ..................................................................63223.5 Zinc Alloys..............................................................................................637 14. 23.5.1 Zinc5% Aluminum Hot-Dip Coatings............................... 637 23.5.2 Zinc55% Aluminum Hot-Dip Coatings.............................639 23.5.3 Zinc15% Aluminum Thermal Spray .................................. 640 23.5.4 ZincIron Alloy Coating ........................................................ 64123.6 Cast Zinc.................................................................................................. 643Chapter 24 Niobium (Columbian) and Niobium Alloys .....................64524.1 Corrosion Resistance .............................................................................64624.2 NiobiumTitanium Alloys.................................................................... 64824.3 WC-103 Alloy .........................................................................................64924.4 WC-1Zr Alloy .........................................................................................64924.5 General Alloy Information ................................................................... 649Chapter 25 Lead and Lead Alloys ............................................................. 65125.1 Corrosion Resistance ............................................................................. 651Reference ............................................................................................................ 654Chapter 26 Magnesium Alloys................................................................... 65526.1 Corrosion Resistance ............................................................................. 655Chapter 27Comparative Corrosion Resistance of NonferrousMetals and Alloys.................................................................... 657Reference ............................................................................................................ 721Index ................................................................................................................... 723 15. 1Fundamentals of Metallic CorrosionThere are three primary reasons for concern about and the study ofcorrosionsafety, economics, and conservation. Premature failure of bridgesor structures due to corrosion can also result in human injury or even loss oflife. Failure of operating equipment can have the same disastrous results. Several years ago, the National Institute of Standards and Technology(formerly the National Bureau of Standards) estimated that the annual costof corrosion in the United States was in the range of $9 billion to $90 billion.These gures were conrmed by various technical organizations, includingthe National Association of Corrosion Engineers. Included in this estimate was corrosion attributed to chemical processes;corrosion of highways and bridges from deicing chemicals; atmosphericcorrosion of steel fences; atmospheric corrosion of various outdoorstructures such as buildings, bridges, towers, automobiles, and ships; andinnumerable other applications exposed to the atmospheric environment. Ithas been further estimated that the cost of protection against atmosphericcorrosion is approximately 50% of the total cost of all corrosion-protectionmethods. Corrosion is the degradation of a materials properties or mass over timedue to environmental effects. It is the natural tendency of a materialscompositional elements to return to their most thermodynamically stablestate. For most metallic materials, this means the formation of oxides orsuldes, or other basic metallic compounds generally considered to be ores.Fortunately, the rate at which most of these processes progress is slowenough to provide useful building materials. Only inert atmospheres andvacuums can be considered free of corrosion for most metallic materials. Under normal circumstances, iron and steel corrode in the presence ofboth oxygen and water. If either of these materials is absent, corrosionusually will not take place. Rapid corrosion may take place in water, inwhich the rate of corrosion is increased by the acidity or velocity of the water,by the motion of the metal, by an increase in the temperature or aeration, bythe presence of certain bacteria, or by other less prevalent factors.Conversely, corrosion is generally retarded by lms (or protective layers) 1 16. 2 Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metalsconsisting of corrosion products or adsorbed oxygen; high alkalinity of thewater also reduces the rate of corrosion on steel surfaces. The amount ofcorrosion is controlled by either water or oxygen, which are essential for theprocess to take place. For example, steel will not corrode in dry air andcorrosion is negligible when the relative humidity of the air is below 30% atnormal or reduced temperatures. Prevention of corrosion by dehumidica-tion is based on this. All structural metals corrode to some degree in natural environments.However, bronzes, brasses, zinc, stainless steels, and aluminum corrode soslowly under the condition in which they are placed that they are expected tosurvive for long periods of time without protection. These corrosion processes follow the basic laws of thermodynamics.Corrosion is an electrochemical process. Under controlled conditions it canbe measured, repeated, and predicted. Since it is governed by reactions on anatomic level, corrosion processes can act on isolated regions, uniform surfaceareas, or result in subsurface microscopic damage. Complicate these formsof corrosion with further subdivisions, add just basic environmentalvariables such as pH, temperature, and stress, and the predictability ofcorrosion begins to suffer rapidly.1.1 Forms of CorrosionThere are nine basic forms of corrosion that metallic materials may besubject to:1. Uniform corrosion2. Intergranular corrosion3. Galvanic corrosion4. Crevice corrosion5. Pitting6. Erosion corrosion7. Stress corrosion cracking8. Biological corrosion9. Selective leaching In addition, there are other forms of corrosion that specic metals oralloys are subject to. Prevention or control of corrosion can usually beachieved by use of a suitable material of construction, use of proper designand installation techniques, and by following in-plant procedures, or acombination of these. 17. Fundamentals of Metallic Corrosion31.1.1 Uniform CorrosionAlthough other forms of attack must be considered in special circumstances,uniform attack is one form most commonly confronting the user of metalsand alloys. Uniform or general corrosion, which is the simplest form ofcorrosion, is an even rate of metal loss over the exposed surface. It isgenerally thought of as metal loss due to chemical attack or dissolution of themetallic component into metallic ions. In high-temperature situations,uniform metal loss is usually preceded by its combination with anotherelement rather than its oxidation to a metallic ion. Combination with oxygento form metallic oxides, or scale, results in the loss of material in its usefulengineering form; scale ultimately akes off to return to nature. A metal resists corrosion by forming a passive lm on the surface. Thislm is naturally formed when the metal is exposed to the air for a period oftime. It can also be formed more quickly by chemical treatment. For example,nitric acid, if applied to austenitic stainless steel, will form this protectivelm. Such a lm is actually a form of corrosion, but once formed it preventsfurther degradation of the metal, provided that the lm remains intact. Itdoes not provide an overall resistance to corrosion because it may be subjectto chemical attack. The immunity of the lm to attack is a function of the lmcomposition, temperature, and the aggressiveness of the chemical. Examplesof such lms are the patina formed on copper, the rusting of iron, the tarni-shing of silver, the fogging of nickel, and the high-temperature oxidationof metals. There are two theories regarding the formation of these lms. The rsttheory states that the lm formed is a metal oxide or other reactioncompound. This is known as the oxide lm theory. The second theory statesthat oxygen is adsorbed on the surface, forming a chemisorbed lm.However, all chemisorbed lms react over a period of time with theunderlying metal to form metal oxides. Oxide lms are formed at roomtemperature. Metal oxides can be classied as network formers, intermedi-ates, or modiers. This division can be related to thin oxide lms on metals.The metals that fall into network-forming or intermediate classes tend togrow protective oxides that support anion or mixed anion/cation move-ment. The network formers are noncrystalline, whereas the intermediatestend to be microcystalline at low temperatures.1.1.1.1 Passive Film on IronIron in iron oxides can assume a valence of two or three. The former acts as amodier and the latter is a network former. The iron is protected from thecorrosion environment by a thin oxide lm l4 mm in thickness with a pcomposition of Fe2 O3 =Fe3 O4 . This is the same type of lm formed by the preaction of clean iron with oxygen or dry air. The Fe2 O3 layer is responsiblefor the passivity, while the Fe3O4 provides the basis for the formation of ahigher oxidizing state. Iron is more difcult to passivate than nickel, because 18. 4 Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metalspwith iron it is not possible to go directly to the passivation species Fe2 O3 .Instead, a lower oxidation state of Fe3O4 is required, and this lm is highly psusceptible to chemical dissolution. The Fe2 O3 layer will not form until theFe3O4 phase has existed on the surface for a reasonable period of time.During this time, Fe3O4 layer continues to form.1.1.1.2 Passive Film on NickelThe passive lm on nickel can be formed quite readily in contrast to theformation of the passive lm on iron. Differences in the nature of the oxidelm on iron and nickel are responsible for this phenomenom. The lmthickness on nickel is between 0.9 and 1.2 mm, whereas the iron oxide lm isbetween 1 and 4 mm. There are two theories as to what the passive lm onnickel is. It is entirely NiO with a small amount of nonstoichiometry, givingrise to Ni3C cation vacancies, or it consists of an inner layer of NiO and anouter layer of anhydrous Ni(OH)2. The passive oxide lm on nickel, onceformed, cannot be easily removed by either cathodic treatment or chemicaldissolution. The passive lm on nickel will not protect the nickel from corrosive attackin oxidizing atmospheres such as nitric acid. When alloyed with chromium,a much-improved stable lm results, producing a greater corrosionresistance to a variety of oxidizing media. However, these alloys are subjectto attack in environments containing chlorides or other halides, especially ifoxidizing agents are present. Corrosion will be in the form of pitting. Theaddition of molybdenum or tungsten will improve the corrosion resistance.1.1.1.3 Passive Film on Austenitic Stainless SteelThe passive lm formed on austenitic stainless steel is duplex in nature,consisting of an inner barrier oxide lm and an outer deposit of hydroxide orsalt lm. Passivation takes place by the rapid formation of surface-absorbedhydrated complexes of metals that are sufciently stable on the alloy surfacethat further reaction with water enables the formation of a hydroxide phasethat rapidly deprotonates to form an insoluble surface oxide lm. The threemost commonly used austenite stabilizersnickel, manganese, and nitro-genall contribute to the passivity. Chromium, a major alloying ingredient, isin itself very corrosion resistant and is found in greater abundance in thepassive lm than iron, which is the major element in the alloy.1.1.1.4 Passive Film on CopperWhen exposed to the atmosphere over long periods of time, copper will forma coloration on the surface known as patina; in reality, the coloration is acorrosion product that acts as a protective lm against further corrosion.When rst formed, the patina exhibits a dark color that gradually turns green.The length of time required to form the patina depends upon the atmosphere, 19. Fundamentals of Metallic Corrosion5because the coloration is given by copper hydroxide compounds. In a marineatmosphere, the compound is a mixture of copper/hydroxide/chloride; inindustrial atmospheres, it is copper/hydroxide/sulfate. These compoundswill form in approximately 7 years. When exposed in a clean rural atmo-sphere, tens or hundreds of years may be required to form the patina.1.1.1.5 Passive Film on AluminumAluminum forms a thin, compact, and adherent oxide lm on the surfacethat limits further corrosion. When formed in air at atmospherictemperatures it is approximately 5 mm thick. If formed at elevatedtemperatures or in the presence of water or water vapor, it will be thicker.This oxide lm is stable in the pH range of 49. With a few exceptions, thelm will dissolve at lower or higher pH ranges. Exceptions are concentratednitric acid (pH 1) and concentrated ammonium hydroxide (pH 13). In bothcases, oxide lm is stable. The oxide lm is not homogeneous and contains weak points. Breakdownof the lm at weak points leads to localized corrosion. With increasingalloy content and on heat-treatable alloys, the oxide lm becomes morenonhomogeneous.1.1.1.6 Passive Film on TitaniumTitanium forms a stable, protective, strongly adherent oxide lm. This lmforms instantly when a fresh surface is exposed to air or moisture. Additionof alloying elements to titanium affect the corrosion resistance because theseelements affect the composition of the oxide lm. The oxide lm of titanium is very thin and is attacked by only a fewsubstances, the most notable of which is hydrouoric acid. Because of itsstrong afnity for oxygen, titanium is capable of healing ruptures in this lmalmost instantly in any environment where a trace of moisture or oxygenis present.1.1.1.7 Passive Film on TantalumWhen exposed to oxidizing or slightly anodic conditions, tantalum formsa thin impervious layer of tantalum oxide. This passivating oxide has thebroadest range of stability with regard to chemical attack or thermalbreakdown compared to other metallic lms. Chemicals or conditions thatattack tantalum, such as hydrouoric acid, are those which penetrate ordissolve the lm.1.1.1.8 Uniform Corrosion RatesWhen exposed to a corrosion medium, metals tend to enter into a chemicalunion with the elements of the corrosion medium, forming stablecompounds similar to those found in nature. When metal loss occurs inthis manner, the compound formed is referred to as the corrosion product 20. 6Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metalsand the surface is referred to as having been corroded. An example of suchan attack is that of halogens, particularly chlorides. They will react with andpenetrate the lm on stainless steel, resulting in general corrosion. Corrosiontables are developed to indicate the interaction between a chemical and ametal. This type of attack is termed uniform corrosion. It is one of the mosteasily measured and predictable forms of corrosion. Many references existthat report average or typical rates of corrosion for various metals incommon media. One such is Reference [1]. Because corrosion is so uniform, corrosion rates for materials are oftenexpressed in terms of metal thickness loss per unit time. The rate of uniformattack is reported in various units. One common expression is mils per year(mpy); sometimes millimeters per year is used. In the United States, it isgenerally reported in inches penetration per year (ipy) and milligrams persquare decimeter per day (mdd). To convert from ipy to mpy, multiply theipy value by 1000 (i.e., 0.1 in.!1000Z100 mpy). Conversion of ipy to mdd orvice versa requires knowledge of the metal density. Conversion factors aregiven in Table 1.1. Because of its predictability, low rates of corrosion are often tolerated andcatastrophic failures are rare if planned inspection and monitoring isimplemented. For most chemical process equipment and structures, generalcorrosion rates of less than 3 mpy are considered acceptable. Rates between3 and 20 mpy are routinely considered useful engineering materials for theTABLE 1.1Conversion Factors from ipy to mdd0.00144Metal Density (g/cc) Density!10L3696!DensityAluminum 2.72 0.529 1890Brass (red)8.75 0.164 6100Brass (yellow) 8.47 0.170 5880Cadmium8.65 0.167 6020Columbium8.40.171 5850Copper 8.92 0.161 6210Coppernickel (7030)8.95 0.161 6210Iron 7.87 0.183 5480Duriron7.00.205 4870Lead (chemical) 11.35 0.127 7900Magnesium1.74 0.826 1210Nickel 8.89 0.162 6180Monel8.84 0.163 6140Silver10.50 0.137 7300Tantalum16.60.0868 11,550Tin7.29 0.198 5070Titanium 4.54 0.317 3160Zinc 7.14 0.202 4970Zirconium6.45 0.223 4490Multiply ipy by (696!density) to obtain mdd. Multiply mdd by (0.00144/density) to obtain ipy. 21. Fundamentals of Metallic Corrosion 7given environment. In severe environments, materials exhibiting highgeneral corrosion rates between 20 and 50 mpy might be consideredeconomically justiable. Materials that exhibit rates of general corrosionbeyond this are usually unacceptable. It should be remembered that not onlydoes the metal loss need to be considered, but where the metal is going mustalso be considered. Contamination of product, even at low concentrations,can be more costly than replacement of the corroded component. Uniform corrosion is generally thought of in terms of metal loss due tochemical attack or dissolution of the metallic component into metallic ions.In high-temperature situations, uniform loss is more commonly preceded byits combination with another element rather than its oxidation to a metallicion. Combination with oxygen to form metallic oxide or scale results in theloss of the material in its useful engineering form as it ultimately akes off toreturn to nature. To determine the corrosion rate, a prepared specimen is exposed to the testenvironment for a period of time and then removed to determine how muchmetal has been lost. The exposure time, weight loss, surface area exposed,and density of the metal are used to calculate the corrosion rate of the metalusing the formula: 22:273WL mpy Z; DATwhere WL, weight loss, gD, density, g/cm3A, area, in.2T, time, days.The corrosion rates calculated from the formula or taken from the tables willassist in determining how much corrosion allowance should be included inthe design based on the expected lifetime of the equipment.1.1.2 Intergranular CorrosionIntergranular corrosion is a localized form of corrosion. It is a preferentialattack on the grain boundary phases or the zones immediately adjacent tothem. Little or no attack is observed on the main body of the grain. Thisresults in the loss of strength and ductility. The attack is often rapid,penetrating deeply into the metal and causing failure. The factors that contribute to the increased reactivity of the grainboundary area include:1. Segregation of specic elements or compounds at the grain boundary, as in aluminum alloys or nickelchromium alloys 22. 8 Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals2. Enrichment of one of the alloying elements at the grain boundary, as in brass3. Depletion of the corrosion-resisting constituent at the grain boundary, as in stainless steel In the case of austenitic stainless steels, the attack is the result of carbideprecipitation during welding operations. Carbide precipitation can beprevented by using alloys containing less than 0.03% carbon, by usingalloys that have been stabilized with columbium (niobium) or titanium, orby specifying solution heat treatment followed by a rapid quench that willkeep carbides in solution. The most practical approach is to use either a lowcarbon content or stabilized austenitic stainless steel. Nickel-based alloys can also be subjected to carbide precipitation andprecipitation of intermetallic phases when exposed to temperatures lowerthan their annealing temperatures. As with austenitic stainless steels, low-carbon-content alloys are recommended to delay precipitation of carbides.In some alloys, such as alloy 625, niobium, tantalum, or titanium is addedto stabilize the alloy against precipitation of chromium or molybdenumcarbides. Those elements combine with carbon instead of the chromiumor molybdenum. All of these factors contributing to intergranular corrosion originate in thethermal processing of materials, such as welding, stress relief, and otherheat treatments.1.1.3 Galvanic CorrosionThis form of corrosion is sometimes referred to as dissimilar metal corrosion,and is found in unusual places, often causing professionals the mostheadaches. Galvanic corrosion is often experienced in older homes wheremodern copper piping is connected to the older existing carbon steel lines.The coupling of the carbon steel to the copper causes the carbon steel tocorrode. The galvanic series of metals provides details of how galvaniccurrent will ow between two metals and which metal will corrode whenthey are in contact or near each other and an electrolyte is present (e.g.,water). Table 1.2 lists the galvanic series. When two different metallic materials are electrically connected andplaced in a conductive solution (electrolyte), an electric potential exists. Thispotential difference will provide a stronger driving force for the dissolutionof the less noble (more electrically negative) material. It will also reduce thetendency for the more noble metal to dissolve. Notice in Table 1.2 that theprecious metals gold and platinum are at the higher potential (more noble orcathodic) end of the series (protected end), while zinc and magnesium are atthe lower potential (less noble or anodic) end. It is this principle that formsthe scientic basis for using such materials as zinc to sacricially protect thestainless steel drive shaft on a pleasure boat. 23. Fundamentals of Metallic Corrosion 9TABLE 1.2Galvanic Series of Metals and AlloysCorroded end (anodic) MagnesiumMuntz metal Magnesium alloys Naval bronze Zinc Nickel (active) Galvanized steel Inconel (active) Aluminum 6053Hastelloy C (active) Aluminum 3003Yellow brass Aluminum 2024Admiralty brass Aluminum Aluminum bronze Alclad Red brass CadmiumCopper Mild steel Silicon bronze Wrought iron 70/30 Cupro-nickel Cast ironNickel (passive) Ni-resistIconel (passive) 13% Chromium stainless steel Monel(active) 50/50 Lead tin solder18-8 Stainless steel type 304 (passive) Ferretic stainless steel 400 series 18-8-3 Stainless steel type 316 (passive) 18-8 Stainless steel type 304 (active) Silver 18-8-3 Stainless steel type 316Graphite(active) Lead Gold TinPlatinumProtected end (cathodic) You will note that several materials are shown in two places in thegalvanic series, being indicated as either active or passive. This is the resultof the tendency of some metals and alloys to form surface lms, especially inoxidizing environments. This lm shifts the measured potential in the nobledirection. In this state, the material is said to be passive. The particular way in which a metal will react can be predicted from therelative positions of the materials in the galvanic series. When it is necessaryto use dissimilar metals, two materials should be selected that are relativelyclose in the galvanic series. The further apart the metals are in the galvanicseries, the greater the rate of corrosion. The rate of corrosion is also affected by the relative areas between theanode and cathode. Because the ow of current is from the anode to thecathode, the combination of a large cathodic area and a small anodic area isundesirable. Corrosion of the anode can be 1001000 times greater than ifthe two areas were equal. Ideally, the anode area should be larger than thecathode area. The passivity of stainless steel is the result of the presence of a corrosion-resistant oxide lm on the surface. In most material environments, it willremain in the passive state and tend to be cathodic to ordinary iron or steel.When chloride concentrations are high, such as in seawater or in reducing 24. 10Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metalssolutions, a change to the active state will usually take place. Oxygenstarvation also causes a change to the active state. This occurs when there isno free access to oxygen, such as in crevices and beneath contamination ofpartially fouled surfaces. Differences in soil concentrations, such as moisture content and resistivity,can be responsible for creating anodic and cathodic areas. Where there is adifference in concentrations of oxygen in the water or in moist soils in contactwith metal at different areas, cathodes will develop at relatively high oxygenconcentrations, and anodes will develop at points of low concentrations.Strained portions of metals tend to be anodic and unstrained portions tendto be cathodic. Sometimes nonmetallic conductors may act as cathodes in galvaniccouples. Both carbon brick in vessels made of common structural metalsand impervious graphite in heat-exchanger applications are examples.Conductive lms, such as mill scale (Fe2O3) or iron sulde on steel, or leadsulfate on lead, are cathodic to the base metal or to some metalliccomponents in their contact. When joining two dissimilar metals together, galvanic corrosion can beprevented by insulating the two materials from each other. For example,when bolting anges of dissimilar metals together, plastic washers can beused to separate the two metals.1.1.4 Crevice CorrosionCrevice corrosion is a localized type of corrosion occurring within oradjacent to narrow gaps or openings formed by metal-to-metal-to-nonmetalcontact. It results from local differences in oxygen concentrations, associateddeposits on the metal surface, gaskets, lap joints, or crevices under a boltor around rivet heads where small amounts of liquid can collect andbecome stagnant. Crevice corrosion may take place on any metal and in any corrosiveenvironment. However, metals like aluminum and stainless steels thatdepend on their surface oxide lm for corrosion resistance are particularlyprone to crevice corrosion, especially in environments such as seawater thatcontain chloride ions. The gap dening a crevice is usually large enough for the entrapment ofa liquid but too small to permit ow of the liquid. The width is on the orderof a few thousandths of an inch, but not exceeding 3.18 mm. The material responsible for forming the crevice need not be metallic.Wood, plastic, rubber, glass, concrete, asbestos, wax, and living organismshave been reported to cause crevice corrosion. After the attack begins withinthe crevice, its progress is very rapid. It is frequently more intense in chlorideenvironments. Prevention can be accomplished by proper design and operating pro-cedures. Nonabsorbant gasketting material should be used at anged joints,while fully penetrated butt-welded joints are preferred to threaded joints. 25. Fundamentals of Metallic Corrosion11 TABLE 1.3 Critical Crevice Corrosion Temperatures in 10% Ferric Chloride Solution AlloyTemperature (8F/8C) Type 316 27/K3 Alloy 82527/K3 Type 317 36/2 Alloy 904L 59/15 Alloy 220S 68/20 E-Brite70/21 Alloy G86/30 Alloy 625 100/38 AL-6XN100/38 Alloy 276 130/55If lap joints are used, the laps should be lled with llet welding or a suitablecaulking compound designed to prevent crevice corrosion. The critical crevice corrosion temperature of an alloy is the temperature atwhich crevice corrosion is rst observed when immersed in a ferric chloridesolution. Table 1.3 lists the critical crevice corrosion temperature of severalalloys in 10% ferric chloride solution. In a corrosive environment, the areas inside the crevice and outside thecrevice undergo corrosion in the same manner. In a neutral chloride solution,the anodic dissolution is supported by the cathodic reduction of oxygen: anodicM/ MnC C neK cathodicO2 C 2H2 O C 4eK/ 4OHKAs the reactions proceed, the dissolved oxygen in the small volume ofstagnated solution inside the crevice is consumed. However, this does notprevent the dissolution reaction inside the crevice because the electronsreach outside the crevice through the metal, where plenty of oxygen isavailable for reduction. A concentration cell (differential aeration) is set upbetween the crevice area and the area outside the crevice. When chloride ions are present, the situation is further aggravated. Theaccumulated cations inside the crevice attract the negatively chargedchloride anions from the bulk solution. Hydroxide anions also migrate,but they are less mobile than chloride ions. The metal chloride formedhydrolyzes to produce metal hydroxide and hydrochloric acid: MCl C H2 O/ MOH C HClThe nascent hydrochloric acid destroys the passive lm and accelerates therate of dissolution of the metal inside the crevice. The cathodic reductionremains restricted to the areas outside the crevice that remain cathodicallyprotected. 26. 12Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals Several steps can be taken to prevent and/or control crevice corrosion:1. Proper design, avoiding crevices, will control crevice corrosion. If lap joints are used, the crevices caused by such joints should be closed by either welding or caulking. Welded butt joints are preferable to bolted or riveted joints.2. Porous gaskets should be avoided. Use an impervious gasket material. During long shutdown periods, wet packing materials should be removed.3. The use of alloys resistant to crevice corrosion should be considered. The resistance of stainless steels to crevice corrosion can be improved by increasing the chromium, nickel, molyb- denum, and nitrogen content. For example, type 316 stainless steel containing 23% molybdenum is fairly resistant, whereas nickel alloys are more resistant than stainless steels.4. Reduction of crevice corrosion can be accomplished, when possible, by reducing the temperature, decreasing the chloride content, or decreasing the acidity.5. The gaps along the periphery of tanks mounted on a masonry platform should be closed with tar or bitumen to avoid seepage of rainwater. Vessels and tanks should be designed to provide complete drainage, thereby preventing the buildup of solid deposits on the bottom.6. Regular inspections and removal of deposits should be scheduled.1.1.5 Pitting CorrosionPitting corrosion is in itself a corrosion mechanism, but it is also a form ofcorrosion often associated with other types of corrosion mechanisms. It ischaracterized by a highly localized loss of metal. In the extreme case, it appearsas a deep, tiny hole in an otherwise unaffected surface. The initiation of a pitis associated with the breakdown of the protective lm on the metal surface. The depth of the pit eventually leads to a thorough perforation or amassive undercut in the thickness of the metal part. The width of the pit mayincrease with time, but not to the extent to which the depth increases. Mostoften, the pit opening remains covered with the corrosion product, making itdifcult to detect during inspection. This, along with a negligible loss inweight or absence of apparent reduction in the overall wall thickness, giveslittle evidence as to the extent of the damage. Pitting may result in theperforation of a water pipe, making it unusable even though a relativelysmall percentage of the total metal has been lost due to rusting. Pitting can also cause structural failure from localized weakening effectseven though there is considerable sound material remaining. Pits may also 27. Fundamentals of Metallic Corrosion 13assist in brittle failure, fatigue failure, environment-assisted cracking likestress corrosion cracking (SCC), and corrosion fatigue, by providing sitesof stress concentration. The main factor that causes and accelerates pitting is electrical contactbetween dissimilar metals, or between what are termed concentration cells(areas of the same metal where oxygen or conductive salt concentrations inwater differ). These couples cause a difference of potential that results in anelectric current owing through the water or across moist steel, from themetallic anode to a nearby cathode. The cathode may be brass or copper, millscale, or any other portion of the metal surface that is cathodic to the moreactive metal areas. However, when the anodic area is relatively largecompared with the cathodic area, the damage is spread out and is usuallynegligible. When the anode area is relatively small, the metal loss isconcentrated and may be serious. For example, it can be expected whenlarge areas of the surface are generally covered by mill scale, applied coatings,or deposits of various kinds, but breaks exist in the continuity of the protectivematerial. Pitting may also develop on bare clean metal surfaces because ofirregularities in the physical or chemical structure of the metal. Localizeddissimilar soil conditions at the surface of steel can also create conditions thatpromote pitting. Figure 1.1 shows how a pit forms when a break in millscale occurs. If an appreciable attack is conned to a small area of metal acting as ananode, the developed pits are described as deep. If the area of attack is rela-tively large, the pits are called shallow. The ratio of deepest metal penetrationto average metal penetration, as determined by weight loss of the specimen,is known as the pitting factor. A pitting factor of 1 represents uniformcorrosion. Pitting corrosion is characterized by the following features:1. The attack is spread over small discrete areas. Pits are sometimes isolated and sometimes close together, giving the area of attack a rough appearance. Electrolyte (water) Fe2+ (rust)Current flowCathode (broken mill scale) Anode steelFIGURE 1.1Formation of pit from break in mill scale. 28. 14 Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals 2. Pits usually initiate on the upper surface of the horizontally placedparts and grow in the direction of gravity. 3. Pitting usually requires an extended initiation period before visiblepits appear. 4. Conditions prevailing inside the pit make it self-propagatingwithout any external stimulus. Once initiated, the pit grows at anever-increasing rate. 5. Stagnant solution conditions lead to pitting. 6. Stainless steels and aluminum and its alloys are particularlysusceptable to pitting. Carbon steels are more resistant to pittingthan stainless steels. Most failure of stainless steels occurs in neutral-to-acid chloride solutions. Aluminum and carbon steels pit inalkaline chloride solutions. 7. Most pitting is associated with halide ions (chlorides, bromides),and hypochlorites are particularly aggressive. Cupric, ferric, andmercuric halides are extremely aggressive because their cations arecathodically reduced and sustain the attack. Performance in the area of pitting and crevice corrosion is often measuredusing critical pitting temperature (CPT), critical crevice temperature (CCT),and pitting resistance equivalent number (PREN). As a general rule, thehigher the PREN, the better the resistance. The PREN is determined bythe chromium, molybdenum, and nitrogen contents: PRENZ%CrC3.3(%Mo)C30(%N). Table 1.4 lists the PRENs for various austeniticstainless steels. The CPT of an alloy is the temperature of a solution at which pitting is rstobserved. These temperatures are usually determined in ferric chloride(10% FeCl3$6H2O) and an acidic mixture of chlorides and sulfates. TABLE 1.4 Pitting Resistance Equivalent Numbers Alloy PREN Alloy PREN 65463.09316LN 31.08 31 54.45316 27.90 25-6Mo 47.4520Cb3 27.26 Al-6XN 46.96348 25.60 20Mo-6 42.81347 19.0 317LN39.60331 19.0 904L 36.51304N18.3 20Mo-4 36.20304 18.0 31733.2 29. Fundamentals of Metallic Corrosion 151.1.6 Erosion CorrosionThe term erosion applies to deterioration due to mechanical force. Whenthe factors contributing to erosion accelerate the rate of corrosion of a metal,the attack is called erosion corrosion. Erosion corrosion is usually causedby an aqueous or gaseous corrodent owing over the metal surface orimpinging on it. The mechanical deterioration may be aggravated by thepresence of a corrodent, as in the case of fretting or corrosive wear.The attack takes the form of grooves, i.e., scooped-out rounded areas,horseshoe-shaped depressions, gullies, or waves, all of which often showdirectionality. At times, attack may be an assembly of pits. Ultimateperforation due to thinning or progression of pits, and rupture due to failureof the thinned wall to resist the internal uid pressure are common. Allequipment exposed to owing uid is subject to erosion corrosion, butpiping systems and heat exchangers are the most commonly affected.Erosion corrosion is affected by velocity, turbulence, impingement,presence of suspended solids, temperature, and prevailing cavitationconditions. The acceleration of attack is due to the distribution or removalof the protective surface lm by mechanical forces exposing fresh metalsurfaces that are anodic to the uneroded neighboring lm. A hard, denseadherent and continuous lm, such as on stainless steel, is more resistantthan a soft brittle lm, as that on lead. The nature of the protective lmdepends largely on the corrosive itself.In most metals and alloys, corrosion rates increase with increased velocity,but a marked increase is experienced only when a critical velocity is reached.Turbulence is caused when the liquid ows from a larger area to a small-diameter pipe, as in the inlet ends of tubing in heat exchangers. Internaldeposits in the pipes, or any obstruction to the ow inside a pipe by a foreignbody, such as a carried-in pebble, can also cause turbulence.Impingement, direct contact of the corrodent on the metal surface, occursat bends, elbows and tees in a piping system and causes intense attack.Impingement is also encountered on the surface of impellers and turbines inareas in front of inlet pipes in tanks and in many other situations. The attackappears as horseshoe-shaped pits with a deep undercut and the endpointing in the direction of ow.An additional subset of erosion corrosion is the case of cavitation that isprevalent in pump impellers. This form of attack is caused by the formationand collapse of tiny vapor bubbles near a metallic surface in the presence of acorrodent. The protective lm is again damaged, in this case by the highpressures caused by the collapse of the bubbles.When two metal surfaces are in contact and experience a very slightrelative motion that results in damage to one or both surfaces, frettingcorrosion, a special form of erosion corrosion, takes place. The movementcauses mechanical damage to the protective lm; this can lead to erosioncorrosion when a corrodent is present. This corrosion usually takes the formof a pitting attack. 30. 16 Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of Metals Attack is further aggravated at higher temperatures and with solutionscontaining solids in suspension. Steam carrying water condensate dropletsprovides an aggressive medium for corrosion of steel and cast iron piping.The impingement of water droplets at the return bends destroys theprotective oxide lm and accelerates the attack on the substrate. Soft and low-strength metals such as copper, aluminum, and lead areespecially susceptible to erosion corrosion, as are the metals and alloys thatare inherently less corrosion resistant, such as carbon steels. Stainless steels of all grades, in general, are resistant to erosion corrosion.The addition of nickel, chromium, and molybdenum further improves theirperformance. Stainless steels and chromium steels are resistant as a result oftheir tenacious surface lms. As a rule, solid solution alloys provide better resistance than alloyshardened by heat treatment because the latter are heterogeneous in nature. Cast irons usually perform better than steel. Alloy cast irons containingnickel and chromium exhibit better performance. Duriron, containing 14.5%silicon, gives excellent performance under severe corrosion conditions. Prevention and/or reduction of erosion corrosion can be accomplished byone or more means: 1. Reduce velocity 2. Reduce or eliminate turbulence 3. Select a harder material 4. Properly design the piping system or the condensers1.1.7 Stress Corrosion Cracking (SCC)SCC is dened as the delayed failure of alloys by cracking when exposed tocertain environments in the presence of static tensile stress. The importanceof a conjoint action of corrosion and stress is reected in the denition; analternate application of stress and corrosive environment will not produceSCC. The stress level at which the failure occurs is well below the stressrequired for a mechanical failure in the absence of corrosion. The minimumstress below which SCC will occur is called the threshold stress, but this maybe as low as 10% of the yield stress in some systems. Corrosion alone in theabsence of stress does not cause SCC. SCC occurs at points of stress. Usually the metal or alloy is virtually free ofcorrosion over most of its surface, yet ne cracks penetrate through thesurface at the points of stress. Depending on the alloy system and corrodentcombination, the cracking can be intergranular or transgranular. The rate ofpropagation can vary greatly and is affected by stress levels, temperature,and concentration of the corrodent. This type of attack takes place in certainmedia. All metals are potentially subject to SCC. The conditions necessaryfor stress corrosion are: 31. Fundamentals of Metallic Corrosion 171. Suitable environment2. Tensile stress3. Sensitive metal4. Appropriate temperature and pH values An ammonia environment can induce SCC in copper-containing alloys,whereas with low-alloy austenitic stainless steels a chloride-containingenvironment is necessary. It is not necessary to have a high concentration ofcorrodent to cause SCC. A solution containing only a few parts per million ofthe critical ion is all that is necessary. Temperature and pH are also factors.There is usually a threshold temperature below which SCC will not takeplace and a maximum or minimum pH value before cracking will start. Normally, SCC will not occur if the part is in compression. Fatigue istriggered by a tensile stress that must approach the yield stress of the metal.The stresses may be induced by faulty installation or they may be residualstress from welding, straightening, bending, or accidental denting of thecomponent. Pits, which act as stress concentration sites, will ofteninitiate SCC. The alloy content of stainless steel, particularly nickel, determines thesensitivity of the metal to SCC. Ferritic stainless steels, which are nickel-free,and the high-nickel alloys are not subject to SCC. An alloy with a nickelcontent greater than 30% is immune to SCC. The most common grades ofstainless steel (304, 304L, 316, 316L, 321, 347, 303, and 301) have nickelcontents in the range of 710% and are the most susceptible to SCC. Examples of SCC include the cracking of austenitic stainless steels in thepresence of chlorides; caustic embrittlement cracking of steel in causticsolutions; cracking of cold-formed brass in ammonia environments, andcracking on monel in hydrouorosilicic acid. Table 1.5 provides partiallisting of alloy systems and the environments that will cause SCC. In severe combinations, such as type 304 stainless steel in a boilingmagnesium chloride solution, extensive cracking can be generated in amatter of hours. Fortunately, in most industrial applications the progress of SCC is muchslower. However, because of the nature of the cracking, it is difcult to detectuntil extensive corrosion has developed, which can lead to unexpected failure. Tensile stresses can lead to other corrosion processes, such as the simplemechanical fatigue process. Corrosion fatigue is difcult to differentiatefrom simple mechanical fatigue, but it is recognized as a factor when theenvironment is believed to have accelerated the normal fatigue process. Suchsystems can also have the effect of lowering the endurance limit such thatfatigue will occur at a stress level below which it would normallybe expected. It is important that any stresses that may have been induced during thefabrication be removed by an appropriate stress-relief operation. Care shouldbe taken so as to not induce a stress as the result of installation. The design 32. 18Fundamentals of Metallic Corrosion: Atmospheric and Media Corrosion of MetalsTABLE 1.5AlloyEnvironment Combinations Causing Stress Corrosion Cracking AlloyEnvironmentAluminum alloys Air with water; potable waters; seawater NaClsolutions; NaClH2O2 solutionsCarbon steels Caustic NaOH solutions; seawater; calcium,ammonium, and sodium nitrate solutions; HCNsolutions; acidied H2S solutions; anhydrousliquid ammonia; carbonate/