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Eugen Volosciuc, APB, Christof Group Giel Notten, NTT Consultancy

Applications of steel 1.4361 (Böhler A610) in highly concentrated nitric acid (CNA)

CNA Storage Tank 500m3, BASF Korea

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• Introduction; survey of applied materials of construction inhighly concentrated nitric acid (CNA)

• Mechanism of corrosion in nitric acid

• Parameters influencing corrosion in CNA

• Metallurgy of Si-bearing stainless steels

• Comparison 1.4361 – A610

• Quality assurance; Huey-test

• Manufacturing of equipment out of A610

• Conclusions

Scope of presentation

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• CNA is very aggressive and hazardous; leakages in piping and equipment will result in serious safety, health and environment consequences.

• The behavior of CNA has forced manufacturers to put a lot of effort in development of suitable materials with excellent corrosion resistance.

• Materials showing corrosion resistance in CNA are:Aluminium, tantalum, borosilicate glass, and glass lined steel, high-silica cast iron (14 – 16 % Si), high-silica stainless steels (3.7 – 4.5 % Si),fully-fluorinated plastics (PTFE and PFA), and elastomers Viton and Kalrez.

• A silicon bearing stainless steel like A610 (1.4361) with strict demands on chemical composition and microstructure fulfils the high requirements.

• It has been a challenge to develop welding procedures resulting in a weld deposit material and heat affected zone with equal corrosion resistance compared to the base material.

Introduction

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Anodic reaction: Me ⇔ Men+ + neCathodic reactions: In e.g. sulphuric acid: 2H+ + 2e- ⇔ H2 (E0 = 0 mV)

In nitric acid: NO3- + 4H+ + 3e- ⇔ NO + 2H2O (E0 = + 940 mV)

In concerning nitric acid circumstances dichromic acid will be formed; the nitrate reduction reaction will be overruled by:

Cr2O72- + 14H+ + 6e ⇔ 2Cr3+ + 7H2O (E0 = + 1330 mV)

The high equilibrium potential of the Redox reaction will shift the corrosion potential further in the transpassivearea of the polarization curve.

Corrosion mechanism of stainless steels in nitric acid

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Parameters influencing corrosion in CNA

• From process side:• Temperature• Presence of wet-dry zones; accumulation of bichromate in crevices• Presence of other metal ions in highest oxidation stage: V5+, Fe3+

• Contaminants like chlorides and fluorides are very detrimental• Liquide velocity in piping

• From material side:• Chemical composition• Microstructure: precipitations, grain size

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Parameters of stainless steel influencing corrosion in CNA

Microstructure:A small grain size highly to be preferred against large grain size. No. according to SEP 1510-61 or ASTM E112: 4 - 6

Benifical alloying elements:• Cr: for formation of passive Cr2O3 layer • Si: 4% beneficial, due to highly oxidixing properties of CNA

oxidation of Si takes place to form ultrathin silicon dioxide layer with a low leakage current density.

• Ni: necessary to form a fully austenitic microstructure

Unwanted elements:• Mo: supports formation of intermetallic phases and shifting

transpassive potential to lower values• Ti and Nb: support dissolution of carbides• C, N: supports formation of chromium carbides• P and S: lead to formation of slag inclusions/precipitations

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CNA Reactor (NO2 oxidation) with corrosion failuremodes at internal 1.4361 vessel

The material 1.4361 puts high demands on the lowest possible heat input and cooling during the welding process (water-cooling) due to the particularly high tendency to form intermetallic precipitates during welding.Severe damage in 7 years on stream

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Condenser for NO2 gasestubeside and shell side 1.4361

Leakages at the tube to tubesheet welds in the upper tube sheet (hot side). The white powder is leaked cooling brine;Detail of a leaked corrosion spot in the weld area. More than 50 such failures were found on this tubesheet after 5 years on streem. The reason was bad tube-tube sheet-welding.

Repair of such failures is extremely difficult and needs experienced, excellent welders.

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Comparison between standard 1.4361; UNS S30600and BOEHLER A610mod. (modified for APB)

Elements

Weight %

Material acc. to Standards Special modifications for APB(values taken from used MTCs)

1.4361(EN10088-1)

UNS S30600(ASTM)

A610mod.

Plate & Pipe GTAW SMAW

C ≤ 0,015 ≤ 0,018 0,005 0,01 0,01Cr 16,5 to 18,5 17,0 to 18,5 17,42 19,5 19,2

Ni 14,0 to 16,0 14,0 to 15,5 14,89 13,2 15,5Si 3,7 to 4,3 3,7 to 4,3 4,19 4,6 4,3Mn ≤ 2,0 ≤ 2,0 0,54 0,8 0,9Mo ≤ 0,20 ≤ 0,2 0 < 0,1 < 0,1P ≤ 0,025 ≤ 0,020 0,013 0,010 0,009S ≤ 0,010 ≤ 0,020 0,0005 0,001 0,003N ≤ 0,11 - 0,018 0,123 -

Cu - ≤ 0,5 - < 0,1 -Others - - - - -

Fe balance balance balance balance balance

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Quality assurance in different steps

• Test Certificates for semi-finished products, welding rods and sticks• Welding Qualification including Huey-Test (15 periods)• Cutting with low heat input• No tack welding • Gas backing during welding of the roots• Welding with low heat input, optimized heat dissipation

and control of the inter pass temperature• Filler- and final passes with water cooling technology• Grinding out of welded roots where possible• Welding of cap passes with water cooling technology• NDE and final treatment of welds and HAZ

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Heat treatment

• For special applications: Solution annealing between 1100°C to 1150°C, followed by rapid cooling in water

• Advantages of post welding heat treatment:• Homogenisation and reduction of tensions• Reduction of the chromium carbide gradient• Reduction of ∆ferrite at the welds and HAZ’s• Dissolving of intermetallic precipitates at grain boundaries

• Disadvantages:• Additional costs• Deformation • Applicable for small parts

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Commercial gains and advantages of BOEHLER A610

• Corrosion rate below 0.1 mm/yr (up to 80°C over the whole range of concentration)

• Better corrosion rates at higher temperatures than aluminium.

• Higher strength values than aluminium, wall thicknesses ratio about 1:8

• No limit in size and no sensitivity against impacts and thermal shocks.

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Further advantages of BOEHLER A610 for pipings

§ Flow velocity in pipings has no influence on the corrosion rate of A610, whereas aluminium material‘s corrosion rates increase with higher flow velocities.

§ BOEHLER A610 has also a better resistance against erosion than aluminium

installed piping for CNA service prefabricated piping spools for CNA service

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Reference List – Highly Concentrated Nitric Acid (CNA)

Reference List for Tanks, Heat Exchanger & Piping out of Material 1.4361 / SA240 S30600 with special analyses

Boehler Brand Name: A 610 for 99% Nitric Acid Application modified acc. to APB´s Technical Order Specification

Customer DescriptionBASF - Germany Nitric Acid Storage Tank 500m3 - Pre-Fabrication and Installation at SiteBASF - Korea Nitric Acid Storage Tank 800m3 - Pre-Fabrication and Installation at SiteBASF - Korea Isometric Pre-Fabrication and Installation at SiteBASF - Germany Isometric Pre-Fabrication and Installation at SiteDu Pont - USA Nitric Acid Storage Tank 500m3 - Pre-Fabrication and Installation at SiteBASF - Germany Tank - VesselPlinke - Germany Heat ExchangerTeam IIP - China Nitric Acid Storage Tank 800m3 - Pre-Fabrication and Installation at SiteTeam IIP - China Isometric Pre-Fabrication and Installation at SiteBASF - Germany Tank - VesselBASF - Germany Tank - VesselBASF - Germany Tank - Vessel - PipingPlinke - Germany DistributorSKWP - Germany Nitrooleum - VesselDAELIM – Korea (Sadara Project) Nitric Acid Storage Tank 500m3 - Pre-Fabrication and Installation at Site Al JubailBASF – Germany TDI - Piping & Fittings for IsometricBASF - Germany DNT - Piping & Fittings for IsometricMeissner - Germany Nitric Acid Storage Tank 60 m3 incl. Piping & Fittings for IsometricDAELIM – Korea (Sadara Project) Piping and Fittings for Isometric

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Conclusions

• Corrosion of stainless steels in CNA is predominantly intergranular.

• Over 75% concentration of HNO3 the corrosion resistance of A610 is evidentlybetter than the corrosion resistance of the nitric acid stainless steels grades like1.4306S (304L NAG; X1CrNi19-11) and 1.4335 (310L NAG; X1CrNi25-21).

• The improved corrosion resistance is a result of formation of a thinprotective SiO2 layer on top of the passive Cr2O3 layer.

• To optimize the corrosion resistance of A610 adapted weldingtechniques with adapted filler materials are required. Water cooling during welding has to be applied, to avoid solution annealing after welding.

• Application of A610 has been applied successfully for numerousequipment in CNA plants like dehydration columns, rectificationcolumns, bleaching columns and storage tanks as well asinterconnecting pipelines.

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APB Apparatebau Schweißtechnik GmbHA member of the Christof Group

Something we are proud of:

Melamine Reactor (304L) for Cornerstone Chemical Company;New Orleans; June 2016

Length: 40 meter / Diamater: 4.5 meter / Weight: 240 ton