duplex in chemical

Duplex Stainless Steels 97 - 5th World Conference

AbstractThe use of duplex stainless steels in thechemical process industry has expandedrapidly over the past few years. They arenow used not only in chloride environ-ments, where they are more resistant toSCC than austenitic stainless steels, butalso in a wide variety of other demandingapplications.In DSM plants, duplex is used especiallyfor chloride-containing process fluidsand for ammonium carbamate solutions.The material has been found to offergood resistance to chloride SCC, al-though such corrosion can and does oc-cur at pH values lower than 3 to 4. Thiswe established both in the field andthrough CERT (constant extension ratetesting).In a strongly alkaline chloride-bearing so-lution, duplex SS gives excellent perform-ance whilst a high-nickel alloy fails afterabout three years because of SCC. Thesefindings are supported by drop evapora-tion tests. In oxygen-free carbamate solutions, du-plex has proved to be even more corro-sion resistant than much more costly ma-terials such as titanium and high-nickelalloys. In the high-pressure synthesissection of urea plants, duplex is an excel-lent alternative to traditional urea gradessuch as type AISI 316 L UG andX2CrNiMoN 25 22 2. These are particular-ly prone to SCC on the steam side of heatexchangers when the boiler feedwater iscontaminated with chlorides. Fabrication of high-pressure vessels induplex is not without difficulty, however.Also, the duplex must meet particularquality requirements to ensure properperformance in carbamate environments.These have meanwhile been defined in aDSM/Stamicarbon material standard.Use of duplex SS can offer significantcost savings. As a case in point, a con-denser, which has given 12 years of satis-factory service, cost US $ 300,000. Thesame vessel in high-nickel alloy wouldhave cost US $ 1,000,000.

Duplex SS, because of its higher strengthfigures, offers a 10% cost saving overaustenitics in the high-pressure pipingused in the urea synthesis section.Sandvik, in collaboration withStamicarbon, the licensing subsidiary ofDSM, has developed a new type of du-plex SS. Named Safurex, this material of-fers optimum corrosion resistance tocarbamate environments.

1 Introduction

At DSM, duplex stainless steels were firstapplied in the mid-nineteen sixties, mainlyfor non-welded parts of carbamate pumpsand valves in urea plants. In the late nine-teen sixties, the drum of a centrifuge in apolyethylene plant was fabricated fromX2CrNiMoCu 21 8 3 2 to replace a type AISI316 drum exhibiting serious chloride stresscorrosion cracking. Large-scale introductionof duplexes began in the early eighties, ini-tially for piping and equipment componentsand later for large-size equipment. At thepresent time, some US $ 60 million worth ofduplex is used in DSM plants.They are used on services such as the fol-lowing:

oxygen-free carbamate solutions at tem-peratures up to about 130 C, replacing ti-tanium and high-nickel alloys;

oxygen-containing carbamate solutionsat temperatures up to about 185 C;

benzoic acid at temperatures of 240 C to260 C, where previously high-nickel al-loys were used;

chloride-containing solutions where thetypical austenitic stainless steels are sus-ceptible to stress corrosion crackingand/or corrosion fatigue;

caustic fluids at temperatures up to200 C;

calcium ammonium nitrate and NPK fer-tiliser;

nitric acid and acidic ammonium nitratesolutions.

Application of duplex stainless steel in the chemical process industryby Giel Notten (DSM Engineering Stamicarbon, the Netherlands)

Keywords: duplex stainless steel, chemical process industry, stress corrosion cracking, selective attack, urea

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Stainless Steel World 1997 KCI Publishing

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Application of duplex stainless steel in the chemical process industry

Various types of duplexes have been usedhere including 22% Cr and 25% Cr gradeslike X2CrNiMoN 22 5 3 and X2CrNiMoCuN25 7 3 2. For nitric acid and acidic ammoni-um nitrate solutions the molybdenum-freetype X2CrNiN 23 4 is used. The workhorseamong the duplexes used in DSM andStamicarbon-licensed plants is X2CrNiMoN22 5 3. This gives good performance in, forexample, a 12% NaCl solution with pH 14 attemperatures up to 200 C. In one instance ithas lasted for nine years without any failurewhile a high nickel alloy failed after 2 yearsdue to stress corrosion cracking. Laboratory stress corrosion cracking exami-nations by means of drop evaporation tests(DET) showed that duplex stainless steelshave a better stress corrosion cracking re-sistance in alkaline (chloride containing) so-lutions than high-nickel alloys. Experience with duplex stainless steels inoxygen-free carbamate solutions was firstreported at the 9th European Congress onCorrosion, 26 October 1989 in Utrecht, theNetherlands (ref. 1). The first large-scale ap-plication of duplex stainless steel on thisservice was a condenser which gave 12years of satisfactory service. The same unitin titanium and a high-nickel alloy had to bereplaced after 5 and 8 years respectively.The duplex cost US $ 300,000 while at thetime a high-nickel alloy unit would have costUS $ 1,000,000.This paper discusses the application andbenefits of duplexes used on urea service. Italso reports on a new duplex stainless steelspecially developed for carbamate servicein urea plants.

2 Stamicarbon CO2-stripping ureaprocess

(Ref. 2) The production of urea is based onthe following reaction mechanism:

2 NH3 + CO2 NH2COONH4

NH2COONH4 CO(NH2)2 + H2O

The process flow diagram of theStamicarbon stripping process is shown infigure 1. The synthesis section consists of aurea reactor (c), a stripper for unconvertedreactants (d), a high-pressure carbamatecondenser (e) and a high-pressure off-gasscrubber (f). In the carbamate condenser (e)CO2 and NH3 are converted into ammoniumcarbamate. In the reactor the ammoniumcarbamate is partly converted into urea andwater.The urea synthesis process takes place at apressure of 140 bar and a temperature of185 C. The bulk of the unconverted carba-mate decomposes in the stripper, whereammonia and carbon dioxide are strippedoff. This stripping is effected by counter-current contact between the urea solutionand oxygen-containing carbon dioxide atsynthesis pressure. The oxygen is neededto maintain a passive (corrosion resistant)layer on the stainless steels in the synthesissection. On leaving the stripper, gaseousammonia and carbon dioxide are con-densed in the high-pressure carbamatecondenser (e) at synthesis pressure. Beforebeing purged from the synthesis section,the inert gases, mainly oxygen and nitrogen,are washed in the high-pressure scrubber (f)

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Figure 1. Stamicarbon CO2- stripping urea proces.


with carbamate solution from the low-pres-sure recirculation section.The urea synthesis solution is highly corro-sive, the most aggressive component beingammonium carbamate. Consequently, ma-terials of construction to be used here mustmeet high standards in terms of composi-tion and quality. Awareness of the importantfactors in materials selection, equipmentdesign, manufacture and inspection, tech-nological design and proper plant opera-tions, together with periodic corrosion in-spections, are the key factors for safeoperation for many years.

3 Corrosion aspects

3.1 Condensation corrosionStainless steels in a corrosive environmentlike ammonium carbamate owe their corro-sion resistance to the presence of a protec-tive oxide layer on the surface. As long asthis layer is intact, the metal corrodes onlyat a very low rate. Passive corrosion rates inliquid phases are generally between 0.01and 0.1mm/year. In gas phases where mix-tures of ammonia, carbon dioxide and watervapour can condense to form carbamatesolutions passive corrosion rates can in-crease to 0.2mm/year. Active corrosionrates in carbamate solutions can be as highas 50mm/year. Stainless steels exposed tocarbamate-containing solutions in urea syn-thesis section can be kept in a passivatedstate by adding a given amount of oxygen.If the oxygen content drops below this limit,acive corrosion starts after some time.Adding oxygen and maintaining a sufficient-ly high oxygen content in the variousprocess streams are prerequisites to pre-venting corrosion of the equipment andpipelines.From the point of view of corrosion preven-tion, condensation of NH3-CO2-H2O gasmixtures to carbamate solutions warrantsextra attention. Despite the presence ofoxygen, a more corrosive condensate is ini-tially formed on condensation. Passivationis believed to take place via a metal ion re-dox system, which is missing in freshlyformed condensate. This accounts for thesevere corrosion sometimes observed incold spots in the channels of HP equipmentand gas lines fabricated from urea grade316L. Such corrosion can be prevented byadequate insulation and tracing.When condensation forms part of theprocess, as in the HP carbamate condenser,

special technological measures must betaken. One such measure is adding an oxy-gen-rich liquid phase containing a metal ionredox system into the condenser, with liq-uid-gas distribution devices preventing dryspots on surfaces where condensationtakes place.The risk of condensation corrosion can alsobe diminished by choosing a more corro-sion-resistant material of construction. Thehigher alloyed austenitic stainless steel typeX2CrNiMoN 25 22 2 is less susceptible tocondensation corrosion than urea grade316L. Type X2CrNiMoN 22 5 3 duplex stain-less steel has proved to be more corrosion-resistant in condensing environments too.Another benefit of duplex stainless steels istheir higher strength. Duplex high-pressurepiping can be much thinner than austeniticstainless steel piping.During a turnaround in October 1996, afterbeing on-stream for about 28 years, thestrip gas line from the HP stripper (d) to theHP carbamate condenser (e) had to be re-placed due to condensation corrosion. Theurea grade 316L line, measuring 219 X22.2mm and 70 m long had a corrosion al-lowance of about 6mm. For duplex stainlesssteel a corrosion allowance of 4mm will do,so leading to 219 X 14mm pipe.Replacement by type X2CrNiMoN 22 5 3duplex (seamless hot extruded) pipe gave acost saving of about 10%.Another advantage of duplex is its high re-sistance to chloride stress corrosion crack-ing. This can be particularly relevant forplants in coastal areas. Such piping in aus-tenitic stainless steel needs to be suitablypainted to minimise the risk of SCC. Duplexstainless steel piping need not be painted.

3.2 Stress corrosion cracking in HP carbamatecondenser

The ammonia and carbon dioxide leavingthe stripper are condensed in the HP carba-mate condenser (e) at synthesis pressure.The heat released in the formation of am-monium carbamate is used for the produc-tion of 4.5 bar steam. Any chlorides enteringthe shell side of the HP carbamate con-denser are likely to initiate stress corrosioncracking of the austenitic stainless steeltubes, starting from the outside surface ofthe tubes. Numerous cases of stress corro-sion cracking are on record. Cracking hasbeen observed in both urea grade 316L andX2CrNiMoN 25 22 2 tubes. An example ofstress corrosion cracking a urea grade AISI316L HP carbamate condenser tube is

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shown in figure 2. The location of the cracksis indicated in figure 3 for expanded-and-welded tubes and for welded tubes.Stamicarbon does not allow the tubes to beexpanded in the tubesheet because thiswould render the leak detection system in-operative.Leakage may result from pinholes in thetube joints. In most cases cracking merelyoccurred in or near the top tubesheet. The

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Figure 2. External stress corrosion cracking of ureagrade AISI 316L HP carbamate condenser tube ( 25 X20mm). Magn. 30 ; etchant oxalic acid; neg. nr.7/83682-24.

Figure 3. The location of cracks in HP carbamate con-denser tubes.

Figure 4. Steam and condensate system in the urea process.


locations have been found to depend on theorigin of the chloride contamination.The following causes of SCC in HP carba-mate condensers have been identified:

Chloride contamination of water used forhydrostatic testing, cleaning or flushing.

Transport and storage in a chloride-con-taining (maritime) atmosphere (infiltrationas a result of breathing due to cyclictemperature changes).

Chloride contamination of boiler feed wa-ter (e.g. leakage of vent condenser insteam condensate tank). The steam andcondensate system in the urea process isshown in figure 4.

SCC in HP carbamate condensers can beavoided by changing the corrosive environ-ment (inhibition) or by using a material thatis more resistant to chloride SCC:

The chloride content of water used forhydrostatic testing, flushing or cleaningshould be less than 1 ppm. As an addi-tional precaution it is advised to inhibitthe water with 2% TSP (trisodium phos-phate).

During transport and storage in a chlo-ride-containing atmosphere the equip-ment should be inerted with nitrogen(0.3-0.5 bar gauge).

The chloride content of the blow-downshould be less than 0.2 ppm. The oxygencontent should be zero; this will be thecase when the blow-down contains ex-cess oxygen scavenger (0.10.5 ppm). Ifchloride contamination cannot be avoid-ed, continuous inhibition with TSP shouldbe considered.

Application of a more SCC-resistant ma-terial like type X2CrNiMoN 22 5 3 duplexstainless steel.

For existing carbamate condensers theremedies were focused on changing the en-vironment but for new equipment the use ofduplex stainless steel was considered. Wehad gained ample experience with suchsteels in terms of SCC resistance.In a naphtha cracking plant of DSM twoprocess steam generators, with U-bundlesin type X2CrNiMoN 22 5 3 duplex stainlesssteel, have been in service since 1988.Conditions on the shell side are even moresevere than in carbamate condensers.Steam is generated from chloride-contain-ing process condensate (average chloridecontent 10 ppm with pH varying from 412)

at a temperature of 180 C. The carbon steeltubesheet is overlay welded with AISI 309L(X2CrNi 24 12). The duplex stainless steeltubes were automatically welded (GTAW)with 309S/309Mo welding consumable. Thetubes were lightly expanded within thetubesheet over a length of 25mm. The thick-ness of the tubesheet is 193mm. A light hy-draulic expansion cannot prevent crevicesbetween the tubes and the tubesheet. Thehorizontal bundle is not fully immersed, ac-cumulation of corrosive components likechlorides is likely to occur in the wet/dryzone. Until now, after about nine years onstream, no corrosion has been observed inthese steam generators. In numerous otherapplications we found that duplex stainlesssteels offer good SCC-resistance at pH val-ues above about 4. At lower pH values theadvantage of duplex stainless steels overaustenitic stainless steel diminishes. Thishas been confirmed in stress corrosioncracking experiments by means of ConstantExtension Rate Testing (CERT) (ref. 1, 3). Duplex stainless steel test coupons havebeen exposed in the HP synthesis sectionof the DSM urea plant since the mid-seven-ties. The average corrosion rate ofX2CrNiMoN 22 5 3 proved to be as follows:

top of reactor: 0.04mm/year top of stripper: 0.08mm/year top of HP carbamate condenser:

0.03mm/year

The corrosion rates of duplex stainless steelin the urea reactor and in the HP carbamatecondenser are comparable with those ofX2CrNiMoN 25 22 2. The measured corro-sion rate of duplex in the stripper is slightlyhigher than that of type X2CrNiMoN 25 22 2steel. Based on these results and experi-ence with chloride-containing environments,we wondered if the chloride stress corrosioncracking problems in the HP carbamatecondenser could be resolved by using du-plex stainless steel instead of urea grade316L or X2CrNiMoN 25 22 2.In order to assess the corrosion resistanceof duplex stainless steel, we installed threetubes of X2CrNiMoN 22 5 3 in the HP car-bamate condenser of the DSM urea plantfor a period of 1.5 years. After this period, amaximum decrease in wall thickness of0.05mm was found. Corrosion was slightlyintergranular and had propagated via theferrite/austenite grain boundaries (figure 5).In October 1993, the first HP carbamatecondenser and HP scrubber with

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duplex stainless steel tubes and tubesheetsoverlay welded with duplex stainless steelwere installed in a urea plant. In September1996, after 3 years on stream, the HP heatexchangers were checked by visual inspec-tion and eddy current measurements of thetubes.This inspection revealed no signs of corro-sion, neither of the overlay welding andtube-to-tubesheet joints, nor of the tubes. A HP carbamate condenser with duplexstainless steel tubes and overlay weldedtubesheets is only slightly more expensivethan the same unit in urea grade AISI 316Land about 5% cheaper than one inX2CrNiMoN 25 22 2.Three other HP heat exchangers with du-plex stainless steel tubes and overlay weld-ed tubesheets are now under construction.

3.3 Corrosion of HP stripper tubesIn the stripper the unconverted carbamate isdecomposed and the ammonia and carbondioxide are stripped off. Stripping is effectedby countercurrent contact between the ureasolution and carbon dioxide at synthesispressure. The stripper is the falling film type.Liquid dividers with gas tubes are posi-tioned on the protruding tube ends with thejoints being sealed with PTFE bushes.Process conditions are severest in the strip-per, especially at the top of the tubes, dueto the relatively low oxygen partial pressureand high tubewall temperature. In theStamicarbon urea stripping plants the strip-per tubes are fabricated from X2CrNiMoN25 22 2. The corrosion rate of this materialin these conditions is 0.05mm/year, which isquite low. If a liquid divider with PTFE bushis not properly seated a higher corrosionrate may occur due to flooding in the tube.Figure 6 shows part of a seriously corrodedstripper tube due to flooding.Given the performance of duplex stainlesssteels in oxygen-containing and oxygen-freecarbamate solutions we feel sure that a du-plex high in chromium and nitrogen and lowin nickel will give good performance even inthese extreme conditions.

4 Material requirements for duplexstainless steels

Experience has shown that duplex stainlesssteels to be exposed to carbamate solutionsmust meet particular requirements. Theymust have a homogeneous ferrite-austenitestructure consisting of a ferritic matrix withaustenite islands fully enclosed by ferrite.

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Figure 5. Slight intergranular attack in duplex stainlesssteel tube material exposed in HP carbamate con-denser. Magn. 700 ; etchant KOH. Neg.nr. 7/82850.

Figure 6. Part of seriously corroded stripper tube( 31 25mm) due to flooding. Neg. nr. 7/83861-32.

Figure 7. Intergranular attack of type X2CrNiMoN 22 53 duplex in Streicher test. Magn. 140 ; etchant LB1;neg. nr. 76350.


The ferrite content of the base material mustbe between 40 and 60% whilst the ferritecontent of the weld and the heat-affectedzones must be between 30 and 70%. A lotof effort has been put into the developmentof a suitable quality test. We found that theStreicher test (ASTM A 262, Practice B) fillsthe bill. Duplex stainless steel test couponsthat showed intergranular attack on expo-sure to carbamate solutions showed also in-tergranular attack in the Streicher test (figure7).We therefore decided that duplex stainlesssteels on ammonium carbamate serviceshould be Streicher-tested in accordancewith ASTM A 262 Practice B. Such steelsshall meet the following requirements:The value of the overall attack in as-deliv-ered and in the final heat-treated condition (e.g. welding) as determined by the gravi-metric method shall not exceed:

1.6 g/m2 h. (10 m m/48 hr) for 22% Cr du-plex types;

0.9 g/m2 h. (6 m m/48 hr) for 25% Cr du-plex types.

The depth of selective or intergranular at-tack as determined by microscopic exami-nation after the Streicher test shall not ex-ceed 100 m m in any direction.Heat exchanger tubes shall be tested forhomogeneity (ferrite-austenite ratio) byeddy-current testing. Longitudinally weldedheat exchanger tubes are not acceptable.These requirements are included in theStamicarbon material specification.

5 Development of a new type of duplexstainless steel for ammonium carba-mate service

Sandvik and Stamicarbon decided to jointlydevelop a new type of duplex stainless steelspecifically for urea synthesis equipmentand piping (ref. 4), This new type of duplex,named Safurex, was to have the followingproperties:

At least the same corrosion resistance tocarbamate solutions as X2CrNiMoN 2522 2.

High resistance to stress corrosion crack-ing.

Superior mechanical properties to ureagrade AISI 316L and X2CrNiMoN 25 22 2.

Good microstructure stability. Good weldability.

For exploratory tests, Sandvik producedseveral heats with different chemical com-positions in a 170 kg ingot HF laboratoryfurnace. From the test results, laboratoryexaminations as well as field tests of sometwenty heats with different chemical compo-sitions the optimum alloy composition withrespect to corrosion resistance, structurestability and weldability was selected. Thiscomposition is used in SafurexTM. It con-tains in % by weight:

C max. 0.05Si max. 0.8Mn 0.34Cr 28-35Ni 310Mo 1.03.0N 0.20.6Cu max. 1.0W max. 2.0S max. 0.010Ce 00.2the balance being Fe.

For a follow-up research programme a 70-tonne AOD (argon oxygen decarburisation)production heat of SafurexTM was made. The investigations in laboratory experimentsand field tests gave the following results:

Safurex meets the following requirementsin the Streicher test to ASTM A 262,Practice B:

overall attack: < 0.7 g/m2/h (0.78mm/year)

selective attack: < 100 m m. Safurexs corrosion resistance to carba-

mate solutions is equal to or better thanthat of type X2CrNiMoN 25 22 2 material.

The stress corrosion cracking resistanceof Safurex is much better than that ofaustenitic stainless steels and equal to oreven better than that of type X2CrNiMoN22 5 3.

The tensile strength of Safurex is abouttwice as high as that of urea grade AISI316L and about 70% higher than that ofX2CrNiMoN 25 22 2.

The elongation of Safurex is >25% (A5). Safurex has a homogeneous microstruc-

ture with a ferritic matrix which fully en-closes the austenite islands.

Annealing and welding tests have showngood microstructure stability.

Welding tests by GTAW using a matchingSafurex filler wire and a matching

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Application of duplex stainless steel in the chemical process industryD97-201PAGE 16


X2CrNiMoN 25 22 2 filler wire haveshown good weldability.

Like all duplex stainless steels, Safurex issusceptible to 475 C embrittlement.However, a holding time of 10 hours at510 C does not cause any loss of ductili-ty.

6 Conclusions

Given their excellent corrosion resis-tance, higher strength and lower cost du-plex stainless steels are an attractive al-ternative to austenitic stainless steel HPpiping in urea plants.

For plants in coastal areas duplex stain-less steel has the added advantage ofhigh chloride stress corrosion crackingresistance.

Duplex stainless steels offer a good solu-tion to stress corrosion cracking prob-lems in the HP carbamate condensers.Such cracking can be initiated by chlo-ride contamination of the condensate.

SafurexTM, is a promising material thatwithstands the severest corrosive (-ero-sive) conditions in carbamate solutions

combined with a high strength and goodstress corrosion cracking resistance.

References1 Experiences with ferritic-austenitic stainless steels

in the chemical process industry. M.J.G. Notten,DSM Stamicarbon, Geleen, The Netherlands.Proceedings PI-038; 9th European Congress onCorrosion. 26 October 1989, Utrecht, theNetherlands.

2 Ullmans Encyclopedia of industrial chemistry; Urea;Vol. A 27. Jozef H. Meessen, DSM Stamicarbon,Geleen, the Netherlands. Harro Petersen, BASFAktiengesellschaft (retired), Ludwigshafen, FederalRepublic of Germany.

3 Stress corrosion cracking of duplex stainless steelin chloride containing media in the chemicalprocess industry; a case study. J.G.A. Aerts andM.J.G. Notten, DSM Stamicarbon, Geleen, theNetherlands. Proceedings Duplex Stainless Steels91, page 11411146. 2830 October 1991,Beaune, Bourgogne, France.

4 Safurex, a new stainless steel for urea service. GielNotten and Jo Eijkenboom, DSM Stamicarbon,Geleen, the Netherlands. Pasi Kangas and AdRaatgeep, Sandvik, Sandviken, Sweden. EighthStamicarbon Urea Symposium 69 May 1996,Amsterdam.

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