Corrosion of DuplexStainless Steels in Seawater

byBengt Wallén,

Avesta Sheffield AB, Research & Development, SE-774 80 Avesta, Sweden

IntroductionDuplex stainless steels were devel-oped in the early thirties in Swedenand in France. The driving force tothe development was the sensitivityto intergranular corrosion of existingaustenitic steels which often con-tained 0.08-0.10% carbon. Theduplex steels had the same carboncontent but proved much less sensitiveto this type of corrosion.

Already before the end of 1930Avesta Jernverk, now part of AvestaSheffield, had developed the grades453 and 453S and in 1932 thesesteels constituted 6.5% of the totalproduction (1). The typical composi-tions of these steels were 25Cr, 5Niwith 0 and 1.5% molybdenum re-spectively, i.e. the latter steel was aforerunner of AISI 329. The steelswere first produced as cast productsbut also as plate. The main applica-tion area was in the sulphite industry.The French correspondence to 453Swas Uranus 50 containing 21Cr, 8Ni,2.5Mo and 1.5Cu (2).

The first duplex steels had about65% of ferrite in the solution an-nealed condition and the high ferritecontent resulted in rather bad me-chanical and corrosion properties inthe heat affected zone after welding.When it was discovered, in the begin-ning of the fifties, that duplex steelshave a good resistance to stresscorrosion cracking (3, 4) the devel-opment of steels with a better weld-ability started.

One of the first new steels wasUNS 31500 (3RE60 = 17Cr, 4.5.Ni,2.7Mo, 0.030 which was laterfollowed by UNS 31803 (2205 =22Cr, 5.5Ni, 3Mo, 0.03C, N). How-ever, none of the duplex steelsexisting in the early seventies wereresistant enough for a general sea-water use and not until the so-calledsuperduplex steels were introduceddid seawater resistant duplex steelsbecome available. These steels allcontain at least 25% chromium andhave increased levels of molybdenumand nitrogen.

Table 1 summarizes various super-duplex steels mentioned in theapproximate order they wereannounced in the literature. The listincludes steels which are produced inmost product forms and have a greatuse in seawater applications as wellas steels which seem to have beenused very little so far. Two of the firstsuperduplex steels were UNS 31260and UNS 32550 which were intro-duced to the market already in theseventies. In the eighties a numberof new superduplex steels wereintroduced, all containing 25-27%chromium, 3-4% molybdenum,0.15-0.30% nitrogen and some withcopper and tungsten additions.

In the following a review over mosttypes of corrosion occurring in sea-water applications is given. With justa few exceptions, only tests using realseawater have been taken into con-sideration. Whenever possible, thebehaviour of superduplex steels iscompared with that of superausteniticsteels. The superaustenitic steels thatare included in any of the succeedingtests are shown in Table 2.

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Table 1.Superduplex stainless steels.

Steel grade Typical composition, %UNS Trade name Cr Ni Mo Cu N W Ref.

S31260 DP 3 1 25 7 3.0 0.5 >0.10 0.4 5S32550 Ferralium 255 2 25 6 3.0 2.5 >0.10 6

Alloy 381 3 25 7 3.9 0.15 7

S31200 UR 47N 4 25 7 3.0 0.18 8

S32550 UR 52N 4 25 7 3.0 1.5 0.18 8

S32760 Zeron 100 5 25 7 3.5 0.7 0.25 0.7 9

S32750 SAF 2507 6 25 7 4.0 0.30 10

Fermanel 2 27 8.5 3.1 1.0 0.23 11

Atlas 958 7 25 7 4.5 0.18 12

DPS 28 1 27 7.5 3.8 0.3 0.30 13

S32520 UR 52N+ 4 25 6.5 3.4 1.5 0.24 14

(1.4469) Märker G-4469 8 26 7.5 4.7 0.27 15

S39274 DP 3W 1 25 7 3.0 0.30 2.0 16

S39277 DTS 25.7 NWCu 9 25 7.5 3.9 1.7 0.28 1.0 17

(1.4501) A911 10 25 7 4.0 0.6 0.23 0.7 18

Steelproducers:

1 Sumitomo Metal2 Langley Alloys3 Climax Molybdenum4 Creusot Loire5 Weir Materials

6 Sandvik Steel 7 Atlas Foundry 8 Schmidt & Clemens 9 CSM10 Böhler Edelstahl

Table 2.Superaustenitic stainless steels used as reference material.

Steel grade Typical composition, %UNS Trade name Cr Ni Mo Cu N

S31254 254 SMO 1 20 18 6.1 0.7 0.20S32654 654 SMO 1 24 22 7.3 0.5 0.50

S34565 Alloy 24 2 24 18 4.5 1.0 0.40

N08926 1925 hMo 2 20 24 6.2 0.9 0.20

N08031 Alloy 31 2 27 31 6.5 1.5 0.20

N08926 UR B26 3 20 24 6.2 0.9 0.20

N08932 UR SB8 3 25 25 4.7 1.5 0.20

N08926 25-6Mo 4 20 24 6.2 0.9 0.20

N08367 AL 6XN 5 20 24 6.2 0.2 0.20

- K-C 32 6 26 37 5.2 1.0 0.20

Steelproducers:

1 Avesta Sheffield2 KruppVDM3 Creusot Loire

4 Inco Alloys5 Allegheny Ludlum6 Schmidt & Clemens

Seawater as acorrosive mediumNatural seawaterIn the mid-seventies Mollica, et al,noticed that stainless steels, indepen-dent of their composition, have sur-prisingly noble potentials in naturalseawater (19). Mollica attributed thisobservation to a microbial slime layer,the biofilm, which is quickly formedon an inert surface. He showed thatthe biofilm has a catalytic effect onthe cathodic reaction in the corrosionprocess, i.e. the oxygen reduction.After this discovery a very large num-ber of investigations have beencarried out to study the nature andeffects of the biofilm.

The noble corrosion potentials,normally in the 300 to 350 mV SCErange, mean that the risk for initiationof localized corrosion such as creviceand pitting corrosion is greater innatural, living seawater than in sterilesolutions like artificial seawater orsodium chloride solutions, where thepotentials are at least a couple ofhundred mV lower. Also due to thebiofilm the rate of the oxygen re-duction is higher in the natural water.At a potential corresponding to thatof a stainless steel specimen attackedby crevice corrosion (≤ 0 mV SCE), thereduction current is about two ordersof magnitude greater than in a sterilechloride solution (20). This means thatalso the propagation rate of anylocalized corrosion is higher in thenatural water.

Even if an active biofilm seems toexist in all seawaters, independent ofthe temperature, heating of the waterwill kill the biofilm and stop its cata-lytic ability. In northern Atlantic thishappens when the water is heated toaround 30°C (21), while in the Medi-terranean the activity is not com-pletely lost until at 40°C (22). Thismeans that natural seawater has itshighest corrosivity at temperaturesslightly below the temperature atwhich the biofilm is killed, e.g. at 25-30°C in the northern Atlantic.

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Microbially induced corrosion (MIC)other than that caused by the biofilmhas been much discussed but seemsnot to be a problem with the super-steels recommended for seawaterapplications (23). One exception,however, is the sulphate reducingbacteria, the effect of which is dis-cussed in section "Sulphite pollutedwater", page 7.

Chlorinated seawaterIn most seawater applications thestainless steels will handle water thathas been chlorinated in order toavoid fouling problems. Chlorine/hypochlorite is a strong oxidant whichdisplaces the corrosion potential ofstainless steels in the noble direction.The high potentials, which are in theorder of 500 to 600 mV SCE, areconsequently more positive thanthose measured in natural water andmake the risk for initiation of creviceand pitting corrosion very great.

On the other hand, chlorine killsthe biofilm which then loses its abilityto catalyse the oxygen reduction.Therefore the cathodic reaction, i.e.reduction of oxygen plus chlorine, isbetween two and three orders ofmagnitude slower than in naturalseawater in the presence of a biofilm(24). This means in practice that thepropagation rate of any localizedcorrosion is much lower in a chlorin-ated water as long as the effectivecathodic area is not very large.

Since there is no temperature-sensi-tive biofilm in a chlorinated water ahigher water temperature will alwaysincrease the corrosivity of the water,and so will an increase in the chlorineconcentration. In practice, the chlo-rine additions are often higher thannecessary. It has been reported that aresidual chlorine level of 0.1-0.2 ppmis sufficient to reduce the microbialactivity on a stainless steel surface toclose to zero (25, 26). If the chlorinedemand of most seawaters is added,addition of less than 1 ppm at theinjection point is normally sufficient.If intermittent chlorination is used, aresidual level of 1 ppm used during30 minutes per day seems enough tostop the microbial activity (26).

Seawater testsWhen stainless steels are used forhandling seawater the main corrosionrisks are crevice corrosion and some-times pitting corrosion in weld areas.Stress corrosion cracking seldomoccurs at the water temperatures nor-mally encountered. One exception,however, is when seawater evapor-ates on a hot wall creating a veryconcentrated chloride solution. Thistype of corrosion will be treated insection "External stress corrosioncracking", page 8.

In this section just crevice andpitting corrosion tests will be treated.The most frequent way of testing thecrevice corrosion resistance is toapply some kind of crevice formerson the surface of the specimens andthen immerse them in the water.Pitting corrosion may occur on thecreviced specimens but, since crevicecorrosion is normally the dominanttype, pitting is often studied onwelded specimens without usingintentional crevices. Tests of this kindare valuable for the screening ofstainless steels in that they indicatethe probability of localized corrosioninitiation. However, the most reliable(and most expensive) way of evaluat-ing the crevice and pitting corrosionresistance of a material is to performtests with prototype systems com-posed of real components. In thefollowing both types of tests will betreated.

Immersion tests— natural seawaterA great number of immersion testshave been reported in the literatureand in Table 3 a compilation of sometests reported in recent years is shown.The table includes the superduplexand superaustenitic steels tested. Theresults are presented as "no corro-sion" = 0, "crevice corrosion" = cc oras "pitting" = p. No attempt has beenmade to classify the degree of attack.

Table 3 does not include loweralloyed steels like S31803 or N08904.However, when these steels havebeen tested they have almost alwayscorroded. In most cases the super-duplex and the superaustenitic steels

behave similarly and are mostly notattacked at all. In the tests reported inref. 34 however, most specimens havebeen attacked by pitting corrosionbeneath salt deposits which formedon the test plates above the water-line. The crevice corrosion reportedin ref. 36 was very mild and onlyoccurred as end grain attack in theholes through which the retaining boltwas passing.

Immersion tests— chlorinated seawaterAs can be seen in Table 4 most ofthese tests have been performed atelevated and controlled tempera-tures, and they involve only a few ofall superduplex steels available. Thesuperaustenitic steel UNS S31254 hasalmost always been included as areference material. UNS S32750 andS31254 seem to have practically thesame corrosion resistance. When thewater is chlorinated to 2 ppm, crevicecorrosion did not occur at 35°C butonly at 45°C. Welded specimens, notequipped with crevices, were notattacked at 45°C, however (33, 39).The second generation superauste-nitic steel UNS S32654 resisted bothcrevice and pitting corrosion at 45°C(38).

A comparison between UNSS32760 and S31254 is reported inref. 34. As in the preceding tests thesuperduplex and the superausteniticsteels have about the same corrosionresistance. The only exception is atthe lowest temperature (30°C) and 3ppm of chlorine where only the super-austenitic steel was resistant. It shouldbe observed that in these tests pittingcorrosion often occurred beneathsalt deposits above the water-line.These attacks will probably lowerthe potential of the specimens andconsequently make the crevice corro-sion results too positive. It is not likelythat the steels are resistant to crevicecorrosion at 70°C and 1.5 ppm ofchlorine.

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Table 3.Results of immersion tests in natural seawater.

Test conditions ResultsTemp. Duration Type of Superduplex steels Superaust. Ref.

°C months specimens S31260 S39274 S31200 S32550 S32760 S32750 S39277 steels

Amb. 12 crevice o 5Amb. 24 crevice o o S08932(o) 8, 27-30

Amb. 3 crevice o o o o S31254(o) 31

S08932 (o)

N08926 (o)

30 6 crevice o 32

35 3 crevice o S31254(o) 33

35 3 welded o35 12 crevice o S31254 (cc) 13

30 3 crevice o/o S31254 (o/o)

40 3 crevice o/p S31254 (o/p) 34

70 3 crevice o/p S31254 (o/o)

35 6 crevice cc o S31254 (cc) 16

Amb. 1 crevice cc o S31254 (o) 35

40 6 crevice o o 17

60 6 crevice cc oAmb. 3 crevice o (cc) S31254 (cc) 36

N08031 (cc)

o = no corrosionp = pitting corrosioncc = crevice corrosion

Table 4.Results of immersion tests in chlorinated seawater.

Test conditions ResultsCl2 Temp. Duration Type of Superduplex steels Superaustenitic steels

ppm °C months specimens S32750 S39274 S32760 S32550 Ref.

2 35 3 crevice o S31254 (o) 332 35 3 welded o 33

2 45 3 crevice cc S31254 (cc) 33

2 45 3 welded o 33

1 30 5 butt welded tubes o S31254 (o) 33

1 40 5 butt welded tubes o S31254 (o) 33

1 Amb. 1 crevice o cc S31254 (o) 35

10 45 3 crevice cc S31254 (cc), S32654 (o) 37

2 55 3 crevice cc S31254 (cc), S32654 (cc) 38

2 55 3 welded p S31254 (p), S32654 (o) 38

* 55 3 crevice cc S31254 (cc), S32654 (o) 38

* 55 3 welded p S31254 (p), S32654 (o) 38

1.5 30 3 crevice o/o S31254 (o/o) 34

1.5 40 3 crevice o/p S31254 (o/o) 34

1.5 70 3 crevice o/p S31254 (o/p) 34

3.0 30 3 crevice cc/p S31254 (o/o) 34

3.0 40 3 crevice cc/p S31254 (cc/p) 34

3.0 70 3 crevice cc/o S31254 (cc/p) 34

2 Amb. 1 crevice o 16

* Intermittent chlorination (2 ppm, 1h/dl during 1 month followed by continuous chlorination (2 ppm) during 2 months.

o = no corrosionp = pitting corrosioncc = crevice corrosion

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Table 5. Environments used in prototype tests in natural and chlorinated seawater.

Test conditions Steels testedType of test Cl2 Temperature Duration Ref.

ppm °C months Superduplex Superaustenitic

Heat exchanger - ≤90 12-24 S31260 40,41Heat exchanger - Amb. →35 12 S31260, S39274 16

Heat exchanger - 50→60 6 S31260, S39274 16

Piping system 0.5-0.8 Amb. 6-12 S32760, S32550 S31254 31

Heat exchanger 0.5-0.8 Amb.→≤50 3 S32550 S31254 31

Piping system 0.5 30 3 S32550, S32760, S31254, S08932,

S32750, (1.4469) N08367, N08926 15, 42, 43

Piping system 1.5 30 3 S32550, S32760, S31254, S08932,

S32750, (1.4469) N08367, N08926 15,42,43

Prototype testsA summary of tests performed withreal components is shown in Table 5.Results from testing of heat exchang-ers and piping systems have beenreported.

Heat exchangers: In ref. 40 and 41,tubes of the superduplex steel UNSS31260 have been roller expandedinto a S31260 tube plate forming amodel heat exchanger that has beentested at different temperatures. It isconcluded that S31260 tubes allowa maximum process fluid temperatureof up to 100°C with a minimum sea-water flow rate of 0.5 m/s. This corre-sponds to a maximum skin tempera-ture of 80°C.

In the same type of heat exchangera comparison has been made be-tween the superduplex steels S31260and S39274 (16). In one test ambienttemperature seawater was heated bysteam (125°C) to 35°C at a flow rateof 1 m/s. S31260 suffered crevicecorrosion in the tube-tube plate jointwhile S39274 was resistant. In asecond test the water (2 m/s) washeated from 50° to 60° by 125°Csteam. None of the steels corroded.However, since the tube plate wasmade of 316, cathodic protection ofthe tube plate was employed. Thismight explain why S31260 did notcorrode in this case.

A similar test is described in ref. 31.Chlorinated, ambient temperatureseawater was used to cool steamhaving a temperature of 50-60°C.Since the water velocity was very low(0.1 m/s) one might assume that theoutgoing water had a temperatureclose to that of the steam. Using

Table 6.Results of comparative testing in parallel loops.Chlorinated seawater, 30°C, 85 days (15, 42, 43)

Steel grade 0.5 ppm Cl2 1.5 ppm Cl2Attacked (out of 12) Attacked (out of 12)

UNS Type Flanges Welds Flanges Welds

S31254 Superaustenitic 0 0 3 0N08367 Superaustenitic 1 0 2 0

S08932 Superaustenitic 3 0 1 0

N08926 Superaustenitic 8 0 8 0

N08926 Superaustenitic 0 0 3 0

K-C 32 Superaustenitic 5 0 7 0

S32550 Superduplex 3 6 2 7

(1.4469) Superduplex 0 0 0 0

S32760 Superduplex 0 2 3 0

S32750 Superduplex 0 1 4 1

S31803 Duplex 7 0 6 0

S31254 tube plates, tubes made ofS32550, S31254, and titanium weretested. Titanium and S31254 showedno signs of corrosion and only onepit was evident on the S32550 tubes.Pitting corrosion under salt depositswas detected, however, on theS31254 tube plate just above thewaterline.Piping systems: In a very extensiveprogramme different piping systemcomponents were tested for 6-12months in natural and chlorinated(0.5-0.8 ppm) seawater of ambienttemperature (31). The programmeincluded several test loops containingflanged pipes, valves, pipe branchingand deadlegs. Pumps were tested inseparate non-metallic systems.

In all tests, components made of316L and S31803 were attacked bysevere crevice corrosion after a shorttime. The highly alloyed steelsS32760 and S31254 performed in an

excellent way, and so did titaniumand S32550 which, however, wereonly tested as pump and valverespectively.

In another programme eleven 2 inchpipe loops, composed of differentstainless steels, were tested in paralleland thus being exposed to exactlythe same environment (15, 42, 43).Each loop consisted of six 1 m longpipes onto which flanges had beenwelded. Two tests were performed,one where the seawater was chlorin-ated to 0.5 ppm residual chlorine andone where 1.5 ppm was used. Sinceboth tests were performed at 30°Cthe environments were considerablymore corrosive than in the precedingtest. The results of the tests are shownin Table 6 where the number offlanges attacked by crevice corrosion,and the number of flange to pipewelds attacked by pitting corrosion,are given.

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As expected, most of the duplexS31803 flanges exhibit crevice corro-sion. The superduplex steels behavevery differently. While S32550 isattacked by crevice corrosion in bothtests, S32750 and S32760 only cor-rode at the higher chlorine level. Thevery highly alloyed cast material1.4469 was resistant in both tests.Also the superaustenitic steels behavedifferently and only S31254 and oneN08926 grade resist crevice corro-sion at the lowest chlorination level.

The results are quite different whenconsidering the joint welds. Thesuperaustenitic steels were neverattacked by pitting corrosion but onlytwo of the duplex steels were resistanteven at the lowest chlorine concen-tration. The fact that S31803 was notattacked is due to the crevice attacks,which exist on all six pipes, and whichbrings cathodic protection to the restof the pipes.

Samples from the same steel gradesas those tested in the loops were usedfor determining CCT (plate) and CPT(butt welded tube) in the laboratory(44, 45). The results do not alwaysshow a good relation with the resultsof the loop tests and this is especiallytrue for the CCT ranking. This em-phasizes the importance of qualifyinga material by testing real componentsunder realistic conditions.

RepassivationIn practice, stainless steels which areused within proven application limitsmay still be attacked during unfore-seen temporary service upsets, e.g.by temperature excursions. Onceinitiated the attack may continue topropagate even when normal serviceconditions are re-established.

The conditions for repassivation ofcrevice corrosion, initiated in sea-water at 75°C and 300 mV SCE, havebeen investigated in some detail (46,47). The stainless steels studied weresuperduplex S32750 and superaus-tenitic S31254, both in the form ofplate, welded plate and cast mater-ial. The repassivation was studied atthree temperatures under conditionssimulating a chlorinated water. Theresults are shown in Table 7.

Table 7.Effect of repassivation conditions on maximum repassivation time and maximumcrevice corrosion depth (46)

Steel grade Repassivation conditions ResultsUNS Product Potential Temp. Time Attack depth

mV SCE °C days mm

S31254 rolled 0 40 49 0.046welded 35 0.035

cast 36 0.042

S32750 rolled 0 40 59 0.010welded 59 0.060

cast >182 2.510

S31254 rolled 600 40 11 0.378welded 16 1.211

cast 17 1.242

S32750 rolled 600 40 25 0.501welded 26 0.598

cast 17 0.930

S31254 rolled 0 15 31 0.006welded 68 0.013

cast 45 0.011

S32750 rolled 0 15 129 0.023welded >131 0.034

cast >131 0.093

S31254 rolled 600 15 <1 0welded 22 0.009

cast <1 0

S32750 rolled 600 15 <1 0welded <1 0

cast 2 0.001

S31254 rolled 200 40 12 0.162S32750 rolled 35 0.080

Due to the low cathodic reactionrate in a chlorinated water (see sec-tion "Chlorinated seawater", page 3)the area ratio between cathode andanode surface is very important.A small cathodic surface, or a smallpipeline diameter giving high ohmicdrops, results in low potentials duringthe repassivation while a large cath-odic surface results in high potentials.As shown in Table 7 the longest re-passivation times for the rolled super-austenitic and the superduplex steelswere 49 and 129 days respectively.For welded and cast specimens therepassivation time was generallysomewhat longer. If small attacks(<0.2 mm) can be tolerated, and ifcast S32750 is excepted, only therepassivation at 40°C and 600 mVSCE is unacceptable.

In the previous investigation thedifference between the superauste-

nitic and the superduplex steels wasrather small. In another repassivationstudy, S32760 specimens, taken from3/4" solution annealed bar, werecompared with S31254 specimens,machined from NPS 6 Sch 80S seam-less pipe produced via the powderroute. The experimental technique wassimilar to that used in the precedingstudy (48).

Contrary to the previous investiga-tion this investigation concludes thatcrevice corrosion is much easier torepassivate in the superduplex than inthe superaustenitic steel. The lowestrepassivation temperature of S32760was 42°C which is above the normaloperating temperatures 130-40°CI.The report concludes that restorationof the normal process conditions,after a short process upset, will resultin repassivation within a few hourseven if crevice corrosion initiates.

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The reason why S32760 is mucheasier to repassivate than S31254,while S32750 is not, is believed to bedue to the tungsten (0.7%) andcopper (0.7%) content in S32760.These additions are said to increasethe resistance to the hydrochloricacid-containing solution formedwithin the corroding crevice. Accord-ing to data given in ref. 48, S32760has surprisingly good resistance tohydrochloric acid, at least on a levelwith that of Alloy C-276 and C-22 infact.

Sulphide polluted waterUnder certain circumstances sulphatereducing bacteria (SRB) may becomeactive and start producing hydrogensulphide. It is known that this sub-stance stimulates the anodic reactionin the corrosion process of stainlesssteels and that their pitting potentialsdecrease when the concentrationincreases (49, 50). However, hydro-gen sulphide is a strong reductantwhich displaces the corrosion poten-tial in the negative direction thusdecreasing the risk for localizedcorrosion. To illustrate which effect isdominant a 95-day test was carriedout in ambient temperature seawatercontaining about 1000 ppm of totalsulphide (Stot- = H2S + HS- + S2-), aconcentration which is higher thanwhat is normally found in SRB-con-taining waters (37). There was nocrevice or pitting corrosion on anyof the steels tested, 316L and S31803included. It is therefore concludedthat the low corrosion potentials( ~ -500 mV SCE) bring chemicalcathodic protection to the stainlesssteels.

Furthermore, there was no sign ofuniform corrosion on the specimensand the weight losses correspond tocorrosion rates less than 0.001 mm/y.Mollica and Ventura determinedanodic polarization curves for arange of steels in SRB-containingseawater with a total sulphide con-tent of 350-500 ppm (51). At the freecorrosion potentials, i.e. -450 to -500mV SCE, all steels, including 316L andS31803, were passive and the corro-sion current densities were in theorder of 10-2 µA/cm2. This corre-

sponds to very low corrosion rates.However, others have found that inSRB-containing seawater (Stot - ≤ 190ppm), even highly alloyed steels likeS32760 show some uniform corrosionat the free corrosion potential whichwas around -500 mV SCE (52).

As pointed out by Mollica, et al,SRB may become active also in sys-tems handling natural, air-saturatedseawater (53). This is especially thecase in areas where the oxygendiffusion is sluggish, e.g. underneathbarnacles and gaskets. In such asystem the major part of the surfacewill be covered by a normal, activebiofilm but at the small spots whereSRB are active the sulphide contentcan be rather high. If the ratio be-tween the "free" surface and the SRBaffected areas is large, the latterareas will be strongly polarized inthe anodic direction.

By means of anodic polarizationcurves Mollica and Ventura deter-mined the critical crevice corrosionpotentials for a range of stainlesssteels in a water containing 350-500ppm of total sulphide (51). Theyfound that even in the case of 316Land S31803, the crevice corrosionpotentials were more positive thanthe free corrosion potentials (-450 to-500 mV SCE) which excludes thepossibility of spontaneous crevicecorrosion initiation. However, there isa high probability of crevice corro-sion if 316L and S31803 are polar-ized to around 0 mV at 15 or 30°Cwhile the superaustenitic steel S31254needs to be polarized to 300-350 mVSCE at 30°C. Neville and Hodgekissalso detected pitting and crevicecorrosion in highly alloyed steels likeS32760 and S31254 after polariza-tion to very positive potentials (≥1000mV SCE) in a water containing ≤190ppm of total sulphide (52).

EnvironmentalcrackingHydrogen embrittlementIn 1989, five incidents of crackingoccurred in cold worked superduplexS32760 and duplex S31803 stainlesssteel used as production tubulars in aNorth Sea offshore field (54). Thefailures, which occurred after removalof the equipment from the well, wereattributed to hydrogen embrittlement.The tubulars were exposed to hot(120°C), inhibited seawater and werepolarized in the cathodic directiondue to galvanic coupling to the out-side carbon steel casing. A labora-tory investigation showed that tubingwhich did not crack contained lessthan 10 wt ppm of hydrogen whilecomponents that cracked containedin excess of 15 ppm. The cracking didnot occur in service but was generallyinduced by the occurrence of localoverload during handling.

Other investigations conclude thatappreciable hydrogen embrittlementin S32760 exposed to seawater mayoccur only if the steels are polarizedto -900 mV SCE or lower but only ifthe material is cold worked to morethan 35HRC and if very high stressesare present (55). Superduplex S31260is also considered to be safe for hy-drogen embrittlement if the cathodicprotection potential in seawater ismore positive than -900 mV SCE (56).However, galvanic coupling to car-bon steel should result in potentialsconsiderably more positive (57, 58).

The effect of hydrogen on boltingmaterials for sub-sea use has beenstudied using different techniques (59,60). Slow strain rate tests and fatiguecrack growth tests at high stress ratioswere applied on a range of alloyssubject to cathodic protection (-1030mV SCE) in artificial seawater. Thetwo methods gave similar ranking ofmaterials regarding the embrittlingeffects of hydrogen and they showa similar sensitivity to the effects ofhydrogen. As can be seen in Table 8the superaustenitic stainless steels,even in 30% cold worked condition(Rp 0.2 > 1000 MPal has no or littleliability to hydrogen embrittlement,while the superduplex steel showssome hydrogen effect.

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Table 8.Slow strain rate (3.6 · 10-6 1/s) results and fatigue crack growth rates, at astress intensity of 15 MPa √m in air and in seawater with cathodic protection(-1030 mV SCE) for some stainless steels (59, 60).

Alloy Mechanical data* Slow strain rate Crack growthTTF Area red rate x 10-8

Elong. Rp 0.2 Rm rel. to air rel. to air m/cycle

UNS C.W. % % MPa MPa % % Air Sea water

S31254 30 8.0 1067 1163 99 ± 2 106 ± 3 2.5 2.5S34565 52.5 361 792 97 ± 3 93 ± 3 4.1 4.9

S34565 30 5.4 1278 1303 96 ± 3 77 ± 4 6.0 7.0

S32750 35.3 631 952 89 ± 4 77 ± 2 3.0 6.0

* Determined by slow strain rate test in air

Fatigue tests of cathodically pro-tected (-1030 mV SCE), threadedbolts made of S31254 and carbonsteel were performed in seawater(61). The results confirm the goodbehaviour of the superaustenitic steeleven when cold deformed to 30%.Both materials give fatigue life curveswell above design curves given indifferent construction codes.

External stresscorrosion crackingDuplex stainless steels are consideredto possess much better stress corro-sion cracking (SCO resistance thanstandard austenitic steels like 304and 316 and e.g. S31803 is consid-ered resistant to SCC in neutral,oxygen bearing chloride solutions, ofthe same strength as seawater, up toaround 150°C (62). Some years ago,however, a big high pressure separa-tor, made of S31803, on a North Seaplatform cracked after only a fewyears in service in spite of a tempera-ture of max. 100°C (63). The crackingoccurred under the thermal insulationwhich had been soaked with sea-water. The water evaporated on thehot steel surface leaving a very con-centrated chloride solution able tocause the cracking.

A test method, designed to simulatesuch evaporative conditions, wasdeveloped in the seventies (64). In thisso-called drop evaporation test adilute sodium chloride solution slowlydrips onto an electrically heatedspecimen. The dripping frequencyis so adjusted that the solutionevaporates completely before thenext drop arrives. In a similar test,

using artificial seawater as an elec-trolyte, the specimens consist ofelectrically heated C-rings, having aconstant temperature (65). The twotests give the same ranking orderbetween different steel grades, i.e.S31254 > S32760 > S31803.

A third test, by means of which it ispossible to determine the approxi-mate temperature limits for externalSCC, has recently been developed(66). It is shown that when evapora-tion takes place at elevated tempera-tures, the solution mainly consists ofvery concentrated magnesium chlo-ride having a low pH. For instance,evaporation at 132°C results in abrine containing 9.5M Cl- at pH 1.5-2.5. In the new test method C-rings,loaded to slightly above the yieldstrength, are immersed in this electro-lyte up to 4000 hours. The tempera-ture is varied until the limit for SCC isfound. This method gives the sameranking order as the two precedingtests and the lowest temperatures atwhich cracking was observed are110°C for the superaustenitic steelS31254 and 100°C for the duplexsteels S32750 and S31803.

Application limitsof super steelsPractical experienceBefore the experience from realapplications was known, especiallyin the offshore industry, many steelproducers were somewhat optimisticregarding the conditions under whichtheir super steels could be used, and

it was then believed that quite hightemperatures could be used withoutrisking crevice corrosion (8, 9, 67, 68).However, the offshore industry mostlyrestricted the use of superduplex andsuperaustenitic steels to 30-35°C and1 ppm of residual chlorine (69).

The first deliveries of super steelsto the seawater systems on offshoreplatforms took place in the mid-eighties and some years later the firstcorrosion failures were reported (70,71). Since then corrosion has beenreported from superaustenitic as wellas superduplex systems (72). In almostall cases the failures have been dueto crevice corrosion. Threadedconnections have been especiallyvulnerable and a warning againsttheir use was given quite early (39).

Crevice corrosion has also beenobserved on the sealing surfaces offlanges. In most cases the attacks canbe explained by temperatures widelyexceeding the design temperature,e.g. in connection with coolers or withheat tracing used on outdoor piping(71). In some cases, however, crevicecorrosion has occurred at tempera-tures appreciably lower than 30°C.Unfortunately it has not been possibleto quantify how many of the approxi-mately 50 000 delivered S31254flanges that have corroded in sea-water systems where the design limitshave never been exceeded. However,the corrosion cases reported haveresulted in the Norwegian offshorestandard (Norsok) stipulating max.15°C and max. 1.5 ppm chlorine forsuperaustenitic and superduplexsteels if crevices are present (73).

In the following some ways of ex-tending the application limits of thesuper steels in seawater systems willbe discussed.

Use of moreresistant materialIn almost all cases the failures ob-served in practice have been crevicecorrosion while pitting corrosion onfree surfaces or in welds seems tooccur very seldom and only at veryhigh temperatures. Therefore, oneway to solve the crevice corrosionproblems is to use a more resistantmaterial in the pipe componentsexposed to crevices. It has been

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shown that a piping system consistingof S31254 pipes and S32654 flangeshas resisted chlorinated (2 ppm)seawater at a temperature of 45°C.So using superaustenitic 6Mo orsuperduplex pipes and 7Mo flangesis a possible way to overcome mostcrevice corrosion problems. (71).

Another way is to weld overlay theparts of a component forming thecrevice with a corrosion resistantnickel-base alloy. However, differentlaboratory tests (Table 9) show thatthe critical crevice corrosion tempera-ture of weld deposits made from Alloy625 or Alloy C-276 fillers are lowerthan that of parent metal of mostsuperaustenitic and superduplexsteels. To get a real improvement overthe resistance of the original flangematerial, fillers containing very highamounts of chromium and molybde-num have to be used (74, 75).

Table 9.Critical crevice temperatures forNi-base overlays on S31254 plateMTI-2 method (74).

Composition, % Type of CCTCr Ni Mo Other filler °C

21.7 61.8 8.9 Nb SMAW 2520.9 63.9 8.6 Nb PTA 25

17.1 64.3 15.5 SMAW 25

15.5 58.2 16.0 W PTA 30

20 18 6.1 Cu Plate 45

21.3 56.3 13.7 Nb SMAW 65

23.3 59.6 15.3 SMAW 80

22.4 60.6 15.4 GTAW 85

24.8 57.8 14.8 SMAW 90

SMAW: Shielded metal arcPTA: Plasma transferred arcGTAW: Gas tungsten arc

Mild start-up periodIt is well known from CCT determi-nations in 6% ferric chloride that thecritical temperatures will be higher ifthe same specimen is used until thecritical temperature is reached thanif new specimens are used at eachtemperature. The explanation to thisis that the specimens become pas-sivated by the oxidizing solution atlower temperatures where no local-ized corrosion occurs. The improvedpassive layer thus obtained has a lowpassive current which reduces the riskfor crevice corrosion initiation (76).

The same effect can be observed inseawater too. Laboratory tests, simu-lating a chlorinated water, show thata slow increase of the potential upto the level of a chlorinated waterresults in higher a CCT than if thehigh potential is applied from thestart (76). Similarly, aged specimens,having a reinforced passive layer,have a CCT which is at least 10°Chigher than that of freshly preparedspecimens (77).

The effect has also been confirmedin field tests performed in chlorinatedseawater at 55°C. The corrosion resis-tance of S32750, S31254 and S32654was considerably improved if the ex-posure started with a period of inter-mittent chlorination (2 ppm, 1h/d)instead of exposing the specimens tothe continuously chlorinated (2 ppm)water from the start (38).

This means, in practice, that thecorrosion resistance of a seawaterpiping system is dependent on theway its first contact with the seawateris managed. The best way is to startwith a low temperature and to use noor intermittent chlorination during thefirst weeks. The worst way is to ex-pose the system to a hot, continuouslychlorinated water from the start.

Cathodic protectionThe critical temperatures for creviceand pitting corrosion in seawater arestrongly dependent on the potential.In a chlorinated seawater (∼600 mVSCE) the CCT is around 30°C forsuperduplex and 6Mo superausteniticsteels. If the potential is kept lowthe CCT is much higher, e.g. 80°C at0 mV SCE (78). If a normal type ofcathodic protection, i.e. with sacrifi-cial anodes, is applied the potentialwill be very low (~ -1000 mV SCE).Since the cathodic current density isdetermined by the limiting currentdensity for oxygen reduction theanode consumption rate will be highin natural as well as in chlorinatedwater.

In a non-chlorinated seawater, theactivity of the biofilm results in highreduction currents (∼10 µA/cm2) evenat 0 mV SCE. However, in a chlorin-ated water, where no active biofilm ispresent, the reduction current at 0 mVis two to three orders of magnitude

lower resulting in a very low anodeconsumption rate (24, 79). This know-ledge is now used in practice in theso-called RCP method where aresistor limits the current from thesacrificial anode. Very low anodeconsumption rates are reported forthe protection of stainless steel pipingin chlorinated seawater even at hightemperatures (78).

Conclusions• Immersion tests in natural and

chlorinated seawater show thatsuperduplex and superaustenitic6Mo steels have about the sameresistance to initiation of creviceand pitting corrosion.

• Prototype tests, with real compo-nents, confirm that the differencebetween the best grades of the twotypes of steel is small but that thereare variations between gradesbelonging to the same group.

• Superduplex and superaustenitic6Mo steels seem to have about thesame ability to repassivate a pro-pagating crevice attack but opin-ions differ here.

• Sulphide containing seawater isless corrosive than natural seawaterbut mixed aerobic-anaerobic con-ditions may be more corrosive.

• Superduplex steels are more sensi-tive to hydrogen embrittlement,caused by cathodic protection,than superaustenitic steels.

• Superduplex steels seem somewhatmore sensitive to external stresscorrosion cracking than superaus-tenitic steels.

• Experience from large scale off-shore installations shows thatcrevice corrosion has occurred inboth superaustenitic 6Mo andsuperduplex seawater systems butthat many failures are due toservice conditions exceeding thedesign limits.

• For both types of steels the applica-tion limits can be extended bydifferent methods.

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This article was first presented as aplenary paper at "Duplex StainlessSteel '97". Published with the kindpermission of the copy right holder,"Stainless Steel World".

Although Avesta Sheffield has made every effort to ensure the accuracy of this publication, neither it nor any contributor can accept any legalresponsibility whatsoever for errors or omissions or information found to be misleading or any opinions or advice given.

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