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Corrosion of DuplexStainless Steels in Seawater
byBengt Walln,
Avesta Sheffield AB, Research & Development, SE-774
80Avesta, 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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AVESTA SHEFFIELDCORROSION MANAGEMENTAND
APPLICATIONENGINEERING
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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) Mrker 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 25.7NWCu 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 7Atlas Foundry 8Schmidt & Clemens
9CSM10Bhler 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 30C (21), while in
the Medi-terranean the activity is not com-pletely lost until at
40C (22). Thismeans that natural seawater has itshighest
corrosivity at temperaturesslightly below the temperature atwhich
the biofilm is killed, e.g. at 25-30C 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 0.1-0.2ppmis 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 1ppm at theinjection point is normally sufficient.If
intermittent chlorination is used, aresidual level of 1ppm 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 35C butonly
at 45C. Welded specimens, notequipped with crevices, were
notattacked at 45C, however (33, 39).The second generation
superauste-nitic steel UNS S32654 resisted bothcrevice and pitting
corrosion at 45C(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 (30C) 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 70C and
1.5ppm 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, 27-30Amb. 3
crevice o o o o S31254(o) 31
S08932 (o)
N08926 (o)
30 6 crevice o 3235 3 crevice o S31254(o) 3335 3 welded o35 12
crevice o S31254 (cc) 1330 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) 16Amb. 1 crevice cc o S31254 (o)
3540 6 crevice o o 1760 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 332 45 3 crevice
cc S31254 (cc) 33
2 45 3 welded o 331 30 5 butt welded tubes o S31254 (o) 331 40 5
butt welded tubes o S31254 (o) 331 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 (2ppm, 1h/dl during 1 month followed
by continuous chlorination (2ppm) 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 16Heat exchanger - 5060 6 S31260, S39274 16Piping
system 0.5-0.8 Amb. 6-12 S32760, S32550 S31254 31
Heat exchanger 0.5-0.8 Amb.50 3 S32550 S31254 31Piping 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 100C with a minimum sea-water
flow rate of 0.5 m/s. This corre-sponds to a maximum skin
tempera-ture of 80C.
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 (125C) to 35C 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 125Csteam. 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
50-60C.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, 30C, 85 days (15, 42, 43)
Steel grade 0.5ppm Cl2 1.5ppm 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(0.5-0.8ppm) 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
0.5ppm residual chlorine andone where 1.5ppm was used. Sinceboth
tests were performed at 30Cthe 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 75C 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 Repassivationconditions 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
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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 1000ppm of
totalsulphide (Stot- = H2S + HS- + S
2-), 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 350-500ppm (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 A/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 30Cwhile the superaustenitic
steel S31254needs to be polarized to 300-350 mVSCE at 30C. 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(120C), 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 15ppm. 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 > 1000MPal 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 15MPa 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 waterS31254 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
150C (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. 100C (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 132C 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 are110C for the superaustenitic steelS31254 and 100C 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 30-35C 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 30C.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.15C and max. 1.5ppm
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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9
shown that a piping system consistingof S31254 pipes and S32654
flangeshas resisted chlorinated (2ppm)seawater at a temperature of
45C.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 10Chigher than that of freshly
preparedspecimens (77).
The effect has also been confirmedin field tests performed in
chlorinatedseawater at 55C. The corrosion resis-tance of S32750,
S31254 and S32654was considerably improved if the ex-posure started
with a period of inter-mittent chlorination (2ppm, 1h/d)instead of
exposing the specimens tothe continuously chlorinated (2ppm)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
30C forsuperduplex and 6Mosuperausteniticsteels. If the potential
is kept lowthe CCT is much higher, e.g. 80C 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 A/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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10
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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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