Corrosion-of-Duplex-Stainless-Steels-in-Seawater

CorrosionofDuplexStainlessSteelsinSeawaterbyBengtWallén,AvestaSheffieldAB,Research&Development,SE-77480Avesta,SwedenIntroductionDuplexstainlesssteelsweredevel-opedintheearlythirtiesinSwedenandinFrance.Thedrivingforcetothedevelopmentwasthesensitivitytointergranularcorrosionofexistingausteniticsteelswhichoftencon-tained0.08-0.10%carbon.Theduplexsteelshadthesamecarboncontentbutprovedmuchlesssensitivetothistypeofcorrosion.Alreadybeforetheendof1930AvestaJernverk,nowpartofAvestaSheffield,haddevelopedthegrades453and453Sandin1932thesesteelsconstituted6.5%ofthetotalproduction(1).Thetypicalcomposi-tionsofthesesteelswere25Cr,5Niwith0and1.5%molybdenumre-spectively,i.e.thelattersteelwasaforerunnerofAISI329.Thesteelswerefirstproducedascastproductsbutalsoasplate.Themainapplica-tionareawasinthesulphiteindustry.TheFrenchcorrespondenceto453SwasUranus50containing21Cr,8Ni,2.5Moand1.5Cu(2).Thefirstduplexsteelshadabout65%offerriteinthesolutionan-nealedconditionandthehighferritecontentresultedinratherbadme-chanicalandcorrosionpropertiesintheheataffectedzoneafterwelding.Whenitwasdiscovered,inthebegin-ningofthefifties,thatduplexsteelshaveagoodresistancetostresscorrosioncracking(3,4)thedevel-opmentofsteelswithabetterweld-abilitystarted.OneofthefirstnewsteelswasUNS31500(3RE60=17Cr,4.5.Ni,2.7Mo,0.030whichwaslaterfollowedbyUNS31803(2205=22Cr,5.5Ni,3Mo,0.03C,N).How-ever,noneoftheduplexsteelsexistingintheearlyseventieswereresistantenoughforageneralsea-wateruseandnotuntiltheso-calledsuperduplexsteelswereintroduceddidseawaterresistantduplexsteelsbecomeavailable.Thesesteelsallcontainatleast25%chromiumandhaveincreasedlevelsofmolybdenumandnitrogen.Table1summarizesvarioussuper-duplexsteelsmentionedintheapproximateordertheywereannouncedintheliterature.Thelistincludessteelswhichareproducedinmostproductformsandhaveagreatuseinseawaterapplicationsaswellassteelswhichseemtohavebeenusedverylittlesofar.TwoofthefirstsuperduplexsteelswereUNS31260andUNS32550whichwereintro-ducedtothemarketalreadyintheseventies.Intheeightiesanumberofnewsuperduplexsteelswereintroduced,allcontaining25-27%chromium,3-4%molybdenum,0.15-0.30%nitrogenandsomewithcopperandtungstenadditions.Inthefollowingareviewovermosttypesofcorrosionoccurringinsea-waterapplicationsisgiven.Withjustafewexceptions,onlytestsusingrealseawaterhavebeentakenintocon-sideration.Wheneverpossible,thebehaviourofsuperduplexsteelsiscomparedwiththatofsuperausteniticsteels.ThesuperausteniticsteelsthatareincludedinanyofthesucceedingtestsareshowninTable2.acom1-1998AVESTASHEFFIELDCORROSIONMANAGEMENTANDAPPLICATIONENGINEERINGacomNo.1-982Table1.Superduplexstainlesssteels.SteelgradeTypicalcomposition,%UNSTradenameCrNiMoCuNWRef.S31260DP312573.00.50.100.45S32550Ferralium25522563.02.50.106Alloy38132573.90.157S31200UR47N42573.00.188S32550UR52N42573.01.50.188S32760Zeron10052573.50.70.250.79S32750SAF250762574.00.3010Fermanel2278.53.11.00.2311Atlas95872574.50.1812DPS281277.53.80.30.3013S32520UR52N+4256.53.41.50.2414(1.4469)MärkerG-44698267.54.70.2715S39274DP3W12573.00.302.016S39277DTS25.7NWCu9257.53.91.70.281.017(1.4501)A911102574.00.60.230.718Steelproducers:1SumitomoMetal2LangleyAlloys3ClimaxMolybdenum4CreusotLoire5WeirMaterials6SandvikSteel7AtlasFoundry8Schmidt&Clemens9CSM10BöhlerEdelstahlTable2.Superausteniticstainlesssteelsusedasreferencematerial.SteelgradeTypicalcomposition,%UNSTradenameCrNiMoCuNS31254254SMO120186.10.70.20S32654654SMO124227.30.50.50S34565Alloy24224184.51.00.40N089261925hMo220246.20.90.20N08031Alloy31227316.51.50.20N08926URB26320246.20.90.20N08932URSB8325254.71.50.20N0892625-6Mo420246.20.90.20N08367AL6XN520246.20.20.20-K-C32626375.21.00.20Steelproducers:1AvestaSheffield2KruppVDM3CreusotLoire4IncoAlloys5AlleghenyLudlum6Schmidt&ClemensSeawaterasacorrosivemediumNaturalseawaterInthemid-seventiesMollica,etal,noticedthatstainlesssteels,indepen-dentoftheircomposition,havesur-prisinglynoblepotentialsinnaturalseawater(19).Mollicaattributedthisobservationtoamicrobialslimelayer,thebiofilm,whichisquicklyformedonaninertsurface.Heshowedthatthebiofilmhasacatalyticeffectonthecathodicreactioninthecorrosionprocess,i.e.theoxygenreduction.Afterthisdiscoveryaverylargenum-berofinvestigationshavebeencarriedouttostudythenatureandeffectsofthebiofilm.Thenoblecorrosionpotentials,normallyinthe300to350mVSCErange,meanthattheriskforinitiationoflocalizedcorrosionsuchascreviceandpittingcorrosionisgreaterinnatural,livingseawaterthaninsterilesolutionslikeartificialseawaterorsodiumchloridesolutions,wherethepotentialsareatleastacoupleofhundredmVlower.Alsoduetothebiofilmtherateoftheoxygenre-ductionishigherinthenaturalwater.Atapotentialcorrespondingtothatofastainlesssteelspecimenattackedbycrevicecorrosion(≤0mVSCE),thereductioncurrentisabouttwoordersofmagnitudegreaterthaninasterilechloridesolution(20).Thismeansthatalsothepropagationrateofanylocalizedcorrosionishigherinthenaturalwater.Evenifanactivebiofilmseemstoexistinallseawaters,independentofthetemperature,heatingofthewaterwillkillthebiofilmandstopitscata-lyticability.InnorthernAtlanticthishap