Effectofagingonthecorrosionresistanceof2101leanduplexstainlesssteelLihuaZhanga,YimingJianga,BoDenga,WeiZhangb,JuliangXua,JinLia,⁎aDepartmentofMaterialScience,FudanUniversity,Shanghai200433,PRChinabInstituteofstainlesssteel,BaosteelCo.,Ltd,Shanghai200431,PRChinaARTICLEDATAABSTRACTArticlehistory:Received7May2009Receivedinrevisedform14August2009Accepted29August2009Themicrostructureandlocalizedcorrosionbehaviorofa2101leanduplexstainlesssteelagedat700°Cwereinvestigated.Theresultsshowedthatchangesinthemicrostructureoftheduplexstainlesssteel,duetotheformationofprecipitates,affecteditspittingcorrosionresistance.Thevaluesofthepittingpotentialandthecriticalpittingtemperaturedroppeddrasticallybeforeagingtimeupto30min.Thepotentiostaticpittingcorrosionmeasurementindicatedmoresensitivetothesmallamountofprecipitatescomparedtothepotentiodynamictest.Pittingnucleatedmainlyintheferritephaseforthesolution-annealedspecimen,whiletheinitiationofpittingcorrosionfortheagedspecimentookplaceatCr-depletionareaaroundtheprecipitates,i.e.inthenewlyformedsecondaryaustenitephase.©2009ElsevierInc.Allrightsreserved.Keywords:DuplexstainlesssteelAgingPrecipitationPittingcorrosion
1.IntroductionDuplexstainlesssteels(DSS)arebeingincreasinglyemployedinchemical,petrochemicals,nuclear,andmarineindustriesprimarilyduetotheirexcellentmechanicalpropertyincombinationwithgoodcorrosionperformance[1].ItiswellestablishedthatDSSexhibitoptimumpropertieswhentheirferritetoaustenite(α/γ)volumeratiosareabout1:1[2]andnoothertertiaryphasesarepresent.However,duringfabrication,suchasweldingandotherthermaltreatments,theferritetoaustenitevolumeratiosmaybechangedalongwiththeprecipitationofvariouscompounds(e.g.chromiumcarbidesandnitrides)andsomeotherintermetallicphases(likesigma(σ),chi(χ),secondaryaustenite(γ2))[3–5].Thesemicrostruc-turalmodificationshaveadetrimentaleffectonmechanicalandcorrosionresistancepropertiesoftheDSS.Asignificantamountofwork,thatstudiedtheeffectofheattreatmentandprecipitationonthecorrosionbehaviorofcommonlyusedDSSgradessuchas2205(UNSS32205),2304(UNSS32304),and2507(UNSS32507),hadbeenpublished[6–9].Since2000,withthehighfluctuationofthenickelprice,DSS2101(EN1.4162,UNSS32101)containinglessthan2%nickelhasmadeitattractive.Thereducednickelcontentiscompensatedbyanincreaseintheamountofmanganeseandnitrogeninthesteeltostabilizetheaustenitephaseinduplexmicrostructures[10].Themolybdenumcontentofthissteelisalsosignificantlyreducedtolimittheprecipitationofdelete-riousphases.Liljas[11]inthereviewarticleonDSS2101exploredtheutilityofthisnewduplexfamilyinawidevarietyofapplications,replacingausteniticsteelsoftype304andalso,tosomeextent,structuralsteels.Westin[12]addressedtheeffectofalloyingelementsandthermalcyclesonphasebalanceandmicrostructureinthehightemperatureheat-affectedzone(HTHAZ)ofDSS2101.Theauthors[13]
alsoMATERIALSCHARACTERIZATION60(2009)1522–1528
⁎Correspondingauthor.Tel.:+862165643648;fax:+862165643648.E-mailaddress:jinli@fudan.edu.cn(J.
Li).1044-5803/$–seefrontmatter©2009ElsevierInc.Allrightsreserved.doi:
10.1016/j.matchar.2009.08.009availableatwww.sciencedirect.com
www.elsevier.com/locate/matcharinvestigatedtheeffectofannealingtemperaturesinthe1000–1200°Crangeonthemicrostructureandcorrosionbehaviorof2101leanduplexstainlesssteel.DespitesomestudiesontheDSS2101,researchaboutthemicrostructureandpropertiesofDSS2101isstilllimited[14,15].Verylittledataareavailableonthecorrosionresistance,especiallythepittingcorrosionintheexistenceofprecipitation.ExposureofDSStotemperatureabove~350°Ccanresultintheformationofprecipitates,dependingonthetemperatureanddurationofexposure[16].Thepresenceoftheseprecipitatesmayleadtochromiumdepletionintheadjacentmatrix,adverselyaffectingthelocalizedcorrosionresistanceofsteels.Sigmaphase(σ)wasfoundtohavethestrongestinfluenceontoughnessandcorrosionresistancebecauseofitsrelativelylargevolumefractionandrapidformation[17,18].Secondaryaustenite(γ2)hadlowerchromiumcontentascomparedtotheformeraustenite,resultinginapoorercorrosionresistance[19].Cr2Nparticleshadbeenbelievedtobenucleationsitesforpitting[20].Therefore,theexaminationoftheeffectofprecipitationonthepittingcorrosionwithinDSSwouldbeveryeffectiveinimprovingtheiroverallcorrosionresistance.Zhang[21]foundthatDSS2101agedat700°Chadminimumimpactenergywhensubjectedtothermalagingattemperaturesfrom500to1050°Cfor30min,andinvestigatedtheprecipitationbehaviorofDSS2101agingat700°Canditseffectonthemechanicalproperties.ThisaiminthisworkconcernedtheinfluenceofprecipitationonthecorrosionbehaviorofDSS2101agingat700°Cinchloridesolution.2.ExperimentalThematerialwasahot-rolled2101duplexstainlesssteelplate(EN1.4162,UNSS32101)withthefollowingchemicalcompo-sitioninwt.%:21.4Cr,5.7Mn,1.2Ni,0.31Mo,0.23N,0.03C,0.4Si,0.019P,0.005S,balanceFe.Thespecimensweresubjectedtosolution-annealedat1050°Cfor30min,followedbywaterquenching.Then,theywereisothermallyagedat700°Cfordifferenttimesinthe3minto240minrange,andquenchedbywater.TheelectrochemicalmeasurementwascarriedoutwithaPARSTAT2273potentiostat.Aplatinumfoilandasaturatedcalomelelectrode(SCE)wereusedasthecounterelectrodeandreferenceelectrode,respectively.Unlessotherwisestatedinthispaper,allpotentialsrecordedwerereferredtoSCE.Thespecimensactingastheworkingelectrodeswereembeddedinepoxyresin.Priortoeachexperiment,theworkingelectrodewasgroundmechanicallyusingsuccessivegradeemerypapersupto1500grit,polishedwith1.5μmaluminawatersuspension,rinsedwithdistilledwateranddriedinair.Inordertoavoidcrevicecorrosion,theinterfacesbetweenspecimenandresinweresealedwithspecialsilicagelsealantanddriedintheair.Theexposedelectrodesurfaceareawas1cm2.Thetestsolution,1molL−1NaCl,wasmadeupfromanalyticalgradereagentsanddistilledwater.Potentiodynamicpolarizationmeasurementwascon-ductedat30±1°Cwithascanrateof0.33mVs−1,from−300mVSCE(lowerthantheopencircuitpotential)tothepotentialwherethecurrentindicatedthatstablepittingortranspassivityhadoccurred.Pittingpotential(Ep)wasidenti-fiedasthepotentialwherethecurrentdensitycontinuouslyexceeded100μAcm−2.Ontheotherhand,potentiostaticmeasurementwasperformedtoobtainthecriticalpittingtemperature(CPT)byapplyingananodicpotentialof250mVSCEandcontinuouslyincreasingtheelectrolytetemperatureatarateof1°Cmin−1untilstablepittingoccurred.TheCPTwasdefinedasthetemperatureatwhichthecurrentdensityreached100μAcm−2.Theelectrolytewasbubbledwithpurenitrogengas(N2)togetridoftheoxygengas(O2)throughoutthewholetest.Eachtypeofelectrochemicalmeasurementswasrepeatedatleastthreetimes.Anycrevicecorrosionobservedonthespecimenaftertestingmeansthatthetestresultswereinvalidandmustbediscarded.Forthemicrostructurestudy,theagedspecimenswereelectrochemicallyetchedina30%KOHsolution.Thisetchingprocessmadeprecipitatesappearblack,austenitephasebrightandferritephasegray.Scanningelectronmicroscope(SEM,FEIQuarter400)withaRobinsonbackscatteredelectrondetectorwasusedtoobservethemicrostructureevolutionduringisothermalagingandthemorphologyofpits.Mean-time,thecompositionofdifferentphaseswasanalyzedbyenergydispersiveX-rayspectroscope(EDS),whichattachedtoscanningelectronmicroscope(SEM).3.ResultsandDiscussion3.1.MicrostructureFig.1showsaseriesofmicrographsofthespecimensagedat700°Cforvariousagingtimes.Thesolution-annealedmaterialwascharacterizedbyaribbonlikemicrostructure,consistingofelongatedausteniteislandsintheferritematrix.NointermetallicprecipitatewasseeninFig.1(a).Agingat700°Cfor6min,fewofnucleatedintermetallicprecipitateswereobservedattheα/γinterfaces.Afteracertainagingtimeof30min,theintermetallicprecipitatesalongthephasebound-arywerenoticeableandsomeprecipitateswerealsoobservedatα/αinterfacesinthespecimen.Withthefurtherincreaseofagingtimeto240min,theamountofintermetallicprecipi-tatesincreaseddrasticallyalongtheα/γinterfacesandα/αinterfaces,asshowninFig.1(d).Forthecompositionanalysisofdifferentphases,EDSwasperformedontheunetchedspecimenagedat700°Cfor240min.Toobtainreproducibility,10differentareaswereanalyzedandtheaveragecompositionofthesephaseswasgiveninTable1.FromthemicrographsandEDSresults,itcouldbefoundthattheprecipitatesformedintheα/γinterfacesandα/αinterfaceshadahigh-chromiumcontent.Meantime,thelighterregionneartheprecipitatesshowedlowerchromiumcontentascomparedtootherphases,i.e.ferritephase,austenitephaseandprecipitates.Thislow-chromiumregionwasregardedastheso-calledsecondaryaustenitephase(γ2)[22].Inrelatedworkperformed[21],theprecipitatesformedinDSS2101,agedat700°C,wereidentifiedasCr2NbyusingX-raydiffractionanalysis(XRD)andtransmissionelectronmicroscope(TEM),togetherwithThermo-Calcsoftware.Hence,fromtheexperimental
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