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This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID : 2300
To link to this article : URL : http://dx.doi.org/10.1016/j.corsci.2007.07.014
To cite this version : Lima - Neto , Pedro de and Farias , Jesualdo P. and Herculano , Luis Flávio G. and Miranda, Hélio C. de and Araújo, Walney S. and Jorcin, Jean-Baptiste and Pébère, Nadine ( 2008) Determination of the sensitized zone extension in welded AISI 304 stainless steel using non-destructive electrochemical techniques. Corrosion Science, vol. 50 (n° 4). pp. 1149-1155. ISSN 0010-938X
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DeterminationofthesensitizedzoneextensioninweldedAISI304stainlesssteelusingnon-destructiveelectrochemicaltechniques
PedrodeLima-Netoa,*,JesualdoP.Fariasb,LuisFlávioG.Herculanoa,b,HélioC.deMirandab,WalneyS.Araújob,Jean-BaptisteJorcinc,NadinePébèrec
a Departamento de Química Analítica e Físico-Química, Universidade Federal do Ceará, Campus do Pici, Bloco 940, 60455-970, Fortaleza, CE, Brazilb Departamento de Engenharia Metalúrgica e Materiais, Universidade Federal do Ceará, Campus do Pici, Bloco 714, 60455-970, Fortaleza, CE, Brazil
c CIRIMAT – UMR CNRS 5085, ENSIACET, 118 route de Narbonne, 31077 Toulouse cedex 04, France
Abstract
Extensionofsensitizedzone(SZ)inweldedAISI304stainlesssteelwasdeterminedbytwonon-destructiveelectrochemicaltests:doubleloopelectrochemicalpotentiokineticreactivationtechnique(DLEPR)andlocalelectrochemicalimpedancespectroscopy(LEIS).Weldingwascarriedoutusingtheshieldedmetalarcwithtwoselectedweldingenergies:thefirstone(0.7kJmm¡1)doesnotpromotethesensitiza-tionofthe304steelanditconstitutesthereferencesampleandthesecondone(2.2kJmm¡1)whichleadstotheprecipitationofchromiumcarbidesinthegrainboundariesaftertheweldingprocess.Thenon-destructiveDLEPRandLEIStestsallowedthelengthoftheSZtobedeterminedandagoodagreementbetweenthetwotechniquesandthemicrostructureofthetwoweldedsampleswasshown.Thepresenceofaninductivelooponthelocalimpedancediagramsseemstoreflectagalvaniccouplingbetweentheweldstring(anode)andtheweldedstainlesssteelplates(cathode)whichwillbeveryprejudicialtoagoodcorrosionresistanceoftheweldedsystem.Theresultsshowedthatthetwoelectrochemicaltestscouldbeappliedinpracticalcasesinindustrialfield.
Keywords: A.Stainlesssteel;B.EIS;B.SEM;C.Welding;C.Intergranularcorrosion
1. Introduction
Itiswidelyknownthatweldedausteniticstainlesssteelscanpresentasensitizedzone(SZ)intheheataVectedzone(HAZ).This isawell-knownphenomenonandconsistsofcarbide precipitation at grain boundaries and chromiumdepletioninadjacentregions,makingthematerialsuscep-tible to intergranular corrosion.The intensityof thisphe-nomenonisaVectedby,amongotherfactors,theamountofcarboninthebasematerialandintheweldedmaterial,thewelding process, the welding energy, the exposure time ofthematerialtothetemperatureinthatsegregationphenom-enatakeplaceandthecarbidemakerelements[1].
doi:10.1016/j.corsci.2007.07.014
* Correspondingauthor.Tel.:+558533669956;fax:+558533669982.E-mail address: [email protected](P.deLima-Neto).
Usually,thesensitizedregionisdeterminedbymethodolo-giesmentionedinASTMA262StandardPractice.However,thisstandarddoesnotquantifythesensitizationdegree,thetestsareofdiYcultpreparation,exclusive foruse in labo-ratoryand theyaredestructive.Foran industrialpracticepoint of view, it is necessary to develop non-destructivemethodsandtestsfortheassessmentofsensitizedzoneinaweldedstainlesssteelonitssiteofoperation.
Doubleloopelectrochemicalpotentiokineticreactivationtest(DLEPR)isapowerfultoolusedrecentlyinourlabo-ratory to evaluate the sensitization toquantify thedegreeofsensitizationoffiveausteniticstainlesssteels(304,304L,316L,321and347)heattreatedintherangeof400°C–600°Cforuntil100hours [2,3], tostudythe lossof thecorrosionresistanceoftheUNSS31803duplexstainlesssteelagedatlow temperatures (350–550°C) [4] and finally, to assessthe eVect of the low-temperatureaging in the corrosion
resistanceofAISI444stainlesssteel[5].Theadvantagesofthistechniqueare:itisquick,non-destructive,couldbeusedforin situmeasurementsandtheresultsareindependentofthesurfacefinishing.
Localelectrochemicalimpedancespectroscopy(LEIS)isalsoanothernon-destructiveelectrochemicaltechniquethathasbeenusedinrecentyearstostudylocalizedcorrosiononbaremetalsurfaceandoncoatedalloys[6,7].Additionally,sensitizationisa localizedphenomenonwhichturns inter-estingtoapplytheLEIStechniquetolocalizethesensitizedregioninaweldingstainlesssteel.
Thus,theaimofthepresentworkistoevaluatethepoten-tialityoftheDLEPRandLEIStechniquestodeterminetheextensionofthesensitizedareainaweldedAISI304stainlesssteel.Thestudywascarriedoutusingtwoselectedweldingenergies:0.7kJmm¡1,whichdoesnotpromotethesensitiza-tionofthe304steelandusedasreference,and2.2kJmm¡1to obtain the precipitation of chromium carbides in thegrainboundariesaftertheweldingprocess.
2. Experimental
2.1. Material and welding process
AISI 304 sheet, obtained from hot lamination process,wassuppliedbyACESITA(Brazil).Thechemicalcompo-sition was given by the manufacturer and is presented inTable1.ThecompositionisinagreementwiththestandardlimitsfortheAISI304stainlesssteel.Thesteelswereweldedusingshieldedmetalarcwelding (SMAW)techniquewiththeARCHESAWSE–308L–16electrodehaving3.25mmdiameter.TheweldingparametersarepresentedinTable2.
2.2. Metallographic etchings
MetallographicetchingaccordingtoASTMA-262wasperformed.MicrographswereacquiredusingaPhilipsXL-30scanningelectronmicroscope(SEM).Theobtainedmicro-structureswereclassifiedintothreetypes:“step”structurewithnoditchesatgrainboundaries;“dual”structure,withsome ditches at grain boundaries; and “ditch” structure,withoneormoregrainscompletelysurroundedbyditches.
Table1ChemicalcompositionofAISI304(%wt)
Element C Mn Si P Cr Ni Mo
wt% 0.02 1.39 0.46 0.03 16.82 10.06 2.03Element V Nb Sn Ti N W Cowt% 0.04 0.02 0.01 0.01 0.03 0.03 0.05
Table2Weldingparameters
Current/A Voltage/V Travelspeed/mmmin¡1
Weldingenergy/kJmm¡1
92 24 200 0.7132 28 100 2.2
SEM micrographs showing the microstructures, classifiedaccordingASTMA-262,canbefoundelsewhere[3].
2.3. DLEPR tests
TheelectrochemicalcellwaspositionedontheHAZofweldedsteeltoanalysetheextensionofthesensitizedregion.Fig.1showsadrawofthesystemusedfortheDLEPRmea-surementsandanOringwasusedtomaintainthesolutioninthecellexposinganareaof1mm2.Fig.2ashowsaschemaofthediVerentpositionsoftheplatewheretheDLEPRmea-surementswereperformed.
Priortoeachexperiment,theweldedAISI304steelsur-face was polished with 400 grit emery paper, degreasedwith ethanol and cleaned in water. The working solutionwas0.5MH2SO4+0.01MKSCN.TheDLEPRtestswereconducted using a Pt foil as the auxiliary electrode and asaturatedcalomelelectrode(SCE)asthereferenceone.Theexperiments were started after nearly steady-state opencircuit potential (Eoc) had been reached (about 10min)followedbythepotentialsweep in theanodicdirectionat1mVs¡1untilthepotentialof0.3V/SCEwasreached,thenthescanwasreverseduntiltheEoc.Apotentiostat/galvano-statAUTOLABPGSTAT30,linkedwithaPCmicrocom-puterandcontrolledby theGPESsoftware,wasused foracquisitionoftheelectrochemicaldata.
2.4. LEIS measurements
LEIStechniquewasusedtocorrelatethelocalelectrochem-icalbehaviourwiththemodificationofthemicrostructurein
Fig.1.DrawoftheelectrochemicalcellusedtoevaluatetheextensionofthesensitizedregioninweldedAISI304SS(DLEPRtests).
Fig.2.Schematicrepresentationsoftheweldedplate.Forthetwoelectrochemicaltechniques,thediVerentpositionswherethemeasurementswerecarriedoutareindicatedonthescheme.
Fig.3.SEMimagesshowingatypicalmicrographofthe304SSweldingwith 0.7kJmm¡1. The micrograph was observed at 4mm from the weldstring.
theHAZofthestudiedweldedmaterial.LEISmeasurementswerecarriedoutwithaSolartron1275system.Thismethodusedafive-electrodeconfigurationanddetailscanbefoundelsewhere[6,7].Asinsuchaconfigurationthelocalmeasuredcurrentdependsontheconductivityoftheelectrolyte.Thus,theexperimentswerecarriedoutin0.001MNa2SO4inorderto improve the resolutionof themethod.The local imped-ancediagramswereobtainedeach0.5mmperpendicularlytotheweldstring(Fig.2b)andwererecordedoverafrequencyrangeof10kHztoaround100MHz.Withtheusedexperi-mental set-up, only the normal component of the currentwasmeasuredandthespatialresolutionwasestimatedtobeabout1mm2foreachmeasurement.
3. Results and discussion
3.1. Microstructure characterization
Fig.3showsaSEMimageofthe304SSsurface,weldedwith 0.7kJmm¡2, and obtained at 4mm from the weldstring.Itcanbeobservedtheabsenceofditchesonthegrainboundaries.TheobservedmicrostructurewasrepresentativeofthatobservedatdiVerentplacesintheHAZofthewelded304SS.Thissurfacemorphology,accordingtoASTM262-
A, is classified as step and associated to a non-sensitizedstainlesssteel.
The modification of the microstructure of the 304 SS,welded with 2.2kJmm¡2, with the distance from the weldstring is shown inFig.4.Themicrostructureat2mmfrom
Fig.4.SEMimagesshowingthemicrostructureatdiVerentdistancesfromtheweldstingforthe304SSweldingwith2.2kJmm¡1:(a)2mm;(b)3mm;(c)6mm;(d)9mmand(e)10mm.
theweld string (Fig. 4a) is similar to thatobserved for the304SSweldingwith0.7kJmm¡2andit isclassifiedasstep.Thisisanindicativethatfromtheweldstringtothisdistance,thetemperatureishigherthan800°C.Fromtheliterature,itisknownthattemperatureshigherthan800°Cpromotethesolubilisationofthechromiumcarbidesinthegrainmatrix,avoidingthesensitization[1].Ontheotherhand,themicro-structureat3mmfromtheweldstring(Fig.4b)presentssomeditchesaroundthegrainscharacterizingadualmicrostruc-ture.ThisindicatesthatthebeginningofthesensitizedzoneintheHAZisatabout3mmfromtheweldstring.At6mmand9mmfromtheweldstring(Fig.4candd),thegrainsare
entirelysurroundedbyditchesandthemicrostructureclassi-fiedasditch,indicatingthattheseregionsofHAZaresensi-tized.TheimagesFig.4banddsuggestthatthetemperatureofthisHAZregionisbetween800°Cand400°C,whichleadsto the precipitation of the chromium carbide in the grainboundaries.Finally,theSEMimageobtainedat10mmfromthe weld string (Fig. 4e) shows a microstructure similar tothatobservedat2mmfromtheweldstring,indicatinganon-sensitizedregioninHAZ,suggestingthatthetemperatureofthisHAZregionislowerthan400°CHAZandthattheendofsensitizedzoneisaround9mmfromtheweldstring.Thesensitizedzonehasanextensionof6mm.
3.2. DLEPR test
0 2 4 6 8 100.0
0.1
0.2
0.3
0.4
Ir /
Ia
Distance from weld string / mm
0.7 2.2
Welding energy / kJ mm-1
BSZ E
SZ
Fig.6.DependenceofIr/Iawiththedistancefromthefusionlineforthetwoweldingenergies.Thebeginning(Bsz)andtheend(Esz)ofthesensitizedregionareindicated.
TypicalDLEPRcurvesobtainedinthenon-sensitizedandsensitizedregionsareshowninFig.5.Ascanbeobserved,thediVerencebetweenplots is that thecurvesobtained inthesensitizedregionshowsawelldefinedreactivationpeakinthereversescan(Fig.5b).Thepeakthatappears inthereversescanisrelatedtothepreferentialbreakdownofthepassivefilmcoveringthechromium-depletedregionofthesteel.Inthistest,thedegreeofsensitizationismeasuredbydeterminingtheratioIr/Ia,whereIristhemaximumcurrentofthereversescanandIaisthemaximumcurrentinthefor-ward(anodic)scan[1].
Thevariationofthegradeofsensitization(Ir/Ia)withthedistancefromtheweldstringforthetwoweldingenergiesis shown in Fig. 6. The curve shows that the steel weldedwith theweldingenergyof0.7kJmm¡1presents the lowerIr/Iavalueswhichremainapproximatelyconstantatabout0.004,independentlyofthedistance.Ontheotherhand,thevalueofIr/Iaforthesteelweldedwith2.2kJmm¡1initiallyisabout0.005until30mmfromtheweldstring,followedbyanincreaseoftheIr/Iavalueswiththedistancereachingamaximumvalueofabout0.4at7mm,andfinally,decreases
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
0.000
0.003
0.006
0.009
0.012
0.015Ia
Ir
I / m
A
E / (V vs. SCE)
E / (V vs. SCE)
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
0.0
0.1
0.2
0.3
0.4
0.5
0.6Ia
Ir
I / m
A
a
b
Fig.5.DLEPRcurvesobtainedinthenon-sensitizedregionat2mmfromtheweldstring(a)andinthesensitizedregionat6mmfromtheweldstring(b)ofthe304stainlesssteelweldingwith2.2kJmm¡1.
forhigherdistancevaluesuntiltoreachtheminimumvalueof0.005at9mmfromweldstring.FromFig.6itwascon-firmedthat:(a)theweldingenergyof0.7kJmm¡1wasnotenoughtopromotethechromiumcarbideprecipitationintheHAZ;(b)thesteelweldedwiththeenergyof2.2kJmm¡1presents precipitation of chromium carbide in the HAZ.Theseresultsare incloseagreementwiththosepreviouslypublishedbyMajidietal.[8].Thus,thebeginning(Bsz)andtheend(Esz)ofthesensitizedzonecanbedeterminedandareindicatedinFig.6.Theextensionofthesensitizedregioninthesteelweldedwith2.2kJmm¡1isabout6mm,whichisingoodagreementwiththeSEManalysis.
3.3. LEIS test
Fig.7showsexamplesofsomelocalimpedancediagramsobtained for the AISI 304 SS welded with both weldingenergy. The three diagrams in each plot were obtained atdiVerentpositionsfromtheweldstring.Forbothsystems,thediagramsarecharacterizedinthehighfrequency(HF)rangebyacapacitivearc(notclearlydefined)followedbyan inductive loop, and in the lower frequency range (LF)byanothercapacitivepart.Theoriginofthefirstcapacitiveloopisnotyetexplained.Thepresenceofaninductivelooponthelocalimpedancediagramwasrecentlyexplainedbythe influenceofcurrentandpotentialdistributionsassoci-atedwiththegeometryoftheelectrode[9–11].Inaddition,inarecentwork,inductiveloopsintheHFpartofthelocalimpedancediagramswerealsoobservedforthecouplingofpurealuminiumwithpurecopperwhentheprobeanalyzesthecopperbehaviour[12].Inthiscase,thepresenceoftheinductiveloopwasconnectedtotheexistenceofagalvaniccoupling between the two materials, with the Cu playingtheroleofcathode.ThecouplingbetweenCuandAlwouldleadtoparticularcurrentandpotentialdistributionsassoci-atedwiththisconfiguration(electriceVect).Byanalogywiththiswork,itisprobablethatinthepresentstudyagalvaniccouplingphenomenonbetweentheweldstringandthestain-
Fig.7.Localimpedancediagramsobtainedonthewelded304SSsurfaceatdiVerentdistancesfromtheweldstring:(a)weldingwith0.7kJmm¡1and(b)weldingwith2.2kJmm¡1.
Fig. 8. Comparison of the normalized impedance modulus measured at0.5Hzwiththedistancefromtheweldstring.
lesssteelisalsooccurring.Thisassumptionwasreinforcedbythevisualobservationoftheplatesaftertheelectrochemi-caltests.Corrosionproductswereclearlyvisibleclosetotheweldstring,suggestingthatthestainlesssteelplatecouldbeactingascathodeinthecorrosionprocess.
Independentlyoftheweldingenergy,thediagramsplot-ted close to the welding (d=0mm) are relatively similar.ThisisexplainedbythefactthatthiszoneisaVectedinthesamewaybytheweldingprocess.InFig.7a,itcanbeseena purely capacitive behaviour (low frequency part of theimpedancediagrams)whentheprobemovesawayfromtheweldstringandadditionally,thediagramsarerelativelysim-ilarforthetwopositionsoftheprobe.Onthecontrary,fortheAISI304SSweldedwith2.2kJmm¡1,afrequenciesshiftisobservedontheLFcapacitivepartofthediagramswhenthepositionoftheprobeischanged(Fig.7b).
Tocomparemoreeasilytheresultsobtainedforthetwosamples,thefrequencyof0.5Hz,correspondingtothecapac-itivepartinLFrange,waschosen.Fig.8presentsthevaria-tionofthemodulusofthenormalizedimpedanceat0.5Hzalong a line perpendicular to the weld string for the twoweldingenergies.Measurementscarriedoutontheweldedplatewith0.7kJmm¡1constitutedthereference.Theimped-ancemodulus for the reference increasesgraduallyon thefirsttwomillimetresstartingfromtheweldstring,thenitsvalue remains relatively constant. For the welding energyof2.2kJmm¡1,thevariationoftheimpedancemoduluscanbe clearly connected to the localization of the sensitizedregion, since it is observed a reduction in the impedancemodulusforthesensitizedregion,comparedwiththeSEMimage(ingreyedonthefigure).ItisobservedinFig.3cthattheweldingmicrostructureofthestainlesssteelisthemostsensitized at 6mm. This point corresponds to the lowestvalueoftheimpedancemodulusonFig.8.At10mmfromtheweldstring,themicrostructureistypicalforanon-sen-sitizedmaterial (Fig.3e).Thispointcorresponds inFig.8toahighervalueofthemodulus.Theincreaseinthemod-ulus inthevicinityoftheweldstringis inagreementwiththemicrostructureobservedontheFig.3a,whichshowedthatinthisHAZregion,thechromiumcarbideprecipitatesweredissolvedinthegrainmatrix.Fortheweldingenergyof 2.2kJmm¡1, the values of the modulus are lower thanthoseobtainedfortheweldingenergyof0.7kJmm¡1.ThisresultcanbeexplainedbysomediVerencesinthepropertiesoftheoxidesformedonthestainlesssteelsurface.ThesameconclusionsfromFig.8canbemadebyfollowingthevaria-tionoftheimpedancephasealongthelineperpendiculartothewelding,showninFig9.IntheHAZ,phasevaluesarestronglydiVerentiatedcomparedtothoseobtainedforthesystemwhichisnotsensitized.
Thus,theanalysisofthelocalimpedancediagramshowsthat the sensitized zone in the HAZ was detected by theLEIS technique at a frequency of 0.5Hz. To visualize the
Fig.9.Comparisonof the impedancephasemeasuredat0.5Hzwith thedistancefromtheweldstring.
HAZ, it is necessary to work in a well defined frequencyrange (about the Hertz), which corresponds to interfacialphenomena.
4. Conclusions
Thenon-destructiveDLEPRandLEIStestsallowedthelengthoftheSZtobedeterminedandaverygoodcorrela-tionbetweenthetwotechniquesandtheASTM262Stan-dard Practice was observed. The presence of an inductivelooponthelocalimpedancediagramsprobablyreflectagal-vaniccouplingbetweentheweldstring(anode)andplatesoftheweldedstainlesssteel(cathode)whichwillbeverydetri-mentalforagoodcorrosionresistanceoftheweldedsystem.
Inthefuture, itwillbenecessarytoobtainmoreinforma-tionfromlocalimpedancespectraparticularlyregardingthehigh frequencypartof thediagrams.Theresultsobtainedshow the feasibility of developing the DLEPR and LEIStestsinindustrialfield.
Acknowledgments
TheauthorsthankCNPq,ANP,FINEPandFUNCAP,Brazil,forfinancialassistance.WalneyS.AraújoalsothanksCNPq/CNRSproject490105/2004-1.
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