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Localized Corrosion and Phase Transformation of Simulated Archaeological Iron

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Localized Corrosion and Phase Transformation of Simulated Archaeological Ironof,OF,and,Phase,phase

    Localized Corrosion and Phase

    Transformation of Simulated

    Archaeological Iron

    ChineseJournalofChemicalEngineering,16(2)299.305(2008)

    LocalizedCorrosionandPhaseTransformationofSimulated

    ArchaeologicalIron

    WANGZise(王紫色),XUChunchun(许淳淳)andDONGXiqing(董希青)

    SchoolofMaterialScienceandEngineering,BeijingUniversityofChemicalTechnology,Beijing100029,China

    AbstractThecorrosionbehaviorsofsimulatedarchaeologicalironinsolutionf0.06mo1.LNaC1+0.03mol?L

    Na,SO4+0.01mol?L

    NaHCO)simulatingsoilwatercompositionwasinvestigatedbypotentialdynamicpolariza

    tion,constantpotentialpolarization,andsimulatedoccludedcell(0.C.)galvanostatictests.Xraydifiraction(XRD),

    energYdispersivesizectrometry(EDS),andscanningelectronmicroscope(SEM)wereusedtostudythecorrosion

    morphologyandtheevolutionofcorrosionproduct.Theobiectivewastodiscoverthe

    ansformationprocessof

    archaeologicaliron.anddeterminethedistributionofchlorinatedcorrosionproducts.Theresultsshowedthatthe

    presenceofcrevice,cavities,andchannelsfacilitatesthelocalizedcorrosionunderrusts;theautocatalyticeffectin

    creasestheconcentrationofFle,Cr,andS0

    .andpromoteslocalacidificationwithinthecrevicesandcavities.

Meanwhile,thephasetransformationofcorrosionproductsisconcludedtoproceedbymeans

    oftwoways.Oneis

    mattheferrousionsareansformedintodifferentkindsofFle00HviatheintermediateFe(II)

    Fle(I?)hvdroxylsalt

    (.e.GreenRusts);theotheristhattheFeionsare

    ansformedintoFeCl,,FleCl,andorangepowdersakaganeite atthecrevicesandcavities.

    Keywordscorrosion,corrosionproduct,iron,chlorine lINTRoDUCTIoN

    Thousandsofironartifactsareexhumedeach

    yearallovertheworldfromarchaeologicalsites.Most ofthesearealwaysruinedowingtotheattacksofthe soilenvironmentalfactorsforthousandsofyears.Soil isacomplexenvironment,thepropertiesofwhichare determinedbythegeologicalfactors(soiltypes.pH. electricalresistivity,etc.1,hydrologicalfactors(water flowthroughgroundlayers).andthesoilchemical composition.Eachoftheseplaysaroleinthecorro. sionprocessf11.Therefore,itisdifficulttostudythe corrosionmechanismofarchaeologicalironinthesoil environment.Thecorrosionmechanismofarchaeo. 1ogicalironwasgenerallyconsideredastheselective dissolutionofcastironf2.Nevertheless,thedetailis

    stillfarfromwellunderstoodforlackofexperimental evidence.Now,variousanalyticaltechniquessuchas electronprobemicro.analysis(EPMA).X.raydiffrac

    tion(XRD),Fouriertransforminfraredspectroscopy fFTIR1canbeemployedtoanalyzethecorrosion products,whichwillprovidemoreopportunitiesto explorethecorrosionmechanism.

    Chloridesareoftencarriedbymostoftheiron artifactsexcavatedfromcoastalorinlandregions3.

    Thedeteriorationofarchaeologicalartifactsisoften associatedwiththepresenceofchlorideionsf461.

    Manypublishedresearcharticlesf71havementioned thatorangepowdersaredepositedinthecrevicesof ironartifactswithyellowliquiddroplets.Thedroplets containedFe,Fe",andCl,andthePHwasabout1 orless.Therefore.itisimportanttounderstandthe corrosionprocessesrelatedtochlorineandthelocal distributionofchlorinatedcorrosionproductsinin. vestigatingthecorrosionmechanismsofarchaeologi

    calironandthemorphologicalandphysicochemical propertiesofthecorrosionproducts.

    ThecorrosionofarchaeOlOgicalartifactsinsoil hasbeenstudiedfordifferentpurposes.Itnotonly providesinformationfortheestimationofironcorro

    sionbehaviorinsoil,particularlytoobtainaverage corrosionrates,butalsocontributestopreventdegra

    dationoftheobiectafterexcavationandensureits reliablerestoration.

    Inthearticle,potentialdynamicpolarization,con

    stantpotentialpolarization.andsimulatedoccluded cellf0.C.1galvanostatictestswereusedtoinvestigate thechemicalandelectrochemicalbehaviorsofsimu. 1atedarchaeologicalironinsolutionf0.O6mo1.L NaC1+0.03mo1.L-.Na,S04+O.01mo1.L-.NaHCO1 simulatingsoilwatercomposition.X.raydiffraction (XRD),energYdispersivespectrometry(EDS),and scanningelectronmicroscope(SEM1wereusedto

identifythecompositionofthephaseandthemicro

    structure.Thestudyaimsatdiscoveringthetransfor. marionprocessofarchaeologicaliron,anddetermines thedistributionofchlorinatedcorrosionproducts. 2EXPERIMENTAL

    MostofthearchaeologicalironinancientChina wasgreycastiron.Castiron,withthemasscomposi

    tion(%):C3.OO,Si1.84,S0.089,P0.098,Mn0.82, Febalance.wasusedtosimulatearchaeologicaliron. Thesampleswereprocessedtobe15ITIInX15mrnx3 iilincouponand3mmx3ITIInX101TlInbar.Thesur. facesofthesesampleswerepolishedwithemerypa

    persupto1000gradefollowedbythoroughrinsingin de.ionizedwaterandacetone.Thecorrosionmedia was0.O6mo1.Lcr+O.03mo1.LS0+O.O1

    mo1.LHC0solutiondilutedfromARgradeNaC1, Na2SO4,NaHCO3withde.ionizedwater.20%(vol

    umeratio)H2SO4andNaOHwereusedtoadjustthe Received20070716,accepted20071024.

    SupportedbytheNationalKeyTechnologiesResearchandDevelopmentProgramofthe10th

    Five-yearPlanPeriod(2001BA805B01)

    Towhomcorrespondenceshouldbeaddressed.Email:phdwzs@163.com

    Chin.J.Chem.Eng.,Vo1.16,No.2,April2008 pHto7toeliminatetheeffectofpHontheexperiment. 2.1Potentialdynamicpolarizationcurves ThecorrosioncellusedwasanEG&GPrinceton threeelectrodesystem.15i/lnl~15mmx31TIITIcast ironcouponpolishedwastheworkingelectrodewith anexposureareaof1cm.Aplatinumelectrodeanda

    saturatedcalomelelectrode(SCE)wereusedasthe auxiliaryandthereferenceelectrodes,respectively. ThereferenceelectrodewasconnectedtotheCOrrO

    sioncellwithasaltbridge.Polarizationmeasurements werecarriedoutonaCorrtestSystemfmadeby HuazhongUniversityofScienceandTechnology, China).Ouasisteadystatepolarizationcurveswere obtainedbyscanningcheelectrodepotentialanodi

    callyfrom1.4V(SCE)to+2V(SCE)attherateof O.5mV.s.ortoeachscanning.thespecimens

    werekeptat1.OV(SCE)for3mintoremovethe

    passiveoxidefilm.

    2.2Constantpotentialpolarization

    Thesampleswerekeptrespectivelyat+O.1V (SCE)and+1.2V(SCE)for2h.Thecorrosioncell andthesampleusedweretheSalTleasthatforpoten

    tialdynamicpolarization.

    2.3Simulatedoccludedcellgalvanostatictest Thesetupusedintheexperimentisdescribedin Fig.1.Theexposedareaoftheoccludedspecimen f3mmx3mlTl~10miYlbar1wasabout50miYl'withthe remainingsurfaceshieldedbysiliconepaste.Surface pretreatmentofthespecimenwasthesameasbefore. Theoccludedspecimenandthebulkspecimenhave thecompositionincommonwithanexposedareara

    tioof1:8O.Theglasscontainerwasfilledwith2000 mlofthesolutionanditspHwasadiustedto7.OO.The occludedchannelwaspluggedwithfilterpaperpulp. About1mlofthebulksolutionwasiniectedintothe 0.C.Anodiccurrentof0.5mA.cm'wasappliedto

    theoccludedspecimentosimulatethegalvanostatic currentinandoutofthe0.C.TheconcentrationsofC1.

    S0,andpHintheO.C.weremonitoredatdifferent timeintervals(whenthepHislessthan6.HC0i doesnotexistinthesolutionanddoesnotneedto measureitsconcentrationf81).ThepHvalueofthe bulksolutionwasalsomonitoredbyapHs225pH

    meteratambienttemperature.TheconcentrationofCl wasanalyzedusingthepotentiometrictitrationtech

    nique.Theconcentrationofsolwasanalyzedusing

    animprovedgravimetricmethod.

    2.4X-raydiffractiontest

    Thesamplesafterconstantpotentialpolarization andsimulatedoccludedcelltestwererinsedby deionizedwateranddriedforXRDtest.X.raydif- fractionfD,MAX25OOPCmodel1wassetupasfo1. 1ows:scanspeed1O(.).min1.scanrange3.to90., radiationsourceCuK,operatedat5OkVand200 mA.

    2.5SEM/EDSanalysis

    Thesamplesafterconstantpotentialpolarization andsimulatedoccludedcelltestwereobservedby SEMofCAMBRIDGE$250,andthecomposition wasanalyzedbyEDSofLINKAN10000.

    3RESU1SANDDISSCUSIoN

    3.1Greenrust

    Thepolarizationcurveofcastironinsimulated soilsolutionisshowninFig.2.Itseemedthatthean. odiccurvewascharacterizedbythestateofactive dissolution.Afterthepotentialinexcessof+1V-the

    currentdensitywasmaintainedatabout1OA-cm' andseldomvaried.Atthistime.f1ocksofgreenrust wereobservedinthesolution.Theseweresuspended inthesolutionneartheelectrode.andwerecontinu

    ouslydiflusedfromtheelectrode.Thetrendofthe curveindicatedthatthegreenrusthadlittleprotection totheelectrode,andcastironstilldissolvedwiththe velocityofabout10A.cm.Afterscanning.agreat

    dealofgreenrustandwhiteprecipitatesinthesolu

    tionwasfound.

    >

    

    J

    Figure2Polarizationcurveofcastironinsimulated soilsolution

    Chin.J.Chem.Eng.,Vo1.16,No.2,April2008301 Tomakeclearthecompositionandtheformation processofwhiteprecipitatesandgreenrust,thesam

    pieswerepolarizedfor2hin+0.1VrSCE,point1in Fig.2)and+1.2V(SCE,point2inFig.2)andthen rinsedbyde.ionizedwateranddriedforXRDtest. TheXRDtestrevealedthatthegreenrustonthe samplepolarizedat+1.2VwasmadeupofGR(C1, andGR(S0),wit}lthemolecularformulaFe

    Fe"(OH)8C1?2H20[9]andFeFe(OH)12S04?2H2O f10],respectively.Cementiteremainedonthesample surfaceaftercorrosion,andthus,therearepeaksof Fe3CinFig.3.

    20/(.)

    Figure3XRDspectraofsamplespolarizedat+O.1V(a) and+1.2V(b)

    l~greenrust(CI);2--greenrust(SOl);BFe3c;I_Fe00H

    Thedissolutionofcastironandthedepolariza

    tionofoxygenoccurredinthecorrosionasbelow: FeFe+2e(1)

    02+H20+4e40H(2)

    Hence,ferroushydroxideFe(OH)2willbesus. pendedinthesolution.Inthepresenceofvariousani. onssuchasCl,S0,orHC0,theaqueousSUS.

    pensionofferroushydroxideFe(OH1,usuallyoxi. dizesintoferricoxyhydroxidesviaanimtermediate Fe(II).Fe(III1hydroxy1.saltdesignatedasGreen Rust(GR),e.g.GR(C1),GR(s0),orGR(c0)[11].

    Thereactions[12]are

    GR(C1):4Fe(OH)2+C1+2H20

    Fe~Fe(0H)8C1-2H2O+e

    GR(SO]):6Fe(OH)2+s0+2H20

    Greenrustonthesamplepol+0.1Vis_

    arizedat

    mainlyGR(C1),whereasGR(S0)isnotlargely

    formedatthismoment.Greenrustisunstableinair, anditwillbeoxidedintoakindofyellowrust,which isamixtureof6c.Fe00Handy-Fe00Haccordingto X.raydiffraction,asshowninFig.4.

    2O4O6O8O

    20/(.)

    Figure4XRDspectraforyellowrust

    L—吖一FeOOH;BFe3C;(1rc【一FeOOH

    3.2Themorphologyofrustsamples

    Figure5showsthemicrophotographofcastiron polarizedat+1.2Vafterremovingtherust.Thesur. faceofthesampleisloose,porous,andcrackedowing tosevereattack.asshowninFig.5(a).Referringto Figs.5(b)and5(c),theinsolublestructure,micro.cavity, andchannelareclearlyobserved.Thecross.sectionin Figs.5(d)providesaparticulardescriptionofpores andchannelsintherustediron.Thechannels,which are10——20gmwideandroughlYperpendiculartothe metal/rustinterface,connectwithothercrevicesand poresindifferentdirectionsandextendthroughtothe metal/rustinterface.Thus,itcanactasthepathfor somedissolveddeleteriOUSspeciestothemetalcore. Althoughcastironisgenerallycharacterizedby uniformcorrosion,thephenomenonoflocalizedcor

    rosionactuallystillexists.Alargenumbersofoc. cludedzones,formedbypores,crevices,andchannels belowtherustlayers.playanimportantroleinthe furtherdevelopmentofcorrosion.

    (3)3?3ThepHchangeinandouto?C?

    FeFe(0H)12SO4-2H2O+2e(4)

    GR(CO32):6Fe(OH)2+HC0;+2H20

    FFe(0H)12c0?2H20+H'+2e(5)

    Whensolorc0isprovided,GR(C1),ow.

    inganaverageoxidiationnumberforFleOf+2.25.can beoxidizedintoGR(S0)orGR(C0)compounds

    f10],Which,owingalargeroxidationnumberforFeOf +2-33.isrelativelymorestablethanGR(C11: ThesimulatedoccludedcellrO.C.1galvanostatic testisoneofthemostimportantmethodstostudythe

    localizedcorrosionbehaviors,andabundantinforma. tionaboutthechemicalchangeswithintheoccluded zonesintheproceedingoflocalizedcorrosionispro

    vided.Fig.6showsthePHvaluesVS.timeinandout oftheoccludedcel1.ItisevidentthatthepHinthe 0.C.dropscontinuouslyandreaches4.8after120h. Thisisbecausethelocalizedcorrosionoccurredinside theoccludedcel1.Thelocalizedcorrosioninsidethe occludedzonesresulted

    differentialoxygencells

    fromtheestablishmentof

    andwascontinuedbythe

    lO{Fe~Fe?(0H)8C1.2H2O}+2Fe+7(c0;orSO42)+02

    7{Fe4HFe2m(OH)12(co3~.rsol).2H2O}+4H2O+10C1(6)

    ^=lI10I_

    ^1_【?I10I_

    302Chin.J.Chem.Eng.,Vo1.16,No.2,April2008 Figure5Micrographoftherustsamplespolarizedat+1.2V 10

    9

    8

    7

    6

    5

    4

    t/h

    Figure6pHchange.timeinandoutO.C.

    _intheO.C.:?inthebulksolution

    simulationoftheautocatalysisprocess,whichpromoted

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