掃描振動電極(SVET)方法表征涂層的防腐蝕性能

1、CorrosionScience52(2010)26362642ContentslistsavailableatScienceDirectCorrosionSciencejournalhomepage:iSVETmethodforcharacterizinganti-corrosionperformanceofmetal-richcoatingsMaochengYan*,VictoriaJ.Gelling*,BrianR.Hinderliter,DanteBattocchi,DennisE.Tallman,GordonP.BierwagenDepartmentofCoatingsandPoly

2、mericMaterials,NorthDakotaStateUniversity,Fargo,ND58108,USAarticleinfoabstractThegalvanicinteractionbetweenametal-richcoatingandtheunderlyingmetalsubstratewascharacter-izedbyanewanalysismethodbasedonthescanningvibratingelectrodetechnique(SVET).Thetotalano-diccurrentatvariousimmersionperiodswasevalua

3、tedbyintegratingtheanodiccurrentdensityonSVETmaps.Zinc-richpaints(ZRPs)coatedonasteelpanelwereusedtodemonstratetheexperimentalapproach.Theanti-corrosionperformanceoftheZRPwasanalyzedbasedontheintegratedanodiccur-rentandtheexperimentalEOCiIntdiagram.Closelycorrelativebehaviourwasfoundbetweentheinte-g

4、ratedanodiccurrentandtheopen-circuitpotential.PublishedbyElsevierLtd.Articlehistory:Received17January2010Accepted12April2010Availableonline28April2010Keywords:A.Metal-richcoatingA.SteelB.SVETB.MicroelectrodetechniqueC.Galvanicinteraction1.IntroductionAsoneofthemostcosteffectivemethodsforcorrosionpro

5、tec-tionofmetallicobjects,organiccoatingsprovidecorrosionprotec-tionmainlybyfourways:abarriereffect,sacricialcathodicprotection,corrosioninhibitorrelease,andanodicprotection.Metal-richcoatings(MRCs)1,2areaclassofcorrosionprotectioncoatingscontainingsacricialmetalpigmentsthataremoreelec-trochemically

6、reactivethantheunderlyingmetalsubstrate,whichinhibitcorrosionbyprovidingsacricial/cathodicprotectiontothemetalsubstrate.MRCsaregenerallydesignedwithhighvolumefractionofmetalpigment(nearcriticalpigmentvolumeconcentra-tion,CPVC)dispersedinnon-conductivepolymerorinorganicmatrix.Themosteffectiveandcommo

7、nlyusedMRCsforsteelsareZn-richprimer(ZRP)coatings35.Mostrecently,Mg-richcoatingshavebeendevelopedandfoundtoprovidesimilarprotec-tiontoaerospaceAlalloys68.Variouselectrochemicalmethodshavebeenemployedtoassesstheanti-corrosionperformanceofmetal-richcoatings,suchascor-rosionpotentialmeasurements,electr

8、ochemicalimpedancespec-troscopy(EIS)3,912,electrochemicalnoisemethods(ENM)andgalvaniccouplingmeasurement3,13.Murray9,14reviewedelectrochemicalmethodsusedforevaluatingorganicanti-corrosioncoatings.Sekine15gaveareviewoncharacteristicsofvariouselectrochemicalmeasurementmethodsandeventheircorrelations.T

9、hedevelopmentofmicroelectrodetechniquesandscanningelec-trodetechniqueshasmadeitpossibletomeasureelectrochemicalprocessesonalocalscale,whichhasyieldednewtypesofinforma-tionrelevanttothelocalelectrochemicalprocessesoncorrodingsurfacesandadvancedtheinvestigationsoflocalizedcorrosion.Amongallthetechniqu

10、esaretheScanningVibratingElectrodeTech-nique(SVET),theScanningReferenceElectrodeTechnique(SRET),LocalElectrochemicalImpedanceSpectroscopy(LEIS),andtheScan-ningKelvinProbe(SKP).SVETwasoriginallydevisedfordetectingtheextra-cellularcur-rentnearlivingcellsinthe1970s16.Itwasrstlydevelopedtostudylocalized

11、corrosionprocessesbyIsaacsinthe1980s17,18.Theelectrochemicalprocessofcorrosioncontainsanioniccurrentowintheelectrolytebalancedbytheelectronowthroughthemetal.Theioniccurrentowcausesapotentialgradi-enttoexistinthesolutionattheelectrochemicallyactivesite.SVETwasdesignedtodetectthepotentialgradientviaam

12、ovablevibratingmicroelectrode.Theelectrodepotentialdifferencebetweenthetwoextremepointsofitsvibration,r/,isrecordedattheextremesofthevibrationamplitude,generatingasinusoidalACsignal.TheACsignalisthenconvertedtotheioncurrentdensity(i)byacalibrationprocedure10,19.Thelocalcurrentisrelatedtor/andtheelec

13、trolyteconductivitykbyOhmslaw20i¼Àjr/ð1Þ*Correspondingauthor.Tel.:fax:*Correspondingauthor.E-mailaddresses:Maocheng.Y(M.Yan),V.J.G(V.J.Gelling).0010-938X/$-seefrontmatterPublishedbyElsevierLtd.doi:10.1016/j.corsci.2010.04.012SVETs

14、ystemsaredesignedtooscillatetheprobeinaLissajousmodesothatbothparallelcomponentixandperpendicularcompo-nentizofthecurrentcanbeobtainedbypartialdifferentiationof(1)withrespecttoxorz.M.Yanetal./CorrosionScience52(2010)2636264226371.1.SVETforcharacterizinganti-corrosionperformanceofcoatingsSVEThasenjoy

15、edwideacceptanceasapowerfulelectrochemi-caltechniqueforevaluationofcorrosioninhibitor,detectionofcor-rosionactivityandquanticationofcorrosiondefectsincoatings.SVEThasbeenusedintheresearchofvarioustypesofcorrosion,suchaspitting21,cut-edgecorrosion2224,galvaniccorrosion18,microbiologicallyinuencedcorr

16、osion(MIC)25,weldcorro-sion,andstresscorrosioncracking(SCC)26.Inthecaseofcorro-sionofacoatedmetal,SVETisabletogivedetailedinsightsintotheelectrochemicalinteractionsbetweenacoatinganditssub-strateatadefect,whichhasbeenprovidedvaluableinformationontheanti-corrosionmechanismbyacoating,includingthegener

17、-ationanddevelopmentofdefects,andtheinuenceofpigments/inhibitorsoncorrosionofsubstrateatadefect27,28.Inpreviousstudiesfromthislaboratory,seriesofcoatingsforAlalloy(AA)2024-T3hasbeencharacterizedbySVET.Tomonitorboththecorrosionactivityofthesubstrateandthepossiblegal-vanicinteractionbetweenthecoatinga

18、ndthesubstrate,themea-surementwasconductedinthevicinityofascratchexposingtheunderlyingsubstrate.TheSVETresultsforpolypyrroledepositedonAA2024-T3showedthatalargeanodiccurrentoccurredatthedefectduetoanodicdissolutionofthealloyandthatthecatho-diccurrentwasratheruniformlydistributedoverthepolymersur-fac

19、e,whichimpliesthatap-dopedCPwouldpromoteactivedissolutionoftheAA2024-T3substrateatthedefectifthepassiv-ationcouldnotbeobtained29,30.Mostrecently,interestinginteractionsbetweenneutralorn-dopedpoly(2,3-dihexylthieno-3,4-bpyrazine)andAA2024-T3hasbeendemonstratedbySVET31.Then-dopedconjugatedpolymerexhib

20、itedtheabilitytosac-riciallyprotecttheexposedAlalloyinadefect.Foraredoxinactivebarriercoating,suchasaplainepoxycoat-ingonsteelorAA2024-T3,theSVETshowedthatbothanodiccur-rentandcathodiccurrentwerelocatedatthescribe32.Duetothehighimpedance/low-conductivityoftheintactbarriercoating,acompletecorrosionce

21、ll,ifany,wouldbeestablishedwithinthedefect,andnocurrentwasdistributedonthecoating.Inthecaseofmetal-richcoatings,suchasMg-richprimercoatedonAlalloys,theSVETresultsexhibitedawell-denedcathodiccurrentpeakabovethescratch.Theanodiccurrent(relatedtotheanodicdisso-lutionofthesacricialpigment)distributedont

22、heprimer,whichdemonstratesthatthesacricialpigmentfunctionsbythecathodicprotectionmechanism1,33,34.Inthiswork,aSVETmethodisprovidedforfurtherunderstand-ingthegalvanicinteractionbetweenametal-richcoatingandthemetalsubstrate,andevaluatingtheanti-corrosionperformanceofthemetal-richcoating.Azinc-richprim

23、er(ZRP)coatedonsteelwasexaminedtodemonstratetheefcacyofthemethod.SeveralcharacteristicindexesareobtainedfromtheSVETcurrentdensitymaptocharacterizethegalvanicinteractionbetweentheZRPanditssubstrate.Thetotalanodiccurrent(andhencethecorrosionrate)overthescanareaisevaluatedbyintegrationoftheoverallanodi

24、ccurrentdensityontheSVETmaps.Thevariationsoftheto-talanodiccurrentandthatofopen-circuitpotential(EOC)wereanalyzedasafunctionoftheimmersiontime.Additionally,theSVETcurrentindexeswerecomparedwiththegalvaniccurrentob-tainedbyzeroresistanceamperometry(ZRA).2.Experimental2.1.Materialsandelectrodepreparat

25、ionAnepoxyresin(Epon828,fromHexion)andamodiedpoly-amide(Epikure3175,fromHexion)curingagentweremixedina1:1.1stoichiometricratio.Thisratioresultsintheoptimalbarrierpropertiesandhardnessoftheprimerbygivingneartothemax-imumamountofcrosslinking.Methylisobutylketone(MIBK)wasusedassolvent.Then,zincpowderwa

26、saddedtothesolu-tionandwasstirredtoformathickmortar-likemixture.Asteelpanel(R-35,fromQ-Panel)waspretreatedbygrindingwith400-and600-gritSiCsandpaper,followedbydegreasingwithhexane.Thecoatingswereappliedusingadrawndownbaratawetthicknessof200lm.Thecoatedpanelswereplacedinaconvec-tionovenat70°Cfor2

27、4hafterashingoffforapproximately30min.Fortheprimersofpigmentvolumeconcentration(PVC)lowerthan25%,thezincpigmentwasrstlydispersedinMIBKforfulldispersion.Thedrylmthicknesseswereintherangeof140190lm.2.2.SVETmeasurementanddataanalysisThecurrentdistributionovertheinterfaceofsolution/ZRP(67%PVC,subsequent

28、lyreferredtoasZRP67)wasmeasuredusingaSVETsystemfromApplicableElectronics(USA).ThePtIrmicroelectrode(MicroprobeInc.)witha10lmdiametertipwhichwasplatinizedtoa20lmdiametersphere.Themicroprobewasvibrated$200lmabovethesampleswiththeamplitude20lmalongtheXandYdirections.Apairofplatinizedplatinumwireswasuse

29、dasboththereferenceandbathgroundelectrodes.Theprobemade20Â20measurementsineachscan($600s),generatinga400-pointmeshacrossthesurface.Scanswereinitiated5minafterimmersionandrepeatedevery60min.TheZRP67sample(1Â1cm2)wasmaskedbyapolyestertape,exposinganopenareaof3Â3mm2asthescan-ningarea.Ana

30、rticialscratchwasintroducedinthecenterofthescanningarea.Forcomparison,thesameSVETmeasurementwasalsoconductedonanunscratchedZRP67.AllSVETmeasurementswereperformedatthefree-corrosionconditioninacellcontaining$5mL3.5wt.%NaClaqueoussolution.TheSVETcurrentdensitymappingandthestatisticalanalysisofthedataw

31、ereperformedwithOriginsoftware.Thecurrentdensi-tiesweredisplayedinthree-dimensional(3D)maps,showingthespatialdistributionofthecurrentdensityasafunctionofthe(x,y)positioninthescanregiononZRP.ThecurrentvaluesintheSVETmaparepositiveforanodiccurrentsandnegativeforcatho-diccurrents.Thecontourmapofthecurr

32、entdensitiesisatthebot-tomofthe3Dmap.TheSVETcurrentdensityvectorimagessuperimposingthemeasuredcurrentvectorontoanopticalimageofthesampleshowedimagesofthesamplesurfaceaswellasthelocationsofanodic/cathodicarea.BasedontheSVETcurrentdensitymap,theanodiccurrentden-sitypeak(iA,max),thecathodiccurrentdensi

33、typeak(iC,max),theaveragecurrentdensity(iAve)andtheintegratedanodiccurrent(IInt)wereusedtocharacterizetheanti-corrosionpropertiesofthecoating.IIntwasevaluatedbyintegrationoftheoverallanodiccurrent(IA)onaSVETcurrentdensitymap,whichistheoreticallyequaltothetotalcathodiccurrent(IC)overtheZRPsurface.Spl

34、it-tingthescanarea(S,3Â3mm2)into20Â20smallsquares,wecalculatetheanodicorcathodiccurrentoneachsquare,andsumalltheresultingcurrentstoobtainIInt(lA)onthescanarea,asshownbyPIAInt¼Sin¼ÀSPiCnð2ÞwhereiAistheanodiccurrentdensity(iAP0),iCthecathodiccur-rentdensity(iC<0)a

35、ndnthenumberofmeasurementpointsineachscan(n=400).2.3.GalvaniccouplingmeasurementGalvaniccoupling(boththemixedpotential,EMix,andthecou-plingcurrent)betweenthecoatedmetalandthebaresubstrate2638M.Yanetal./CorrosionScience52(2010)26362642wasmeasuredusingaGamryPC4/300potentiostatinazeroresis-tanceammeter

36、(ZRA)mode.Theexperimentswerecarriedoutinatwo-compartmentenclosedcelldescribedinourpreviouswork29.Thetwo-compartmentcellpermittedcarefulcontroloftheatmosphericconditionsineachcompartment.Theworkingelec-trodeinonecompartmentwasZRP67-coatedonthesteel(subse-quentlyreferredtoastheZRP-compartment)andthewo

37、rkingelectrodeintheothercompartmentwasthebaresteel(subse-quentlyreferredtoasthesteel-compartment).TheexposedareaofZRP67was1.0cm2andthebaresteelwasinapinholeof0.04cm2(simulatingacoatingdefect),yieldinganarearatio(ZRP67tosteel)ofca.25.Tosimulatetheconditionforatop-coatedsample,whereatopcoatwouldprotec

38、ttheprimerfromdi-rectoxygenaccess,thesolutioninthecoating-compartmentwaspurgedwithN2,whilethesolutioninthealloy-compartmentwaspurgedwithair.3.Resultsanddiscussion3.1.SVETcurrentdensitymapsTheSVETcurrentdensitymapsforthebaresteelin3.5%NaClsolution,aspresentedinFig.1,showedseveralanodiccurrentpeakstha

39、tappearafterseveralminutes,duetopossiblepittingnucleation.After30min,theseanodiccurrentpeakscombinedintoonebroadanodicpeak,wheredarkcorrosionproductsbegantoap-pearonthesteelsurface.Fig.2displaysSVETcurrentdensitymapsabovethedefectonthescratchedZRP67(67%PVC)aftervariousimmersionperiodsin3.5%NaClsolut

40、ion.Thecathodiccurrentwasmainlylocatedatthescratchwhereawell-denedcathodicpeakexistedthroughoutthe5-dayimmersionperiod.TheanodicareaappearedatdifferentsitesonZRP67duringtheimmersion.Atthebeginningoftheimmersionfrom0.1to4h,anodicareaswerefoundtoinitiateonlyatthecornersofthescratch,asshowninFig.2aandb

41、.After5hoftheimmersion,anodicareaswerescatteredaroundthescratch(Fig.2c).After10hoftheimmersion,signicantchangesoccurredbothincurrentdistributionandinvalue,aspresentedinFig.2d.Theanodicactivitymovedfromoneareatoanothernearthescratch;thecathodicareaincludedtoalmostallthescratchandawell-denedcathodicpe

42、akwasobserved.After20hoftheimmersion(Fig.2eandf),thecathodiccurrentdecreasedwithtime,andtheanodiccurrentwasevenlydistributedoverthesur-faceoftheprimer.Thevariationsoftheanodiccurrentpeak(iA,max),thecathodiccurrentpeak(iC,max)andtheaveragecurrentdensity(iAve)inSVETmapsareshowninFig.3.TheiA,maxdecreas

43、edovertheimmersiontimeonthewhole,exceptfortwopeaksappearingat3and10h.TheSVETexperimentwasconductedunderfree-corrosioncondi-tion(withoutexternalpolarizationapplied)wheretheanodiccur-rentsandcathodiccurrentsabovetheZRP67arebalancedandthenetcurrentshouldbezero.Itshouldbenotethat,inascanningplaneaboveth

44、efree-corro-sionsurface,theintegratedanodiccurrentIAshouldbetheoreti-callyequaltotheintegratedcathodiccurrentICintheabsolutevalueandhencetheaveragecurrentdensity(iAve)shouldbezero.ButdeviationsbetweenIAandICareusuallyobtainedbySVET,whichcausesiAvetodeviatefromzero.Thedeviationsmaybeattributedtothefa

45、ctthatthecurrentdensityonaSVETmapwasnottakenatthesametime.Thecorrosionbehaviourandcurrentdistributiononthescanareaarechangingduringscanning(onescantakes$10min).3.2.IntegratedanodiccurrentoftheZRPobtainedbySVETTheZRPpaintsunderstudyhereareheterogeneoussystemswithporesandzincparticlesdistributedrandom

46、lyinthebinder.Theprobablereactionsontheprimerinanelectrolyteareasfol-lows:Zincdissolutiontotheoxide(ZnO)/polymeric-binderlm,electrochemicaldissolutionoftheactivezincparticles,andoxygenreductiononzincparticlesoronthesubstratethroughpores35.Akeyaspectoftheabovementionedmechanismsisthegalvanicinteracti

47、onbetweenZRPandthemetalsubstrate.ThegalvanicinteractionbetweenZRPandsubstratemaybeinuencedbyanyorallofthefollowingfactors:theelectrochemicalstateofzincpar-ticlesatZRP/solutioninterface,reactiveZn/Fearearatio(SZn/Fe)interface,aswellasthediffusionprocessthroughthecoatingandthedepositofZncorrosionprodu

48、cts3,12.Theelectrochem-icalbehaviourandcathodicprotectionperformanceofZRPshavebeenwellstudiedbycorrosionpotentialmonitoring3,EIS3,12,36,conductiveatomicforcemicroscopy(AFM)2,aswellasthescanningelectronmicroscopy(SEM)4,12.Inthiswork,theelectrochemicalandcorrosionperformanceoftheZRPwascharacterizedbyt

49、heSVETmethod.TheintegratedanodiccurrentIIntabovetheZRP67obtainedbytheSVETmethodispresentedinFig.4asafunctionoftheimmer-siontime,togetherwithEOCmeasuredunderthesameconditions.ForthescratchedZRP67,closelycorrelativebehaviourwasfoundbetweenEOCandthetotalanodiccurrent.Mostobviously,twosig-nicantanodiccu

50、rrentpeaksappearedduringtheimmersionex-actlybeforeandaftertheEOCpeakoccurred.Forcomparison,IIntandEOCfortheunscratchedZRP67arealsopresentedinFig.4.ThetrendofIIntoftheunscratchedZRP67wassimilartothatofthescratchedZRP67butwithmuchloweramplitude.ThemostM.Yanetal./CorrosionScience52(2010)263626422639pos

51、itivepotentialofZRPs(unscratched)reachedat$3himmersionandthispotentialwasobservedtodependsignicantlyonthePVC,asshowninFig.5.ThemostpositivepotentialsforZRPswithPVC5%,15%,45%and67.5%were0.166,0.00,À0.60andÀ0.91V,respec-tively.ThefollowingseveralstageswereclearlyrecognizedfrombothIIntandEOCs

52、howninFig.4forthescratchedZRP67.2640M.Yanetal./CorrosionScience52(2010)263626423.2.1.TheactivatingstageTheEOC(ca.À0.9Vinitially)shiftedgraduallytowardsnegativedirectionduringtherst1himmersionasaresultoftheactivationofZnparticlesthroughdissolvingZnOlmorsoakingthebinderlm,approximatingÀ1.0V(

53、EOCofthezincparticles)at$1h.Inthefollowing3h,theEOCshiftedinthepositivedirection,implyingthatthesteelsubstratebegantobewettedbytheelectrolytethroughpores(decreasingtheratioofSZn/Fe).TheIInt($1.2Â10À2lAinitially)slightlydecreasedintherst1himmersion.Then,itgraduallyincreasedasaresultofthedis

54、so-lutionofZnO/zincparticlesattheZRPsurface.From3himmer-sion,IIntdecreasedsharplyaccompanyingwiththerapidincreaseofEOCduetothewettingprocessofthesteelsubstrateuntilEOCreachingapeak(À0.71V)at5h,wherethesteelsubstratewasex-pectedtobetotallywetted.Acurrentvalley(7.3lA)exactlycorre-spondedtotheEOCp

55、eakat5h.Oncethesubstratewaswetted,galvanicinteractionbetweenzincparticlesandsteelsubstratewouldbeexpectedtooccur.Theperiodofthebeginning$5himmersioncanbereferredtoastheactivatingstage,wherethemainprocesswasthedissolutionoftheZnOlmandthenthezincparticlesreactionwiththeelectrolyte.Intheactivatingstage

56、,theelectrochemicalreactionprocessmainlyoccurredontheZRP/solu-tioninterface.3.2.2.ThesacricialprotectionstageEvenafterthesteelsurfacebecamecompletelywettedat$5hinFig.4,somezincparticleswerestillcoveredbythickoxidesorbybinderlms.Zincparticlescontinuedtobeactivatedbythedis-solutionoftheZnOlmand/orgalv

57、anicallycouplingtothesub-strate.Atthebeginningofthegalvanicstage,corrosionproductwasformedinporesintheprimer,whichtendedtosealthepores,reducingthenumberandsizeofpores3.ThisprocessshiftedEOCtowardnegativedirectionsandincreasedIIntsharplyuntilattain-ing2.3Â10À2lAat$11h.TheuctuationofEOCinthe

58、rangeofÀ0.78toÀ0.87Vinthevicinityof9handthesignicantdecreaseinIIntat$10hmightbeattributedtotheaccumulationofZncorrosionproductwhichimprovedthebarrierpropertyofthecoating.FortheunscratchedZRP67,theEOCuctuationat$8himmersiondisappeared,whichfurtherimpliedthattheEOCuctuationmightberelatedtoth

59、ede-positofthezinccorrosionproductinthedefectanditsadhesiontothesurface.3.2.3.ThebarrierstageTheEOCcontinuedtodecay,withsomeuctuations,beginningfrom$11h(Fig.4).IIntdecayedtoalowvalueof1.2Â10À2lA,whereitremainedforthefollowingdurationoftheexperiment.ThedistinctdecreaseinIIntmaybeattributedtothezincdepletingand/ortheimprovedbarrierpropertyoftheprimer.Thebarrierpropertiesoftheprimerwouldimprovebydepositofthecorrosionproductontheprimer