Corrosion+Kinetics.ppt

4.CorrosionKinetics,Attheelectrode/electrolyteinterface,achargeseparationbetweenthemetalsurfaceandtheelectrolyteoccurs.Thespatialregioncorrespondingtothechargeseparationiscalledtheelectricaldoublelayer.Itisusuallyseparatedintotwoparts,theHelmholtzlayerorcompactdoublelayerandtheGouy-Chapmanlayerordiffuselayer.Thechargeattheinterfaceestablishanelectricfield.Withinthecompactlayer,theelectricfieldreachestheorderof108to109V/m,andhencehasaninfluenceonthechargetransferreaction.Sincecorrosionisanelectrochemicalprocessinvolvingthechargetransferreaction,itsrateissignificantlyinfluencedbytheelectrodepotentialortheelectricfieldacrossthedoublelayer.,HelmholtzModel,SternModel,4.1TheElectricalanalogueofdoublelayerTheelectricaldoublelayerischaracterizedbytwolayersofoppositechargefacingeachother,asinacapacitor.Theelectricalcurrentcan,however,passacrossthemetal-solutioninterface,althoughthereissomeresistancetoit.TheelectrodecanthenberepresentedbyanelectricalanaloguecomposedofacapacitorparalleltoresistanceRFcalledFaradaicresistance.TheRFiscalledalsothepolarizationresistanceorcharge-transferresistance.,Whenanelectricalcurrentisimpressedontheelectrode,theRFmustbeovercome.Thisgeneratesadditionalvoltageandcausesashiftintheelectrodepotential.Atrest(opencircuit),Theelectrodehasachargedlayer;intheabsenceofanelectriccurrent,thecapacitorCdlischarged.Thecurrentimpressedontheelectrode,it,isdividedintotwoparts.it=iF+ich,M,S,WheniF:Faradaiccurrentich:currentofchargeaccumulatedinthecapacitorUsuallyiFich.TheelectrodepotentialisproportionaltothechargeQofthedoublelayer.Thus,theelectrodepotentialchangesunderanelectriccurrentacrossthedoublelayer;E=Eeq+(i)(i)=theadditionalvoltageduetothecurrentflow.,4.2ChargetransferoverpotentialorActivationpotentialForchargetransferreactioninmetal/solutioninterface:M(inlattice)M+z(hydratedinsol.)+ze-,Actually,M,latticeM,adsorbedM+z,sol.,Themetalatomsontheelectrodesurfaceareinenergywellsassociatedwiththelatticestructure,andinordertopassintothesolutiontheyhavetoovercometheactivationenergy.,Thechemicalfreeenergychangeduetothedissolutionordepositionofthemetalisbalancedbyanequivalentquantityofelectricalworkdonebytheionsincrossingtheelectricfieldimposedbytheequilibriumelectrodepotential.MM+z+ze-.atequilibriumorreversiblepotential.Reactionrate=kxConcentration=amountproducedperunitareaperunittime=moles/cm2sec,Therateofmetaldissolutionorreductionmaybeexpressedasacurrentdensity(A/cm2)accordingtotheFaradyslaw;i=Q/At=zFm/At,wherez=numberofelectrons,M=numberofmolesF=Faradysconstant(=96500C/mole)A=Surfaceareaonwhichreactiontakeplaces.Attheequilibriumelectrodepotential,thefluxofchargethroughthedoublelayeristhesameinbothdirectionsandwecallthistheexchangecurrentdensity,io.,Atequil.,ia=ic=io,andanodicreationrate=cathodicreactionrate=io/zFAnodicrx.rate=kaxCM=fCMexp(-G*/RT)where,ka=rateconstant=fexp(-G*/RT)CM=concentrationofsurfaceactiveatomf=frequencyofactivatedcomplex1012/secexp(-G*/RT)=Probabilityofreactionk=f(poolreactantspecies)(probabilityofreaction),LetscalculateCMapproximately.Ifthelatticeconstantis3,thenumberofsurfaceatoms:Ns=(10162/cm2)/(92/atom)1015atoms/cm2Thus,themolesofatoms/cm2=Ns/NA(NA=AvogadroNo.)However,surfaceatomsadjacenttothecrystaldefectshaveahigherprobabilityofreactionthanothersormoreactive;Fractionofsurfaceatomswhicharelikelytobesurfaceactive,=10-3to10-4dependingoncrystalstructure,defectdensityandgrainsize.,Therefore,CM=(Ns/NA)=molesofsurfaceactiveatoms/cm2,ia=io=zFx(anodicreactionrate)=zFfCMexp(-G*/RT)Cathodicrx.rateatequil.=ic/zF=kcCM+zVL=fCM+zVLexp(-G*/RT)where,CM+z=conc.ofmetalionatOHPVL=volumeofthedoublelayer/cm210-8cm3Thus,ic=io=zFfCM+zVLexp(-Gc*/RT),G*canbechangedinelectrochemicalreactionsbyexternallyappliedpotential(Eapp).Thechangeinelectrodepotentialfromtheequil.valuetoacquireanetcurrent(ie,externallymeasurablevalue)iscalledpolarization.ie=f(Eapp-Eeq)=f(),Ifanodicpolarizationisappliedtothemetalelectrode,whathappenstotheenergywellcurve?TheenergyofM(metal)increasesbyzaFandthemetalionsbecomemoreunstable(highenergystate).,Foranodicreaction,G*a=G*+(1-)zFa-zFa=G*-zFaForcathodicreaction,G*c=G*+(1-)zFaia=zFfCMexp(-G*a/RT)=zFfCMexp-(G*-zFa)/RT=zFfCMexp-(G*/RT)exp(zFa/RT)=ioexp(zFa/RT)ic=zFfCM+zVLexp(-G*C/RT)=zFfVLCM+zexp(-G*/RT)exp-(1-)zFa)/RT=ioexp-(1-)zFa/RTie,a=ia-ic=ioexp(zFa/RT)-exp-(1-)zFa)/RT.Volmer-Butlerequation.1)fora0.03V.highfieldapproximation.ie,aia=ioexp(zFa/RT)fora-0.03V.cathodicpolarizationie,c=ic-iaic=ioexp-(1-)zFa)/RTAtsufficientlylargeoverpotential,theierelationshipbecomesexponential.,2)whenaisverysmall,.|a|0.01V.lowfieldapproximationByusing:ex=1+x+x2/2.=1+x,whenx1,ie,a=ioexp(zFa/RT)-exp-(1-)zFa)/RT=io(zFa/RT).alinearrelationshipbetweenaandie,a.,4.3PolarizationDiagram(Evansdiagram)o.Polarization:Anelectrodeisnolongeratequil.whenanetcurrentflowsfromortothesurface.Theextentofpotentialchangecausedbynetcurrenttoorfromanelectrode,measuredinvolts,isoverpotential().=f(ie),1)Foranodicpolarizationie,a=ia-ic=ioexp(zFa/RT)-exp-(1-)zFa)/RT=ioexp(zFa/RT)athighfieldapprox.a=2.3RT/zFlogie,a-2.3RT/zFlogio=Balogie,a/io.Tafelequation.2)Forcathodicpolarizationie,c=ic-iaic=ioexp-(1-)zFa)/RTathighfieldapprox.c=-2.3RT/(1-)zFlogie,c/io=Bclogie,c/io,where,io=exchangecurrentdensityEr=equil.potential,orrestpotentialBa,Bc:Tafelconstant,-0.05VB0.15V,a,c,ER,logio,log|io|,E,Anodiccurrent,Cathodiccurrent,Ba=2.3RT/zF=0.059/z,Bc=-2.3RT/(1-)zF=-0.059/(1-)z,4.4ConcentrationPolarizationOneoftheassumptionsinthederivationoftheVolmer-Butlereq.istheuniformityofconcentrationneartheelectrode.Thisassumptionfailsathighcurrentdensitiesbecausemigrationofionstowardstheelectrodefromthebulkisslowandmaybecomeratedetermining.Alargeroverpotentialisthenneededtoproduceagivencurrentbecausethesupplyofreducibleoroxidizablespecieshasbeendepleted.Thiseffectiscalledconc.polarization.Forreductionreaction:M+z+ze-MEr=E+RT/ZFlnCB.(1)Whenapassageofexternalcurrentismadethroughthecell,theinterfacialconc.changestoavalueofCS,resultinginchangeofelectrodepotential.,M+z+ze-MEp=E+RT/zFlnCS.(2)Conc.overvoltage,c=Ep-Er=RT/zFlnCs/CB.(3),Ficks1stlaw:J=-DC/x=-D(CS-CB)/=i/zFwhereD=diffusivityofM+zion,=Nernstlayer,about0.1mm.therefore,i=DzF/(CB-CS).(4)WhenCS0,iiL:limitingcurrentdensity,Thus,iL=DzFCB/.(5)(5)(4)i=iL(1-CS/CB),CS/CB=(1-i/iL).(6)(6)(3)c=RT/zFln(1-i/iL)=0.059/zlog(1-i/iL)at298K.(7),iLisincreasedbyhighersolutionconcentration,CB;highertemperaturewhichincreasesdiffusivity,D;highersolutionagitation,whichdecreases.ConcentrationpolarizationonlybecomesimportantwhenthecurrentdensityapproachesiL,Foranodicdissolutionprocess:MM+z+ze-Duringcorrosion,conc.polarizationforanodicdissolutioncanbeignoredbecauseanunlimitedsupplyofmetalatomsisavailableattheinterface.But,athighcorrosionrates,theconc.ofM+zionintheanolyteissignificantlyincreasedandexertabacke.m.f.whichresultsinanodicconc.polarization.,4.5CombinedPolarizationT=ct+cAtlowreactionrates,ctcontrolsthereactionrates.Athighreactionrates,ccontrolsthereactionrates.1)foranodicdissolution,cissmall;diss.=Balogi/io2)forreductionprocess;red.=Bclogi/io+2.3RT/zFlog(1-i/iL),4.6ResistanceOvervoltageResistanceOvervoltage(R)arisesfromthepassageofelectriccurrentthroughanelectrolytesolution(withlowconductivity)surroundingtheelectrode.ThisiRdropmaybeoffsetbyclosingthetipofLuggincapillarytothesurfaceoftheelectrode.Significantwhensurfaceoxidefilmsformsontheelectrodesurfaceasaresultofelectrochemicalreaction.R=i(Relectrolyte+Rfilm),4.7MixedPotentialTheory-presentedbyWagnerandTraudin1938.-basedonthefollowinghypotheses;1)Allfreecorrosionreactionsinvolveatleastoneanodicandonecathodicprocess.2)Duringthecorrosionofanelectricallyisolatedmetal,thetotalpartialanodiccurrentmustequaltothetotalpartialcathodiccurrent:Aaia=AcicatE=Ecorr3)Formetals,theelectrical(Galvani)potentialofthemetalattheanodicsiteisequaltothatatthecathodicsiteduetotheirverylowresistivities.Thismaynotbevalidforsemiconductors,particularlyiftheanodicandcathodicsitesarewidelyseparated.,ConsiderZnundergoingactivecorrosioninadeaeratedHClsolutionat25C:,Forhydrogenreductionreaction,Er,H2=0.059V,io,H+/H2(onZn)=10-11A/cm2,Bc=0.12V/decadeForZnoxidation,Er,Zn=0.94V,io,Zn2+/Zn=10-7A/cm2,Ba=0.06V/decade,Amixedelectrodeisanelectrodeormetalthatisincontactwithtwoormoreoxidation-reductionsystems.Corrosionpotential(Ecorr)andcorrosioncurrentdensity(icorr)aredeterminedatthepointwherethetotalratesofoxidationandreductionareequal.icorr=therateofZndissolution=iH2(Zn)atEcorr,E(SHE)V,log|i|A/cm2,4.8ImportanceofexchangecurrentdensityConsiderthecorrosionofironindeaeratedacidsolution.,1)ForFeoxidation:Fe2+2e-Fe.EFe2+/Fe=0.44VEr,Fe=EFe2+/Fe+0.059/2logaFe2+=0.44+0.059/2log10-6=0.62Vio,Fe2+/Fe=10-6A/cm2Ba=0.08V/decade2)Forhydrogenreduction:2H+2e-H2Er,H2=0.059pH=0.059VatpH=1io,H+/H2(onFe)=10-6A/cm2Bc=0.15V/decade,Ecorr=corrosionpotential,restpotential,opencircuitpotential,mixedpotentialHydrogenovervoltage=EcorrEr,H2=Ecorr(0.059pH),ComparisonofcorrosionbehaviorbetweenZnandFe1)Er,Zn=0.94Vicorr,ZnThisisduetothelowerexchangecurrentdensityforhydrogenevolutiononZncomparedtothatonFe.i,eio,H+/H2(Zn)io,H+/H2(Fe),Theexchangecurrentdensity(io,H+/H2)forhydrogenevolutionreactionishighlysensitivetothenatureofthemetalsubstrateonwhichthereactionoccurs,andaremarkedlyreducedbythepresenceoftraceimpuritiessuchasarsenic,sulfur,andantimonycompounds.Forcorrosionofmetalsinacid.H2=EcorrErH+/H2=Ecorr+0.059pHH2(Zn)RF(Pt)Exchangecurrentdensity,io(A/cm2)dependson:electrodecompositionredoxreactiontemperaturesurfaceroughness,4.9EffectofoxidizerThedrivingforcorrosionisincreasedbytheadditionofastrongeroxidizerthatisaredoxsystemwithahalfcellelectrodepotentialmuchmorenoblethanthatofanyotherspresen