Corrosion-KineticsVIP专享VIP免费

http://corrosion.kaist.ac.kr4.CorrosionKinetics4.CorrosionKineticsAttheelectrode/electrolyteinterface,achargeseparationbetweenthemetalsurfaceandtheelectrolyteoccurs.Thespatialregioncorrespondingtothechargeseparationiscalledtheelectricaldoublelayer.Itisusuallyseparatedintotwoparts,theHelmholtzlayerorcompactdoublelayerandtheGouy-Chapmanlayerordiffuselayer.Thechargeattheinterfaceestablishanelectricfield.Withinthecompactlayer,theelectricfieldreachestheorderof108to109V/m,andhencehasaninfluenceonthechargetransferreaction.Sincecorrosionisanelectrochemicalprocessinvolvingthechargetransferreaction,itsrateissignificantlyinfluencedbytheelectrodepotentialortheelectricfieldacrossthedoublelayer..+++++Mx-----MSSx---+++++----------'Stuck'ionsScatteredionsMSMSHelmholtzModelSternModelhttp://corrosion.kaist.ac.kr4.1TheElectricalanalogueofdoublelayerTheelectricaldoublelayerischaracterizedbytwolayersofoppositechargefacingeachother,asinacapacitor.Theelectricalcurrentcan,however,passacrossthemetal-solutioninterface,althoughthereissomeresistancetoit.TheelectrodecanthenberepresentedbyanelectricalanaloguecomposedofacapacitorparalleltoresistanceRFcalledFaradaicresistance.TheRFiscalledalsothepolarizationresistanceorcharge-transferresistance.e-e-e-e-e-M+zM+zElectricaldoublelayerEquivalentCircuitCdlRFWhenanelectricalcurrentisimpressedontheelectrode,theRFmustbeovercome.Thisgeneratesadditionalvoltageandcausesashiftintheelectrodepotential.Atrest(opencircuit),Theelectrodehasachargedlayer;intheabsenceofanelectriccurrent,thecapacitorCdlischarged.Thecurrentimpressedontheelectrode,it,isdividedintotwoparts.it=iF+ichMShttp://corrosion.kaist.ac.krWheniF:Faradaiccurrentich:currentofchargeaccumulatedinthecapacitorUsuallyiF>>ich.TheelectrodepotentialisproportionaltothechargeQofthedoublelayer.Thus,theelectrodepotentialchangesunderanelectriccurrentacrossthedoublelayer;E=Eeq+(i)(i)=theadditionalvoltageduetothecurrentflow.Electrode-ElectricAnalogueCdlRFitichiFhttp://corrosion.kaist.ac.kr4.2ChargetransferoverpotentialorActivationpotentialForchargetransferreactioninmetal/solutioninterface:M(inlattice)M+z(hydratedinsol.)+ze-Actually,M,latticeM,adsorbedM+z,sol.http://corrosion.kaist.ac.krThemetalatomsontheelectrodesurfaceareinenergywellsassociatedwiththelatticestructure,andinordertopassintothesolutiontheyhavetoovercometheactivationenergy.ElectrolyteMetalGc*Ga*GchM+GelectM+++++++++MS10ÅHelmholtzDoubleLayerGouyChapmanLayerE=M-sG=Gchem+GelectroIHPOHPMM+IHPOHPXGG*http://corrosion.kaist.ac.krThechemicalfreeenergychangeduetothedissolutionordepositionofthemetalisbalancedbyanequivalentquantityofelectricalworkdonebytheionsincrossingtheelectricfieldimposedbytheequilibriumelectrodepotential.MM+z+ze-.....atequilibriumorreversiblepotential.Reactionrate=kxConcentration=amountproducedperunitareaperunittime=moles/cm2sec•Therateofmetaldissolutionorreductionmaybeexpressedasacurrentdensity(A/cm2)accordingtotheFarady’slaw;i=Q/At=zFm/At,wherez=numberofelectrons,M=numberofmolesF=Farady’sconstant(=96500C/mole)A=Surfaceareaonwhichreactiontakeplaces.Attheequilibriumelectrodepotential,thefluxofchargethroughthedoublelayeristhesameinbothdirectionsandwecallthistheexchangecurrentdensity,io.http://corrosion.kaist.ac.krAtequil.,ia=ic=io,andanodicreationrate=cathodicreactionrate=io/zFAnodicrx.rate=kaxCM=fCMexp(-ΔG*...