碱土金属离子与EDTA对纯铝在碱性溶液中的协同缓蚀作用.doc

碱土金属离子与EDTA对纯铝在碱性溶液中的协同缓蚀作用No.3SHAO,HaiBoeta1.:TheCooperativeInhibitionEffectsofAlkalineEarthMetalIonsandEDTAontheCorrosion313andNa2SnO3+In(OH)3wereeffectiveinhibitorsystems.Itwasfoundthattheadditionofzincoxidetoalkalinesolutionsignifi?cantlyreducedthecorrosionofaluminumelectrode.Intheeffortstodecreasethecorrosionrateofalumihum,itwasalsofoundthatcalciumionsincombinationwithcertaincomplexesarehelpful.InthepresentstudytheinfluencesofalkalineearthmetalhydroxidealoneandincombinationwithEDTAin4mol?L一KOHsolutionwereinvestigatedbymeansofhydrogencollection,polarizationcurvesandelectrochemicalimpedancespectroscopy,andthealuminumsurfacewassubsequentlyobservedbySEMandanalyzedbyEDAX.1ExperimentalmethodsElectrochemicalmeasurementswerecardedoutinaclassicalthree?electrodeglasscel1.Theworkingelectrodewasmadeofspecpurealuminumrod.6.35mmindiameter.Theelectrodesurfacewaspolishedby1000#waterproofabrasivepaper,degreasedinacetoneandrinsedindeionizedwater.ThecounterelectrodewasaplatinumfoilandthereferenceelectrodewasaHg/HgOelectrode.AllthesolutionswerepreparedbyreagentsofA.R.gradeanddeionizedwater.Themeasurementsystemconsistsofapotentiostat(EG&Gmodel273A)andalock-inamplifier(model5210),controlledbyamicrocomputerwithcertainsoftware+Thepotentiodynamicpolarizationcuwesweremeasuredatascanningrateof1mVs一andinthepotentialrangefrom一2100mVto-1000mVHg/HgO).Electrochemicalimpedancespectroscopy(EIS)wasobtainedintheffequencyrangeof100kHzto10mnzandata.c.signalamplitudeof5mV.AIltheabovemeasurementsweretakenat25.Beforeeachmeasurementtheelectrodewaskeptintestsolutionfornolessthanhalfanhour,duringwhichasteadyopencircuitpotentialwasobtained.SEMandEDAXanalyseswereperformedwithanFEISIRIONSEMsystem,beforewhichthespecimenswerekeptimmersedinthesolutionsforeighthours,thenrinsedindeionizedwateranddriedinair.2ResultsanddiscussionThecorrosioncurrentdensitiescanbecalculatedbytheresuitsofhydrogencollectionatOCP(opencircuitpotentia1),asdescribedinRef.【141.Table1showsthecalculatedcorrosioncurrentdensitiesandinhibitionefficienciesofpurealuminumin4molLKOHsolutioncontainingalkalineearthmetalhydroxidesandEDTAwithdifferentconcentrations.Inthissolution,thealkalineearthmetalionsalealmostcompletelychelatedbyEDTA,andtheconcentrationsoffreeionsarelowbecauseofthehighconcentrationofOH一.ThereforethetotalconcentrationsofalkalineearthmetalionsalealmostequaltothatofEDTA.ItcanbeseenfromTable1thatthecurrentdensityisgreatlydecreasedbytheadditionofCa(OH)2andSr(OH)2,andtheirinhibitioneffectsarefurtherenhancedbytheadditionofEDTA,whileEDTAitselfhasonlyalittleinhibitioneffect.WhentheconcentrationofEDTAis0.O1mo1.L.,Ca(OH)2andSr(OH)2respectivelyshowthelargesteffect.SaturatedCa(OH)2isfoundtoshowlargestinhibitioneffectamongalkalineearthmetalhydroxidesfollowedbyotherhydroxidesintheorderCa(OH)2>Sr(OH)2>Ba(OH)2>Mg(OH)2.Andwhenaddedwith0.0lmolLEDTA,theyalsofollowtheaboveorder.itisbelievedthatthecooperativeactionofalkalineearthmetalionsandEDTAmaybeduetotheformationofachelatecomplexbetweencationsandEDTA.Andthechelatecomplexmightbephysicallyadsorbed,withcationsdisposedtowardsthemetalsurface.TheresultsofthepolarizationcurvesshowthattheOCPshiftstothepositivedirectioninsaturatedSr(OH)2_containingTaIble1Thecorrosioncurrentdensiesandinhibitionefficienciesofpurealuminumin4mol?L一KOHwithinhibitorscalculatedbyhydrogenevolution314ActaJP.-Chim.Sin.(Wulisolution(Fig.1a),whileitshiftstothenegativedirectioninsaturatedCa(OH)2-containingsolution(Fig.1b).ItcanalsobeseenfromFig.1athatboththeanodicandcathodicprocessesofaluminuminthesolutionwithsaturatedSr(OH)2areinhibited.ButwithincreasingconcentrationofEDTA,theinhibitioneffectontheformeriSmuchgreatertllantllatontllelatter.WhileinsaturatedCa(OH)z-containingsolution,withincreasingtheconcentrationofEDTA,theinhibitioneffectonthelatterismuchgreaterthantllatontlleformer.Thisindicatesthatmoststrontium.EDTAchelatecomplexesmayadsorbontheanodicactivesitesandthe6耋墨6三岫6三岫g(f/(mA-cm)Fig.1PolarizationcurvesofAIin4mol?L一KOHsolutionswithorwithoutaddi廿YeS(a)EDTA+Sr(OH)2;(b)EDTA+Ca(OH)2;(c)EDTA+Ca(OH)z+Sr(OH)2No.3SHAO,HalBoela1.:TheCooperativeInhibitionEffectsofAlkalineEarthMetalIonsandEDTAontheCorrosion315Table2SomeparametersfittedfromtheEISinFig.2showninFig.3,whereR.isthetotalohmicresistanceofthesolution,CPEistheconstantphaseelementrelatedtothedoublelayercapacity,RIischargetransferresistanceoftheelectrode.Table2showssomefittedparametersofEIS.ThefitteddataIvalues)accordwiththeforegoingdiscussion.Afterbeingkeptimmersedin4mol.L一KOHsolutionwithadditivesfor8h,theelectrodesurfacewasobservedbySEMandexaminedbyEDAX.OnlyaluminumandoxygenweredetectedbyEDAX,witharatioabout2:3,whichindicatedthatthepassivefilmcontainedonlyAl2Oa.Alkalineearthmetalwasnotfoundtoparticipateinformingfilm.Thisfactindicatesthatalkalineearthmetalions?EDTAchelatecomplexesareinterfaceinhibitorsratherthaninterphaseonesandtheymayproduceinhibitioneffectbyadsorbingontheactivesitesofaluminumelectrode.3ConclusionsThecorrosionofaluminumin4mo卜L一KOHsolutioncanbegreatlyinhibitedbytheadditionofalkalineearthmetalhydroxides,andcalciumhydroxideisfoundtogivelargestinhibitioneffect.TheirinhibitionisenhancedbytheadditionofEDTA.whileEDTAitselfhasonlyalittleinhibitioneffect.TheanalysesonEISandEDAXrevealthatalkalineearthmetalionsandEDTAchelatecomplexesmayproducetheinhibitioneffectonthecorrosionofaluminumbyadsorbingontheactivesitesofaluminumsurface.AcknowledgementTheauthorsgratefullyacknowledgethefinancialsupportoftheChineseStateKeyLaboratoryforCorrosionandProtection.ReferenceslMacDonald,D.D.;English,C.J.App1.Electrochem.,1990,20:4052Licht,S.;Marsh,C.J_Electrochem.Soc.,1992,139:L1093Shao,H.B.;Zhang,J.Q,;Wang,J.M.;Cao,C.NActaPhys.ChinSin.,2003,19(4):372邵海波,张鉴清,王建明,曹楚南.物理化学(WuliHuaxueXuebao),2003,19(4):37214Wang,X.Y.;Wang,J.M.;Shao,H.B.;Zhang,J.Q.;Cao,c.N.App1.Electrochem.,2005,35:2135Zaromb,S.J.Electrochem.Soc.,1962,109:11256Trevethan.L.;Bockstie,D.;Zaromb,S.ElectrochemSoc.,1963,110:2677Krishnan,M.;Subramanyan,N.Corros.Sci.,1977,17:8938Paramasivam,M.;Suresh,G.;Muthuramalingam,B.;Iyer,V.S.;Kapali.V.J-App1.Electrochem.,1991,21:452