Corrosion Inhibition Mechanism with Spilt Cell Technique of Rare Earth Metal on LC4 Al Alloy

Corrosion Inhibition Mechanism with Spilt Cell Technique of Rare Earth Metal on LC4 Al Alloy Corrosion Inhibition Mechanism with Spilt Cell Technique of Rare Earth Metal on LC4 Al Alloy Availableonlineatwww.sciencedirect.com ?…cE @……? JOURNALOFRAREEARTHS24(2oo6)89—96 JOURNALOF AEAS CorrosionInhibitionMechanismofRareEarthMetalonLC4AIAlloy withSpiltCellTechnique GuBaoshan(顾宝珊),LiuJianhua(刘建华) (,.School(!,.MaterialsScienceandEngineering,BeqingUniversityofAeronautwsandAstronautws,Bering 100083,China;2.DepartmentofSurfaceTechniqueandCorrosionEngineering,CentralIron&SteelResearch Institute,Beijing100081,China) Received3July2005;receivedinrevisedfrom12August2005 Abstract:AnewmethodofstudyingthecorrosioninhibitionmechanismofrareearthmelaI(REM)onLC4AIalloywith thespiltcelltechniquewasstudied.Theprincipleandexperimentalmethodofthespiltcelltechniquewereanalyzed.By measuringthechangeofnet— electriccurrentbetweenthetwoelectrodescausedbythechangeoftheamountofoxygenin thesolutionandtheadditionofCeCI,theinflnenceofcorrosiveperformanceofCeCIonLC4super—poweraluminum"alloy inthe0.1nlol?L0NaClsolutionwasinvestigated.Meanwhile,【heconditionalchangesofpHvalues,CeCI3solution,ad— ditimandtimeofperformancewerealsostudied.Finally.theaturesofelectrodesurfacewerer evealedbyusingSEM andX—rayenergy— dispersivespectrometry(EDS).Bycombiningthesewithotherelectricchemicaltechniques, suchaspo— tential-timecurve,polarizationcurveeta1. Keywords:spiltcelltechnique;ceriumconversioncoatings; CLCnumber:0646.6;TG133.2Documentcode:A nhibitionmechanism;aluminumalloys;pdreearths ArticleID:1002—0721(2006)01—0089—08 Inmid一1980s,Hintonetal.6jfoundthata smallamountofCeC11canobviouslyreducetheCOITO— sionrateof7075aluminumalloyinNaC1solutionand cerium(?)saltisregardedastheidealsubstituteof chromates(?).Ceriumsalt,asanenvironment—fa— voredinhibitorofaluminumalloy,hasgainedmore andmoreattention.However.itsinhibitionmecha— nismisstillunknown.Basedonthecathodiccoating theory.Hintonetalputforwardrestrictioncathodic reactiontheorythatrareearthcanimprovetheeOITO— sionresistingpropertyofaluminumalloys.Atpresent, theresearchoninhibitionmechanismofceriumsalt hasgaineduniveIalattentiont12].TheTecentre— searchshowsthatCeisrichinthesecond.phasegrains ofaluminum[?,. Whenaluminumissubjectedt0lo- ealizedcorrosion,themajorityofthesecond—phase grains,wherethecathodicreactiongoeson,greatly restraintheoccurrenceofthecathodicreactiondueto theexistenceofCeoxidefilm.Therefore.itisbe— lievedthatcerium(?)saltisakindofcathodicin— hibitor. Amajorcomplicationinthestudyofcorrosionis theinabilitytoseparatetheanodicandcathodicsur— facereactions.Inthispaper,theanodicandcathodic reactionscanbeseparatedbythespiltcelltee— hniquewhichisacombinationofthegalvanicCOrrO— siontechniqueswithadifferentialaerationcel1.Bv measuringthechangeofnet—electriccurrentbetween thetwoelectrodescausedbythechangeoftheamount ofoxygeninthesolutionandtheadditionofCeC13,a researchwasdoneintotheinfluenceofcorrosiveper— formaneeofCeC13onLC4super—poweraluminumalloy Correspondingauthor(E?mail:gubs@263.net) Biography:GuBaoshan(1967 一),Male,Doctor,Seniorengineer;SpecializinginsurfacetechnologyandemTosionengine ering Copyright@200x,theChineseS~ietyofRareE',rrths.PublishedbyElsevierB.V.Allrightsre served inthe0.1me1.LNaClsolution.Finally,thefea— turesofelectrodesurfacewiththehelpofSEMandX— rayenergy,dispersivespectrometry(EDS)werere— vealed.Bvcombiningthesewithotherelectrochemical techniques,suchaspotential—timecurve,polarization curveetc,theauthorsmadeafurtherapproachintoCe saltinhibitionmechanism. 1Experimental Theexperimentmaterial,LC4super.poweralu— minumalloybelongingtoA1一Zn—Mg—Cu.wasprovided byBeijingUniversityofAeronauticsandAstronautics. TheH.cellusedintheseexperimentscontaineda porousglassfrit.rheglassfritprovidesionicsolution contactthatisnecessaryforelectrochemicalmeasure. mentsandatthesametimepreventsbulkmixingof thesolutions.Anodicandcathodicprocesseswere probedusingacombinationofadifferentialaeration cellandagalvaniccorrosionanalysistechnique.Two LC4aluminumalloyelectrodeswithidenticalareas wereheldat山esamepotentialusingapotentiostat whilethecurrentflowingbetweenthetwoelectrodes wasmonitoredwithazeroresistanceammeter.Follow ingdeaerationofonesideofthespliteell.theeffectof inhibitorswastestedbyinjectingsmallamountsofin. hibitorsolutionsintoonesideofthecel1.Anelectro— chemicalcorrosionmeasurementsystemandsoftware byself-manufacturewereusedinallthespiltcellmen— surements.Thepotentialofthecoupledelectrodeswas measuredandcomparedwiththesaturatedcalomel electrode(SCE).'rheeffectofcorrosioninhibitors wasmanifestedasachangeinthecurrentflowbe— tweenthetwoelectrodes.Testsolutionwas0.1 no1.LNaClandtesttemperaturewas25?. AnEG&GPotentiostat/GalvanostatModel273 andM352corrosionsoftwarewasusedinelectrochemi. (a) O.1mel JOURNALOFRAREEARTHS,Vo1.24,No.1,Feb.2006 calmeasurements.Inalleases,thesaturatedcalomel electrodes(SCE)wereusedasthereferenceelectrode, aplatinumsheetwasusedastheauxiliaryelectrode, LC4aluminumalloywasusedasworkingelectrode, whilethetesttemperaturewas25?.Corrosionpoten. tialandpolarizationcurveswereperformedusingan EG&GPotentiostat/GalvanostatModel273.Surface topographyandelementanalysiswerecompletedbyS一 530scanningelectronmicroscopy(SEM)andLink ISISenergyspectrometer. 2ResultsandDiscussion 2.1Spiltcellmeasurements Adifficultyinthestudyofcorrosionisthepres— enceofbothoxidationandreductionreactionsonthe samemetalsurface.BvcombiningthegalvanicCOrrO. siontechniqueswithadifferentialaerationcell,the anodicandcathodicreactionscanbeseparatedand theeffectofinhibitorscanbeprobed.Schematicof thespilteellexperimentisillustratedinFig.1. 3'heexperimentalsetupisillustratedinFig.1 (a).Inthepresentstudy,onesideofanH—cel1isde. aeratedandacurrentisgeneratedduetothereduction ofoxygenatoneelectrode(electrodeI,right)O2+ 2H20+4e—+4OH一,anddissolutionofaluminumat theotherelectrode(electrode?,left)A1—3e—}Al?. AsFig.1(b)shows,itisexpectedthatdeaeration shouldproduceacurrentbetweenthetwoLC4electro. des(netcurrent).Inordertodistinguishbetweenthe twosidesofthespiltcell,theywillbereferredtohere asIandI1withcellIcontainingaerated0.1 me1.LNaClsolutionandcellIIcontainingdeaerat— ed0.1me1.LNaClsolution.Electronicflowfrom electrodeItoelectrodelIisdefinedasapositive currentandthereverse(IIto工)anegativecurrent. Fig.1Schematicofsplitcellexperiment (a)Schematic;(b)Effectofdeaeration DuBSeta1.CorrosionInhibitionMechanismofRareEarthMetal(REA1)oilLC4A1Alloy Fig.2showsthatthecurrentandpotentialre- sponseduetodifferentialdeaerationarepredictable andreproducible.Bydeaeratingandre-oxygenating thesolutions,thecurrentcanbeeliminatedandeven reversed.Aswecanreadilyperceive.thereisnonet currentflowobservedbetweenthetwoelectrodesa1. thoughlocalizedcorrosioncontinuesontheindividual electrodes(Fig.2(c)).Whenthecell工isdeaerat ed,thecurrentobservedisconsistentwiththeex砌. pieinFig.1(b),andelectronflowsfromelectrode工 to1I(Fig.2(d)).Cell工andIIaredeaeratedand nonetcurrentflowisobservedbetweenthetwoelec. trodes(Fig.2(e)).Cell工isre.oxygenatedandcell IIisdeaerated.netcurrentflowisobservedbetween thetwoelectrodes(Fig.2(f))andelectronflowsfr0m electrode1Ito工. 2.2Effectofcorrosioninhibitors CorrosioninhibitorsandtheireffectonLC4COrrO. sioncanbestudiedusingthesetupdescribedinFig.1 (b).Ifaninhibitorofaluminumdissolutionisinicoted intocell1Iofthesplitcel1.theobservedcurrent shoulddecreasetozero.Ifacathodicinhibitorisadd. edtocell工ofthespiltcel1.theobservedcurrent shouldagaindecreaseanddependontheinhibitoreffi. 《 b -J Z 91 ciency,thecurrentmaybeeliminated.Fig.3illus. tratesthepredicatedbehaviorofinhibitorsusingthe spiltcellmethod.T0testthehypothesis.aninhibitor thathasbeenpreviouslycharacterizedwaschosen. Potassiumdichromate(K2Cr207)actingasananodic inhibitorwaschosen.Fig.4showstheeffectofaddit. ionofa0.5%K2Cr2O7/0.1mo卜LNaClsolutionon theanodicreactionofcell1I.BecauseK2Cr2O7isan oxidizer,thepotentialofthecoupleinereasesupon addition.When0.5%K2Cr207isiniected,thenet, currentdecayssignificantly.Theinhibitionbehavioris consistentwiththepreviousobservationsofK,Cr,O1as ananodicinhibitoronaluminum.Theresultsfor K2Cr207appeartoconfirlnthehypothesisforspiltcell behavioroutlinedinFig.3(a). Ultimately,thismethodwasdevelopedtoprobe thecorrosioninhibitionmechanismofcerium(m). Fig.5showstheeffectofcerium(m)onthecathodic processonLC4.Inthiscase,about0.5%CeC11/0.1 mo1.LNaClsolutionwasinjectedtoceU工.After CeC13solutionwasinjected,thenetcurrentbetween electrodesIand1Idecreasesslowlytozero,butthe potentialofthecouplewasmovedtonegativeuponad- dition.Thisresultisincorrespondencewiththetestof .l'imc/l0:s (c) rime/l()s Fig.2Effectofdeaerationonsplitcellresponse(p2asdeaerated) (a)Netcurrent-timecurve;(b)Potentia1.timecurve;(c)CellIand? notdeaerated;(d)CellIdeaerated;(e)CellIandII deaerated;(f)CellIre-oxygenatingandcellIIdeaerated 一叫0??0意?l0 Na( ibitor l\,,e:一l o 暮 i.2 主 . 4 Nono!ctlrrofln('~~ . ,D【^【4OFRAREEARTHS,Voi.24,No.1,Feb.2006 ().Jmo Cathodic NLlnvtctlrr~lltmn, Fig.3Hypothesizedeffectofcorrosioninhibitorsonsplitcellresponse (a)Anodicinhibitor;(b)Cathodicinhibitor 一 (I___—.^一'I-'.AnodicinhibitiontestinjectCr(),solu!iontocellIl/()234 rimc川);s rimc/l【)s Fig.4EffectofK2Cr207onaluminumdissolutionreactionon LC4 ConcentrationofK2Cr207is,0.5% (a)Netcurrent.timecurve;(b)Potential—timecurve time.potential,anodictreatment,whichshowsthatthe iniectionofrareearthsaltmakesthecorrosionpoten— tialofLC4morenegative.AsisshowninFig.1(b), ifcell工iscathode,thereactionismainlyO2+2H2O +4e—}4OH.Thedecreaseofthenetcurrentshows thatthecathodicreactioniscurbed.oroxygendepo— larizationisinhibited.Thistestresultisalsoadirect proofoftheinhibitionofCe(?)onthecathodeofLC4 surface.Meanwhile.theresultshowsthattheinhibi. tionofCe(11)hasaslowstart,differentfromtherap— idfunctionofthestrongoxidantK2Cr207. 2.3EffectofCeCI3concentrationandpH Inordertodoafurtherresearchintotheinhibi. tionmechanismandfindouttheperfectbalancebe— tweenCeC13concentrationanditsinhibitiononcathod一 Iinl~/l0s l'ime/l0j Fig.5EffectofCe(Il1)onaluminumdissolutionreactionon LC4 ConcentrationofCe(Il1)is一0.5%. (a)Netcurrent—timecurve;(b)Potentia1.timecurve iereaction.asmallamountof10%CeCl1/0.1 mol?LNaClsolutionwasinjectedintocellIunder variouspHconditions.Thevariationofnetcurrentin differentconcentrationofCeC13wasrecorded.Thetest resultsareshowninFig.6.WhenpH=6.0,CeC13 concentrationover0.8%itcancompletelyinhibitthe cathodicreactionogLC4in0.1mol?L_.NaC1solu. tion.WhenpH=5.0andconcentrationofCeC11i8 largerthan0.8%.itcaninhibittheanodeofLC4in 0.lmol?L,NaC1solution.buttheinhibitioni8not complete.WhenpH=4.0andconcentrationofCeC1 islargerthan0.4%.itcancompletelyinhibitthean. odeofLC4.WhilepHis3.0.CeC1cannoteffectively inhibitthecathodicreactionofLC4.Fromthetestre. suititcanbeconcludedthatin0.1mol?LNaC1solu— tion,theinhibitionofCeC11oncathodicreactionofLC4 《t1i一l1:}芍Z一?一>墨一(i 一=)?一>时I|l'v(i DuBSeta1.CorrosionInhibitionMechanismofRareEarthMetal(REM)onLC4AlAlloy iscloselymimedtothepHvalueofthesolution.When pHislargerthan3.0,CeC13callhavealleffectiveinhi— bition.WhenpHisbetween3.0and6.0andhappensto be4.0,inhibitionofCe(1]I)isatthebest,requiringthe minimumconcentrationofCeC13tobe0.4%. 2.4Efiectofimmersiontime Fig.7showstheopen.circuitedpotentialofLC4 aluminumvarieswithtimeintestsolution.Itcanbe seenfrO111thediagramthatin0.111101?LNaC1solu. tionthepotentialofLC4varieslittle.Intherangeof ?10111V,ittakestheprocessofuniforilldissolution. Afterover20daysimmersion,thetestedsurfaceof thesamplebecomesdarkandiscoveredwithpitsof differentsizes.Butafter0.25%CeC11isiniectedinto 0.11110l?LNaClsolution.thepotentialvariationof LC4isgreat.First,withinjectionof0.25%CeC1. theinitialpotentialofLC4isdecreasedby60mV. Duringtheinitialstageofthetest(about0,60h), theopen—circuitedpotentialfluctuatesgreatly,then thepotentialkeepsdropping.About120hlater,it dropstothebottom.Subsequentlythepotentialclimbs graduallytillitstayssteadyintheend.Fig.7(2) showsthattheexistenceofrareearthCe(11)saltre. ducestheself-corrosionpotentialofLC4.Andthere existsadynamicprocessofnmtualcompetitionbe. 《 _兰 耋 弓 g Z Concentrationot(:eCI" ConcoltrationofCcCI/% Fig.6CurveillustratingtheeffectofCeC13onanodic (a)pH=3.O;(b)pH=4.O; tweenthedissolutionofalunfinmnalloyandthedepos. itofCe(?)saltinthetestsolution,inwhichdissolu. tionofalunfinmnalloyplaysamajorroleinthebegin- ning,butwithceriumconvemioncoatingsdeveloping, thedissolutionofaluminunlisrestrainedandthecor. respondingpotentialreachesthebottom.LaterCeox. idefihngraduallydissolves.Afterover20daysi111. mersion,thetestedsampleLC4takesashiningsur. face.Measuredbysight,thesurfaceofthesamplehas noobviouspits.Therefore,itshowsthatafterCeC1is injectedinto0.111101.LNaClsolution,thecorrosion 0falunlinunlalloyofLC4canbeeffectivelyinhibited. 2.5Electrochemicalmeasurements Fig.8showsthepolarizationcurvescorresponding toLC4sampletreatedbyfu11inunersionina0.111101? LNaC1+0.05%ofCeC1solutionduringtheindi. catedperiods.Incurve1.thesampleisuntreatedby CeC1andistestedin0.111101?L_.NaC1solution,the othersolutionis0.11130l?LNaCl十0.05%CeC1. Asisshownbythefigure,thecorrosionpotentialE. andthepitcorrosionpotentialE.ofthesample (Fig.8(1))whichisnottreatedbyrareearthsolu— tion,arealmostidentica1.Atier0.05%CeC11isin. iectedinto0.111101?LNaC1solution,thecorrosion C()nccntrati0l1ot'CcCI,, CollCClttrationofCcCl/% dissolutionreactiononLC4aluminumatvaryingpH (c)pH=5.O;(d)pH=6.0 ,,j岂荽u 《Iu0上1嚣Z 《r1c.JJ兀u:l potentialE.ofLC4movestowardthenegativeby about100mVandcorrosioncurrentalsodropsbytwo order8ofmagnitude.Withthetimeoffullimmersion in0.1mo卜LNaCl+0.05%CeC1solutionpro— longed,thesurfaceofLC4graduallyformsrareearth conversioncoatings.Afterthat.thecathodicbranchin thepolarizationcurvehassmallcurrent.Corrosionpo— tentialE.movestowardthenegative,thusseparating E—fromEpi,(rig.8:E一E.J一0,EPifl2一一2 ?5mV,Epit. 3一E... 3?15mV,E..t4一E.440 mV).Itisgenerallybelievedthatthelargerthegap betweenE.andEc.is,thebetterthecorrosion—re— sistancepropertyofthealuminumalloywillbe,espe— ciallythecorrosionresistanceofpit.Themovementof corrosionpotentialtowardthenegativemakesthetest— edsampleofaluminumalloyinpassivationandthe corrosionwillnotOccurintheopen—circuitedsitua. tion.AfterbeingtreatedbyCeCI,.LC4formsconv. ersioncoatingofrareearthonitssurface.Thetestre— suhofpolarizationcurveshowsthatconversioncoating ofrfl/'eearthexertsagoodinhibitiveeffectoncathode. ,/l()jinin Fig.7E…一tdia~amscorrespondingtoLC4aluminumal— loysampletreatedinthefollowingconditions (1)0.1mol-LI1NaC1;(2)0.1mol-LNaCI+0.25% Cea3 ? 2 > ×1…l×l0l×1O1×l0I×l0 i/area/(A-Clll!) Fig.8PolarizationcurvescorrespondingtoLC4sampletreated byfullimmersionina0.1mol?L一NaC1+0.05%of CeC13solutionduringtheindicatedperiods(Curve1: sampletestedin0.1tool?LNaC1solution) JOURNALOFRAREEARTHS,tlo1.24,No.1,Feb.2006 2.6Surfaceanalysis Fig.9(a)and(b)showthepatternofthesam. pleofLC4afterbeingimmersedin