Progress in Organic Coatings ,Volume 66, Issue 3,Pages 199-205

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ProgressinOrganicCoatings

journalhomepage:www.elsevier.com/locate/porgcoat

CompositeanticorrosioncoatingsforAZ91Dmagnesiumalloywithmolybdateconversioncoatingandsiliconsol–gelcoatings

JunyingHu,QingLi∗,XiankangZhong,LiangZhang,BoChen

SchoolofChemistryandChemicalEngineering,SouthwestUniversity,TianshengRoad,Chongqing400715,China

articleinfoabstract

ThisstudyevaluatedthecorrosionresistanceofAZ91Dmagnesiumalloycoatedbycompositecoatingswhichconsistedofamolybdateconversioncoatingandthreelayersofsiliconsol–gelcoatings.Formolyb-dateconversiontreatment,variousconditionsincludingthepHofthemolybdatebaths,immersiontimeandbathtemperaturewereinvestigatedusingelectrochemicalmeasurements.ThecorrosionresistanceoftheAZ91Dmagnesiumalloywasimprovedtosomeextentbytheconversioncoatingwiththeoptimalconversionparameters(7.3g/L(NH4)6Mo7O24·6H2OsolutionwithpH5for30minat30◦C).

InordertogetafurtherimprovementofcorrosionprotectionforAZ91Dmagnesiumalloy,threelayersofsiliconsol–gelcoatingsweresuccessfullydepositedonthemolybdateconversioncoatingpre-appliedtoAZ91Dalloy.Thesurfacemorphologyandthecorrosionprotectionperformanceofthecompositecoatingswereinvestigatedindetailusingscanningelectronmicroscope,electrochemicalimpedancespectroscopyaswellaspotentiodynamicpolarizationtests.TheresultsdemonstratedthatthecompositecoatingscouldgreatlyimprovethecorrosionprotectionperformanceoftheAZ91Dmagnesiumalloy.

© 2009 Elsevier B.V. All rights reserved.

Articlehistory:

Received14November2008

Receivedinrevisedform18February2009Accepted22July2009Keywords:

MagnesiumalloyMolybdateSiliconsol–gel

Corrosionresistance

1.Introduction

Magnesiumandmagnesiumalloysexhibitanattractivecombi-nationoflowdensityandhighstrength/weightratiomakingthemidealcandidatesforlight-weightengineeringapplications.How-ever,magnesiumandmagnesiumalloysarerelativelyreactiveandtendtosufferseverecorrosionduringservice[1,2].Toincreasethecorrosionresistanceandtoimprovethepaintadhesionprop-ertiesofmagnesiumalloys,chemicalconversiontreatmentsarecommonlyappliedtothesurface.Thepropertiesofconversioncoatingsarecloselyrelatedtotheirmicrostructureandcompo-sitions,whichinturn,stronglydependonthecompositionofthebasemagnesiumalloys,thepre-treatmentsurfacecleaning,thetypeandcompositionofthesolution,andthecorrespondingoperatingparameterssuchassolutiontemperatureandtypeanddegreeofagitation[3,4].Themostpopularandeffectiveconver-sionprocesstoproduceaprotectivelayeronthesurfaceofmetalsisbasedontheimmersionofthealloysinabathcontainingchro-mateions.Thehexavalentchromiumisreducedduringcorrosiontoformaninsolubletrivalentchromiumspeciesthatterminatestheoxidativeattack[1].Thesecoatingshavebeenshowntopro-videgoodcorrosionprotectionformagnesiumanditsalloysin

∗Correspondingauthor.Tel.:+8602368253884;fax:+8602368367675.E-mailaddresses:liqingswu@yeah.net,akang420@yeah.net,chenbo11-1984@163.com(Q.Li).

0300-9440/$–seefrontmatter© 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.porgcoat.2009.07.003

mildserviceconditions.However,theuseofchromatebathsisbeingprogressivelyrestrictedduetothehightoxicityofthehex-avalentchromiumcompounds.Inordertoavoidthisdrawbackthenewenvironmental-friendlypre-treatmentssuchasstannate,rareearthsaltandphosphate/permanganateconversiontreatmentshavebeenactivelydevelopedintherecentdecades[5–10].

Molybdates,tungstates,permanganatesandvanadates,includ-ingsimilarchemicalelementstochromium(groupsVIandVIIoftheperiodictable),werethefirstchemicalstriedasalternatives[11,12].MolybdenumiswellknownasalocalizedcorrosioninhibitorwhenpresentinelectrolyteasMo(VI)orasanalloyingelementinsteel[13].Becauseofthestrongoxidationcharacter,itsreductionpro-ductionisstabilizationandcanformapassivefilm.Likechromate,molybdateactsasanoxidantinthechemicalconversiontreat-ments,itsadsorbentproductioncaninhibitthepenetrationoftheinimicalionssuchasCl−toprotectthesubstrate.

MolybdatetreatmentshavebeenappliedonZn,Alalloyandsteelsubstratestoimprovethecorrosionresistanceofthosesub-strates[14–16].Molybdatetreatmentshavebeenalsoappliedfordecorativeandsolarabsorptionfunctionalpurposes[17].However,onlyfewworksarereportedontheapplicationofmolybdate-basedconversioncoatingsonmagnesiumalloys.Themainproblemwhichcanarisewhenpreparingmolybdate-basedcoatingonthemagne-siumalloysubstrateisthepresenceofporesandcrackswhatleadtoseriousdeteriorationofprotectivepropertiesofpre-treatmentlayer.Therefore,inordertoavoidthisdisadvantage,oneofthemeasuresistoprepareasealingcoatingtocoverontheporesand

200J.Huetal./ProgressinOrganicCoatings66 (2009) 199–205

Fig.1.Flowchartofpriorsurfacetreatmentsandthecompositecoatingspreparation.

cracks.Amongthevariouscoatingtechniquesthatareavailableforthispurpose,acoatingobtainedwithsol–gelmethodisofspecialinterestespeciallyintoday’senvironmental-protectivesociety.Adhesionofsol–gelcoatingstometalormetaloxidesurfacesisespeciallyfavorable,becausecovalentbondsareformedbetweenthesubstratesurfaceandthesol–gelfilm.Varioussol–gelproce-duresformagnesiumwerereportedintheliteratures[18–23].Inthispaper,anewlyenvironmental-friendlycompositeanti-corrosioncoatingstechniquewasdevelopedforAZ91Dmagnesiumalloytoimproveitscorrosionresistance.Themolybdateconver-sioncoating,whichactsasabarriertoenvironment,improvedthecorrosionresistanceofthemetaltosomeextent,thensiliconsol–gelcoatingwasdepositedonthemolybdateconversioncoatingthroughsol–geltechniquetogetafurtherimprovementofcorro-sionprotectionformagnesiumalloy.Electrochemicaltechniqueshavebeenemployedtocharacterizethecorrosionpropertiesofthesecoatings.2.Experimental2.1.Materials

Die-casteAZ91Dmagnesiumalloywithachemicalcomposition(wt.%)ofMg–8.77Al–0.74Znwasusedinthisstudy.Fig.1outlinestheproceduresforthemechanicalandcompositecoatingstreat-ment.

2.2.Molybdateconversioncoating

Themolybdateconversioncoatingwasobtainedbyimmersionofthesamplesinaerated7.3g/L(NH4)6Mo7O24·6H2Oaqueoussolu-tionaddedwith8g/LKMnO4andmoderateamountofNaFadditiveatdifferenttemperatures(20,30and50◦C).Dippingtimeswere

10,30and60min,respectively.ThepHofthemolybdatebathwasadjustedto3and5byphosphoricacid.Afterconversion,thesam-pleswererinsedindistilledwater,anddriedfor30mininovenat60◦C.

2.3.Siliconsol–gelcoating

Siliconsolwaspreparedfromtetraethylorthosilicate(TEOS),3-glycidoxypropyltrimethoxysilane(GPTMS)andethanol,whichweremixedinamolarratioof0.25:0.75:10,respectively,withthewaterbeingaddeddropwiseintothemixture.Aceticacidwasaddedtopromotethehydrolysisreactionat60◦C,thenammoniawasaddedtoacceleratethecondensationreactionafterhydrolysisfor30min.Theapplicationofsiliconsol–gelcoatingonthecon-versioncoatingwasconductedbyusingaspincoatingtechnique.Coatedsampleswerecuredinanovenat100◦Cfor60min.2.4.Analysisandmeasurements

TheFouriertransforminfrared(FT-IR)spectrumrecordedwithIR-10300(PerkinElmer,America)hasbeencarriedouttoanalyzethestructureofsiliconxerogelpreparedbysiliconsolheattreat-mentat100◦C.

ThepotentiodynamicpolarizationcurveswereperformedusingaPS-268Bsystem(Zhongfu,Beijing,China).Athree-electrodecell,withthesampleastheworkingelectrode,asaturatedcalomelelec-trode(SCE)asreferenceandaplatinumsheetascounterelectrode,wasemployedinthosetests.Theareaoftheworkingelectrodewas1.0cm2.Anaqueoussolutionof3.5wt.%NaClinwhichpHvaluewasadjustedto7withHClorNaOHwasusedaselec-trolyteintheelectrochemicalmeasurements.Afteraninitialdelayof10mininelectrolyte,potentiodynamicpolarizationcurveswerescannedfrom−1.7Vatarateof1mV/s.TheEISmeasurementswere

J.Huetal./ProgressinOrganicCoatings66 (2009) 199–205201

Fig.2.Potentiodynamicpolarizationcurvesin3.5wt.%NaClsolution.(a)Thebaresubstrate.AZ91DmagnesiumalloyimmersedinthemolybdateconversionbathwithpH3for(b)10min,(c)30minand(d)60min.

Fig.3.Potentiodynamicpolarizationcurvestestedin3.5wt.%NaClsolution.AZ91DmagnesiumalloyimmersedinthemolybdateconversionbathwithpH5for(a)10min,(b)30minand(c)60min.

carriedoutusingIM6esystem(Co.Germany)inthefrequencyrangeof105Hzto50mHz.Thesignalamplitudewas10mV.Allsampleswereimmersedfor30minbeforeimpedancemeasurements.Elec-trochemicalimpedancespectrometry(EIS)dataarepresentedasNyquistplotsandBodeplots.Allofthetestswereperformedatroomtemperature.

Thesurfacemorphologyofthemolybdateconversioncoatingandthecompositecoatingswasobservedusingimagesobtainedfromascanningelectronmicroscope(SEM)S-4800(HITACHI,Tokyo,Japan).Theenergyusedforthisanalysiswas20kV.3.Resultsanddiscussion3.1.Molybdateconversioncoating

Inordertoobtainaneffectiveprotectivemolybdatecoversioncoating,itisanissueofprimeimportancefortheinvestiga-tionoftheoptimalconversionparametersincludingthepHofthemolybdatebaths,immersiontimeandbathtempera-ture.Hereinpotentiodynamicpolarizationmeasurementswereemployedasamaintechniqueforthisinvestigation.Therepresen-tativepotentiodynamicpolarizationcurvesforthesamplestreatedbymolybdate-basedbathswithpH3and5fordifferentdippingtimesarepresentedinFigs.2and3,respectively.Atthesametime,therelevantelectrochemicalparameterscalculatedfromtheTafelplotsarealsolistedinTable1.Thepolarizationresistance(Rp)isdeterminedfromtheslopeofthevoltageversuscurrentdensitycurveatthecorrosionpotential.Inthispaper,thepotentialforcal-culationofRpischosenoveranarrowvoltage,from−10to+20mVrelativetocorrosionpotential(Ecorr).Somerelativelyassumptionsoflinerarityhavebeengiven[24].

Corrosioncurrentdensity(Icorr),corrosionpotential(Ecorr)andpolarizationresistance(Rp)areoftenusedtoevaluatethecorrosionprotectivepropertyofthecoatings.Basedonthesedata,itiseasytodrawaconclusionthatthemosteffectiveprotectivemolybdateconversioncoatingcanbeobtainedinasolutionpH5,immersionfor30min.Additionally,thebathtemperaturedidnothavesignif-icanteffectonpreparingthemosteffectiveprotectivemolybdateconversioncoating.

WecandiscusstheinfluenceofthebathpHonthecorrosionprotectionofthemolybdateconversioncoatingwhenthebathtem-peratureandimmersiontimearetreatedasconstants30◦Cand30min,respectively.AsisshowninFigs.2and3aswellasTable1,itcanbeseenthatthemorecorrosionresistiveconversioncoat-ingswereobtainedusingbathswithpH5incontrasttopH3.TheEcorrforthesamplestreatedinthebathswithpH3and5is−1.450and−1.435V,respectively.AndalowerIcorrofthecoatingcanbeobtainedfrombeingtreatedinpH5solutioncomparedwiththecaseofpH3.

Theimmersiontimealsosignificantlyaffectedtheprotectiveabilitiesofthesamples.Theeffectofimmersiontimeinthemolyb-datebathswasprominentatpH5.Inthiscase,thelongerthetime,thehigherweretheresistances.Butafter30minthesitua-tionchanged,longerimmersiontimesinthebathsproducedalessprotectivefilm,aswasevidentfromthereductionintheEcorrandtheincreaseinIcorrrecordedforthe60minmolybdate-treatedelec-trode.Presumably,thisisbecausethecrackdistributionisincreasedonthesurfacewhenforimmersiontimebeyond30min,whichwassupportedbyimagesofSEMshowninFig.4(c).Sothebestanti-corrosionmolybdateconversioncoatingwasachievedinthebathswithpH5for30min.

Theincreaseofthebathtemperaturefrom20to50◦Cdidnothaveanysignificanteffect.Thepotentiodynamicpolarizationcurveswerenotlisted.Yangetal.[25]alsodemonstratedthatthetemperatureofthevanadium-basedchemicalconversioncoatingonthecorrosionresistanceofmagnesiumalloydidnothaveobviouseffect.

TheSEMimagesshowninFig.4arethemorphologiesofAZ91Dmagnesiumalloyafterimmersioninmolybdatebathfor10,30and60min.After10minofimmersionintheconversionsolution,thenucleationofthemolybdateconversioncoatingwasjuststartedandcoatingthicknesswasnotenoughtocompletelycoverthesurfaceofthesampleshowninFig.4(a).Fig.4(b)displaysthesur-facemorphologyofAZ91Dmagnesiumalloyaftertheconversioncoatingtreatmentinthemolybdatebathforabout30min.Itcanbeseenthatthemolybdateconversioncoatingpresentsnetworkfeature,andthereweremanymicro-cracksonthesurfaceofthecoating.Thesecracksmaybeduetothereleaseofhydrogenfrom

Table1

TheelectrochemicalparametersofpotentiodynamicpolarizationcurvesofpH5and3calculatedfromtheTafelplots.

pH310min

Ecorr(mV/SCE)Icorr(A/cm2)Rp(󰀂cm2)

−1503

17.4×10−5114

30min−1450

9.0×10−5293

60min−1513

14.4×10−5150

pH510min−1510

4.0×10−52

30min−1435

1.76×10−51230

60min−1480

3.8×10−5566

202J.Huetal./ProgressinOrganicCoatings66 (2009) 199–205

Fig.4.SEMimagesonAZ91DmagnesiumalloyinthesolutionofpH5for(a)10min,(b)30min,(c)60min,(d)30minandsiliconsol–gelcoatings.

somechemicalreactionsuchasthereplacementreactionofMginacidsolutionduringtheconversiontreatmentand/orthedehydra-tionofthemolybdateconversioncoatingaftertreatment.Similarphenomenacanbeseenintherareearthconversioncoatingsandpermanganate–phosphateconversioncoatings[6]inmagnesiumalloyaswellasmolybdateconversioncoatingsinzincgalvanisedsteel[11].Fig.4(c)exhibitsthesurfacemorphologyofAZ91Dmag-nesiumalloyafter60minimmersiontreatmentinmolybdatebath.Itisobviousthattheformationofcrackswasmoreseriousforlongerimmersiontimesthanforshortertimes’immersion.

Inspiteoftheachievementforinvestigationtheoptimalparam-etersofthemosteffectiveprotectiveconversioncoatinghasbeenobtained,thecorrosionprotectionofthiscoatingisreallylimitedowingtothedefectsinthecoating.Therefore,furthertreatmentforenhancingthecorrosionprotectionofthemolybdateconversioncoatingisnecessary.3.2.Compositecoatings

ThecompositecoatingsconsistedofaconversioncoatingtreatedbymolybdatebathwithpH5for30minandthreelayersofsiliconsol–gelcoatingspreparedbyaspinningdepositiontechnique.Asatopcoatingofthiscompositecoating,thethreelayersofsiliconsol–gelcoatingplayaverysignificantroleinimprovingthecor-rosionprotectionperformanceofAZ91Dmagnesiumalloy,owingtotheformationofaverystable,continuous,andhighlyadherentprotectivecoatingonthemolybdateconversioncoating.Thesolwasproducedbyatwo-stepprocesswhichincludeshydrolysisviaacidcatalysisandcondensationviabasecatalysis.Asthecrosslink-ingandassemblingoccursuponapplicationofthesiliconsolonthepreparedmolybdatecoatingsimultaneouslywiththeevapora-tionofthesolventandcoatingformationleadingtothecompositecoatings.

Fig.5showstheFT-IRspectraofthesiliconxerogelandthesolprecursors(TEOSandGPTMS).ItisknownthattheSi–O–SiorSi–Ohasanasymmetricstretchingmodeinawideregion.ComparisonoftheIRspectraofthesiliconxerogel,TEOSandGPTMS,siliconxerogelrevealedabroad󰀁as(Si–O–Si)(1000–1110cm−1),the󰀁as(Si–O)oftheTEOSandGPTMSwaspresentat1081and1088cm−1,respectively.Besides,therewas󰀁as(C–O–C)(1194cm−1)intheFT-IRspectraofGPTMSandsiliconxerogel.Basedonthedifferencesliedinhydroxylgroupsinfraredabsorptionband,itwasevidentthattheco-condensationreactionbetweenTEOSandGPTMShadoccurred.Forthesiliconxerogel,theinfraredabsorptionbandofhydroxylgroupappearedat2939cm−1,whereasthebandat2943and2841cm−1attributedtothepresenceofhydroxylgroupinGPTMS.However,thehydroxylgroupinfraredabsorptionbandisnotfoundintheTEOS.

Fig.5.TheFT-IRspectrumofsiliconxerogel,TEOSandGPTMS.

J.Huetal./ProgressinOrganicCoatings66 (2009) 199–205203

Fig.6.Potentiodynamicpolarizationcurvesfor(a)thebaresubstrate,(b)thesingleconversioncoatingand(c)thecompositecoatings.

SEMimageforthecompositecoatingsisshowninFig.4(d).Itcanbeseenthatthecompositecoatingsiscompactandimper-forate.Itwasexpectedthatthecompositecoatingsholdthebestanti-corrosionabilityformagnesiumalloy.Thisobservationwassupportedbythesubsequentelectrochemicalmeasurementresults.

Inordertocharacterizethegeneralcorrosionpropertiesofthecompositecoatings,thepotentiodynamicpolarizationcurveswererecordedinthe3.5wt.%NaClsolution.Therepresentativepolariza-tioncurvesforthecompositecoatingsandtheconversioncoatingareshowninFig.6.Incontrasttothesubstratecoatedwithasin-gleconversioncoating,thecompositecoatingsexhibitlowerIcorrandhigherEcorrindicatingthatthecompositecoatinghasbettercorrosionprotectionperformancecomparedwiththesinglecon-versioncoating.Itdemonstratesthatthesiliconsol–gelcoatingscancovertheporesandcracksonthemolybdateconversioncoat-ingindeedandofferabettercorrosionprotectionperformance.TherelevantelectrochemicalparameterscalculatedfromtheTafelplotsarelistedinTable2.

AsisshowninTable2,comparedwiththebaresubstrate,thesingleconversioncoatinghasalowercorrosioncurrentdensity(Icorr),highercorrosionpotential(Ecorr)andlargerpolarizationresistance(Rp).WhileIcorrforthecompositecoatingisdecreasedapproximatelybytwoordersofmagnitude,Ecorrisshiftedposi-tively190mVandRpisincreasedby35-foldcomparedwiththebaresubstrate,respectively.

Theelectrochemicalimpedancespectroscopyisoneofthemostintensivelyusedandpowerfultechniquesfortheinvestigationandpredictionoftheanticorrosionprotection[26].EISwasusedinthisworktoestimateprotectiveabilitiesoftheconversioncoatingandthecompositecoatingsontheAZ91Dmagnesiumalloyinthe3.5wt.%NaClaqueoussolution.AndEISdataforafreshlypolishedsampleofAZ91Dmagnesiumalloywithnocoatingwasincludedasacomparison.

Thelowfrequencyimpedanceisoneoftheparameterswhichcanbeeasilyusedtocomparecorrosionprotectionperformanceofdifferentsystems.Higherimpedancedemonstratesbetterprotec-tion[26].AsshowninFig.7,thebaresubstrateshowedthepoorestcorrosionresistanceafter30minofimmersioninthe3.5wt.%NaClaqueoussolution.Inthelowfrequencyregion,itcanbeseenthattheimpedancedropsasthefrequencydrops,indicatingthatthesurface

Table2

TheelectrochemicalparametersofpotentiodynamicpolarizationcurvescalculatedfromtheTafelplots.

Ecorr(mV/SCE)

Icorr(A/cm2)Rp(󰀂cm2)Thesubstrate

−1614.01.29×10−51685.20Theconversioncoating−1435.61.76×10−51230.46Thecompositecoatings

−1425.5

3.80×10−7

58,692.10

Fig.7.EISBodeplotsobtainedin3.5wt.%NaClsolutionfor(a)thebaresubstrate,(b)thesingleconversioncoatingand(c)thecompositecoatings.

oxidelayerofthebaresubstrateformedinairisnotprotective[27].Forsamplesoftheconversioncoatingandthecompositecoatings,thelowfrequencyimpedanceinBodeplotcontinuestoincreaseasthefrequencydrops,indicatingastrongresistancetotheflowofionsandelectronsbythecoatings,hencepreventingcorrosion.Especially,thecompositecoatingsincreasetheimpedancefurtherbyatleasttwoordersofmagnitudecomparedwiththebaresub-strate.ThesameeffectsoncorrosionresistancecanbeobservedinNyquistplot(Fig.8).Thesurfaceresistanceofcompositecoatingswasabout4.5×104󰀂cm2,whichisequalto75timestheresis-tanceofbaresubstrate.Thus,itdemonstratesthatthecompositecoatingsserveasaconsiderablyeffectivebarrieragainstcorrosionelectrolyteingressduringEISmeasurement.

Forquantitativeestimationofthecorrosionprotection,exper-imentalimpedancespectrawerefittedusingequivalentcircuits.Theimpedancespectroscopyoftheconversioncoatingwasmod-eledusingtheschemeshowninFig.9(a).Here,constantphaseelementswereusedinsteadofcapacitancesinordertotakeaccountofthedispersivecharacterofthetimeconstantsoriginat-

Fig.8.EISNyquistplotsobtained3.5wt.%NaClsolutionfor(1):(a)thebaresubstrateand(b)conversioncoating,(2)thecompositecoatings.

204J.Huetal./ProgressinOrganicCoatings66 (2009) 199–205

Table3

ThemainfittingparametersforEISdataobtainedfortheconversioncoatingandthecompositecoatings.

Rinter(󰀂cm2)

Rconv(󰀂cm2)Rsol(󰀂cm2)Rpolar(󰀂cm2)Cdl(F/cm2)Conversioncoating39.91366.0Compositecoatings

138.9

1032.0

Fig.9.Theconversioncoatingequivalentcircuit(a)and(b)thefittingresult.

Fig.10.Thecompositecoatingsequivalentcircuit(a)and(b)thefittingresult.

–552.95.03×10−416,720.0

29,260.0

2.98×10−7

ingfromthenonuniformityofthecoatings.Rsoltistheresistanceofsolution;RinterandQinterrefertotheresistanceandthecapac-itanceofsolution/coatinginterface,respectively;RconvandQconvrepresenttheresistanceofconversioncoating,respectively.Thepresenceofseveralporesandcracksisontheconversioncoating,inwhichpenetrationofactivechlorideionsandwaterleadstopar-tialdestruction.Withthedevelopmentofthecorrosionprocess,thepolarizationresistanceRpolaranddoublelayercapacitanceCdlshouldbeaddedinthefittingcircuitduetotheappearanceofanewtimeconstantinthelowfrequencyregion.ThecorrespondingequivalentcircuitfortheEISofthecompositecoatingsisshowninFig.10(a).Comparedwiththefittingcircuitofconversioncoating,RsolandQsolrepresentingtheresistanceandcapacitanceofsol–gelcoating,respectively,areaddedtothisoneduetothedepositionofsiliconcoatingsontheconversioncoating.

BasedonthemainparametersforEISdataobtainedforthebothcoatingslistedinTable3,agreatimprovementoncorro-sionresistanceofthesubstratehasbeenobtainedowingtothecontributionofcompositecoatingswithahighcoatingresistance(R=Rinter+Rsol+Rconv+Rpolar).Namely,thecompositecoatingsactasagoodbarrierforthepenetrationofactivechlorideionsandwater,subsequentlyenhancingtheanticorrosionperformanceoftheAZ91Dmagnesiumalloy.Cdlofthecompositecoatingssampleshowsalmostaverylowvalue(2.98×10−7F/cm2)demonstratesexcellentprotectivepropertieseventheabsenceofactivedefectsatsubstrate/coatinginterface.

4.Conclusions

Compositeanti-corrosioncoatingsforAZ91Dmagnesiumalloyhasbeenevaluatedinthiswork.Thefirstcoatingismolybdatecon-versioncoating,whilethesecondisthreelayersofsiliconsol–gelcoatings(appliedbyspincoatingtechniqueafterthedepositionmolybdateconversioncoatingonAZ91Dmagnesiumalloy).

Themolybdateconversioncoating(7.3g/L(NH4)6Mo7O24·6H2OsolutionwithpH5for30minat30◦C)provideshighercorrosionpotentialalloybutalowercorrosioncurrentdensitytotheAZ91Dmagnesiumalloyasindicatedbyelectrochemicalmeasurementscarriedoutatroomtemperature.Themolybdateconversioncoatingpresentsnetworkfeature,andplentyofmicro-cracksexistsonthesurfaceofthecoatingobservedbySEMimage.

Thethreelayersofsiliconsol–gelcoatingscantotallycoverthecracksproducedonthemolybdateconversioncoatingascanbeseenfromtheSEMimage.Itcanexplainwhythecompositecoat-ingsholdthebestcorrosionresistantcomparedwiththesingleconversioncoatingandthebaresubstrate.Acknowledgements

TheauthorsthankthesupportsoftheNaturalScienceFoun-dationofChongqing,China(CSTC.2005BB4055)andHigh-TechCultivationProgramofSouthwestNormalUniversity(No.XSGX06).References

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