ystallineMaterials,130025,15paper.ThesurfacemorphologiesofthecoatingswerestudiedbySEM1.Introductionresistance,solderability,electricalconductivityordecorativeappearance.Thiscanbeaccomplishedbycoatingthepartswithametalthathasthedesiredpropertiesnecessaryfortheelectrolessnickelplatingandzincimmersion2.Itcanbenotedthatinmanypreviousreportsontheelectrolessplatingonmagnesiumalloys36,thenickelionsweretheplatingbath.Surface&CoatingsTechnology200Magnesiumisbecomingincreasinglysignificantasalightweightmetalstructuralmaterial(withadensityof1.74g/cm3)inmanyindustriesaircraftconstruction,spacetechnology,optics,andautomobilemanufacturing,forexample.However,magnesiumisintrinsicallyhighlyreactiveanditsalloysusuallyhaverelativelypoorcorrosionresistance,whichrestrictstheapplicationofmagnesiumalloysinpracticalenvironments.So,itisoftendesirabletoalterthesurfacepropertiesofamagnesiumormagnesiumalloyworkpieceinordertoimproveitscorrosionandwearspecificapplication1.Sincemagnesiumisoneofthemostelectrochemicallyactivemetals,anycoatingsonmagnesiumalloysshouldbeasuniform,adheredandpore-freeaspossible.Oneofthemostcosteffectiveandsimpletechniquesforintroducingametalliccoatingtoasubstrateistheplatingtechniques,includingelectrolessplatingandelectroplating.Further-more,magnesiumisclassifiedasadifficultsubstratetoplatemetalduetoitshighreactivity.Asforelectroplatingonthemagnesiumalloy,therearecurrentlytwoprocessesusedforplatingonmagnesiumandmagnesiumalloys:directandFESEM.ThencNicoatinghadanaveragegrainsizeofabout40nmandanevident200preferredtexturerevealedbyXRD.ThehardnessofthencNicoatingwasabout580VHN,whichwasfarhigherthanthat(about100VHN)oftheAZ91Dmagnesiumalloysubstrate.TheelectrochemicalmeasurementsshowedthatthencNicoatingonthemagnesiumalloyhadthelowestcorrosioncurrentdensityandmostpositivecorrosionpotentialamongthestudiedcoatingsonthemagnesiumalloy.Furthermore,thencNicoatingontheAZ91Dmagnesiumalloyexhibitedveryhighcorrosionresistanceintherapidcorrosiontestillustratedinthepaper.ThereasonsforanincreaseinthecorrosionresistanceofthencNicoatingonthemagnesiumalloyshouldbeattributabletoitsfinegrainstructureandthelowporosityinthecoating.D2005ElsevierB.V.Allrightsreserved.Keywords:Nanocrystallinenickel；Magnesiumalloy；Electroplating；Electrolessnickel；CorrosionmagnesiumalloywithdifferentthicknesswerealsopresentedintheAbstractNanocrystalline(nc)Nicoatingwasdirect-currentelectrodepositedontheAZ91Dmagnesiumalloysubstrateaimedtoimproveitscorrosionresistanceusingadirectelectrolessplatingofnickelastheprotectivelayer.Ascomparison,twoelectrolessNicoatingsontheHighcorrosion-resistancenanocrmagnesiumChangdongGu,JiansheLian*,JinguoKeyLabofAutomobileMaterials,MinistryofEducation,CollegeofChangchunReceived22March2005；acceptedAvailableonline0257-8972/$-seefrontmatterD2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.surfcoat.2005.07.001*Correspondingauthor.Fax:+864315095876.E-mailaddress:(J.Lian).NicoatingonAZ91DalloyHe,ZhonghaoJiang,QingJiangScienceandEngineering,JilinUniversity,NanlingCampus,Chinainrevisedform4July2005August2005(2006)54135418/locate/surfcoatprovidedbybasicnickelcarbonateinDifferentfromthemethodsmentionedabove,directelectro-lessnickelplatingontheAZ91Dmagnesiumalloywasquicklyaspossiblebetweenanytwostepsofthetreatments.ThedirectelectrolessnickelplatingwiththehardnesstesterwithVickersindenter,ataloadof100ganddurationof15s.ElectrochemicalmeasurementswereperformedonanElectrochemicalAnalyzer(CHI800,Shanghai,China),whichwascontrolledbyacomputerandsupportedbysoftware.LinearSweepVoltammetryexperimentswerecarriedoutina3wt.%NaClaqueoussolutionusingaclassicthree-electrodecellwithaplatinumplate(Pt)ascounterelectrodeandanAg/AgClelectrode(+207mVvs.SHE)asreferenceelectrode.Beforetesting,theworkingelectrodewascleanedinacetoneagitatedultrasonicallyfor10min.Theexposedareafortestingwasobtainedbydoublycoatingwithepoxyresin(EP651)leavinganuncoveredareaofapproximately1cm2.Thereferenceandplatinumelectrodeswerefixedneartotheworkingelectrode(about0.5mm),whichcouldminimizetheerrorsduetoIRdropintheelectrolytes.Duringthepotentiody-namicsweepexperiments,thesampleswerefirstimmersedinto3wt.%NaClsolutionforabout20mintostabilizetheopen-circuitpotential.Potentiodynamiccurveswererecordedbysweepingtheelectrodepotentialfromavalueofabout300400mVlowertoavalueof500600mVupperthanthecorrosionpotential,respectively,atasweepingrateof5mV/s.Thelog(i)EcurveswereTime30min(about15m)Fig.1.ThetechnicalflowchartoftheelectroplatingncNiontheAZ91Dmagnesiumalloy.Technologythicknessofabout10AmonAZ91Dmagnesiumalloy7wasusedastheprotectivelayerforfurtherplatingonthemagnesiumalloy.TheelectroplatingncNicoatingonthemagnesiumalloywasdirect-currentelectroplatedfromabathcontainingnickelsulfate,nickelchloride,boricacidandsaccharinatapHof5.0andatemperatureof50-C.Duringtheelectrodepositionprocess,theanodewasusedanelectrolyticnickelplate.TheoperationofelectroplatingncNicoatingwasundertakenforabout30minwhichwouldgivethecoatingwiththethicknessofabout15Am.Ascanningelectronmicroscope(SEM,JEOLJSM-5310,Japan)andafieldemissionscanningelectronmicroscope(FESEM,JEOLJSM-6700F,Japan)wereemployedfortheobservationsofthesurfaceofthecoatingsandthecross-sectionmorphologyandanEDXattachmentwasusedforqualitativeelementalchemicalanalysis.Crystal-linestructureofthesamplewasstudiedbytheX-raydiffractometer(XRD,RigakuD/max,Japan)withaCutargetandamonochronmatorat50kVand300mAwiththescanningrateandstepbeing4-/minand0.02-,recentlyundertakenbyusingaplatingbathcontainingsulfatenickel7.Intherecentyears,therehavebeenconsiderableinterestsinunderstandingthemechanicalproperties,thecorrosionresistanceandthewearresistanceofncmetalsproducedbyelectrodeposition,forexample814.Accordingtothem,ncmaterialsexhibitedmanyunusualmechanicalandelectrochemicalpropertiescomparedwithconventionalpolycrystallineoramorphousmaterials.Sointroducinganccoatingcombinedthehighcorrosionresistancewithgoodwearresistanceonmagnesiumalloysubstratewouldbeverypromising.Inthepresentpaper,theelectrolessNiplatingfromanacidicbath7wasfirstdepositedonAZ91Dmagnesiumalloyastheprotectivelayerforthefurtherelectroplatingoperation,andthenancnickelcoatingwasdirect-currentelectroplatedontheprotectivelayer.ThemicrostructuresandtheelectrochemicalpropertiesofthecoatingsontheAZ91DmagnesiumalloysubstratewerestudiedbySEM,FESEM,XRDandelectrochemicalmeasurement.2.ExperimentalThesubstratematerialusedwasAZ91Ddiecastmagnesiumalloywithasizeof30C240C25mm.Thealloywasmainlycontainedabout9.1%Al,0.64%Zn,0.17%Mn,0.001%FeandMgbalance.Thesampleswereabradedwithno.1500SiCpaperbeforethepretreatmentprocesses.ThetechnicalflowchartoftheelectroplatingontheAZ91DmagnesiumalloyisshowninFig.1.Thesampleswerecleanedthoroughlywithde-ionizedwaterasC.Guetal./Surface&Coatings5414respectively.ThehardnessofthemagnesiumalloyandthecoatingswereevaluatedusingaHXD-1000micro-AlkalinecleaningNaOH45g/lNa3PO412H2O10g/lTemperature65CAcidpickleCrO3125g/lHNO3(70%V/V)100ml/lTime40sRoomtemperatureFluorideactivationHF(40%V/V)350ml/lTime10sRoomtemperatureElectrolessplatingprotectivelayerref.7Time15min(about10m)ElectroplatingnanocrystallineNiNiSO46H2O250-300g/lNiCl26H2O30-40g/lH3BO330-45g/lC7H5NO3S0.1-0.2g/lCurrentdensity3A/dm2200(2006)54135418measuredandplottedaftertheaboveelectrochemicalmeasurements.ThecorrosionpotentialEcorrandcorrosioncurrentdensityicorrweredetermineddirectlyfromtheselog(i)EcurvesbyTafelregionextrapolation.Acidimmersiontestin10%HClsolutionatroomtemperaturewasundertakentotestthecorrosionresistanceofthencNicoatingonthemagnesiumalloy.Ifthereweremicroporesinthecoatings,thecorrosionsolutionwoulderodethemagnesiumsubstratethroughthepores.Duetothehighchemicalactivityofthemagnesium,theH+inthecorrosionsolutionwouldbereducedbythemagnesiumandturnedintothehydrogengasbubbles.Sothetimeintervalbetweenthestartofthetestandthefirsthydrogengasbubblearisingfromthecoatingsurfacecouldbeusedtodonatethecorrosionresistanceofthecoatingsonthemagnesiumalloysubstrate.Forcomparison,theelectrolessplatingnickelcoatingswithdifferentthickness(10and25Am)werealsotestedinthepaper.3.Resultsanddiscussions(200)peakviatheScherrerequation16:dXRDkkbhcosh1WherekistheX-raywavelength,btheFWHM(fullwidthofhalfmaximum)ofthe(200)diffractionpeak,hthediffractionangleandtheconstantkC2291.FromEq.(1)theaveragegrainsizeofthisncNiwasabout40nm.Inaddition,theXRDresultsalsoshowedthatthencNicoatinghadanevident200preferredtexture.Infact,Nielectro-depositsareknownforgivingnumerous,well-definedpreferredorientationsdependingonelectrodepositioncon-ditions,i.e.electrolytecomposition,temperature,pH,currentdensity,stirringandorganicadditions17,18.The200preferredtextureofthencNiinthisstudymaybeattributabletothegivenelectrodepositionconditionswhichmayleadtohigherelectrodeoverpotentialandreducedconcentrationofNi2+attheelectrodesurface18.Fig.3(a)showedthetypicalsurfacemorphologyoftheas-depositedncNicoating.Itcanbeseenthattheas-depositedsurfaceofthencNicoatingwasverycompactandnocolonystructures,whichwastotallydifferentfromthecauliflower-like(inmicrometersize)surfacemor-phologyoftheelectrolessnickeldeposition(seeFig.3(b).Moreover,theas-depositedsurfaceofthencNicoatingexhibitedaflatandmirror-likeappearanceandthe25000NiC.Guetal./Surface&CoatingsTechnology20304050607080050001000015000200002030405060708005001000150020002500(a)2/deg.20304050607080050010001500(b)Intensity/CPSMg17Al12Mg(c)Fig.2.TheXRDpatternsoftheelectroplatingNiontheAZ91Dmagnesiumalloyatdifferentintervals,(a)AZ91Dmagnesiumalloy3.1.MicrostructuresandhardnessofthecoatingsFig.2(a)showedthepatternofXRDoftheAZ91Dmagnesiumalloy,whichindicatedthatthesubstratealloyconsistedofprimarya(Mg)grainssurroundedbyaneutecticmixtureofaandb(Mg17Al12)15.ItalsocanbeseenfromtheXRDpatternofFig.2(b)andthemorphologyofFig.3thatafterelectrolessNiplatingforabout15min,theAZ91Dmagnesiumalloywasfullycoveredbytheelectrolessnickeldeposition.Thephosphoruscontentintheelectrolessdepositionwasverylow,becauseNiwasfirstdepositedonthesurfaceofmagnesiumalloyaccordingtothedepositionmechanismofelectrolessplatingonthemagnesiumalloy7.TheXRDanalysisresultsoftheas-depositedelectroplatingncNicoatingwasshowninFig.2substrate,(b)electrolessplatingonthesubstratefor10min,(c)theelectroplatingncNicoating.(c).ThegrainsizedofncNicanbedeterminedfromtheFig.3.Thesurfacemorphologyofa)thencNicoatingandb)theelectrolessnickelplatingontheAZ91Dmagnesiumalloyforabout10min.200(2006)541354185415grainsizeofthedepositscouldnotberesolvedbyconventionalSEMobservations.Inordertomakeaclearobservationofthefinegrainstructure,specimenofthisncNiwaspolishedandcorrodedbydilutenitrate/ethanolsolutionbeforetheSEMobservation.Thesurfacemor-phologyofthencNiafterabovepretreatmentswasshowninFig.4.TheveryuniformncgrainstructurescanbeseenonthesurfaceofncNiafterslightlycorroded.Fig.5(a)showedthecross-sectionmorphologyofthencNicoatingontheAZ91Dmagnesiumalloy.ToindicatethesubstrateandtwotypesoftheNilayersonthecross-sectionofthecoating,thesubstrate,theprotectivelayerandthencNilayerweremarkedbythelinewithtwoarrows,respec-tively,intheFig.5(a).Fromthisfigure,