外文翻译--用自由铬预先处理在 AZ91 D 镁合金上电镀 Ni－P层英文版

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1、Electroless NiP layer with a chromium-free pretreatmenton AZ91D magnesium alloyW.X.Zhang,J.G.He,Z.H.Jiang,Q.Jiang,J.S.LianKey Laboratory of Automobile Materials(Jilin University),Ministry of Education,Changchun 130025,ChinaDepartment of Materials Science and Engineering,Jilin University,Changchun 13

2、0025,ChinaReceived 1 July 2006;accepted in revised form 28 September 2006Available online 27 November 2006AbstractA phosphate-manganese conversion film was proposed as the pretreatment layer between NiP coating and AZ91D magnesium alloysubstrate,to replace the traditional chromium oxide plus HF pret

3、reatment.The subsequent NiP deposited on the layer was also characterized byits structure,morphology,microhardness and corrosion-resistance.The pretreatment layer on the substrate not only reduces the corrosion ofmagnesium during NiP plating process,but also reduces the potential difference between

4、the matrix and the second phase.Thus,a NiP coatingwith fine and dense structure was obtained on the AZ91D magnesium alloy,which shows better corrosion resistance than the NiP with chromiumoxide plus HF as pretreatment.2006 Elsevier B.V.All rights reserved.Keywords:Chromium-free pretreatment;Magnesiu

5、m alloy;Corrosion resistance1.IntroductionMagnesium and its alloys play an important role in manyfields,such as aerospace,electronics and automobile fieldsowing to their unique characteristics of higher strength-to-weight ratio and a good damping capacity.However,theapplication of magnesium alloys h

6、as been limited due to theundesirable properties,including poor corrosion and wearresistance.The corrosion of magnesium alloys depends ontheir metallurgy and environmental factors.To improve thepractical usage of magnesium alloys,many researchers haveattempted to develop anticorrosive and high wear-

7、resistancestrategies 18.Electroless deposition is a variety of chemical depositiontechnology,involving the deposition of metals from solutiononto surfaces without applying an external electric voltage 9and is thought to be the simplest and most economic methodto finish steel,aluminum,copper,plastics

8、 and many othermaterials.Another advantage of the electroless depositiontechnique is that good quality deposits with uniformity can beobtained without special requirements for substrate geome-tries and capability of depositing on either conductive ornonconductive parts.Among the plating metals,elect

9、rolessnickel has exhibited more popularity due to its excellentproperties such as high hardness,wear and corrosion resis-tance and has attracted extensive interests from the academeand the industry 1014.However,the electroless plating on magnesium alloys,hasmany challenges in the processing of plati

10、ng and there is limitedreports on magnesium alloys 1,1518.The magnesium alloyis extremely susceptible to galvanic corrosion that pit severelyon the metal resulting in an unattractive appearance as well asdecreased mechanical properties.The most difficult part ofplating magnesium is developing an app

11、ropriate pretreatmentprocess,once a suitable undercoating is in place many desiredmetals can be plated.There are currently two general solutionsto treating magnesium prior to plating:zinc immersion andconversion treatment in a fluoride-containing bath 3.It isnoted that in many previous reports on th

12、e electroless plating onmagnesium alloys 1,1518,the magnesium alloy was etchedin a solution of chromium oxide and nitric acid and soaked inHF solution to form a conversion film before electroless platingSurface&Coatings Technology 201(2007) author.Department of Materials Science and Engineering,Jili

13、n University,Changchun 130025,China.E-mail address:(J.S.Lian).0257-8972/$-see front matter 2006 Elsevier B.V.All rights reserved.doi:10.1016/j.surfcoat.2006.09.312NiP.Nevertheless,metal finishing industries have to look foralternative materials or specifically deposition methods to re-place the hexa

14、valent chromium compounds,which are prog-ressively restricted due to their high toxicity on environment19 and HF also exhibits strong corrosive that cannot be easilycontrolled.Thus,the environmental and health friendly tech-nology have been extensively studied to effectively inhibit themagnesium cor

15、rosion recently 47.Moreover,the nickel ionswere provided by basic nickel carbonate in most NiP platingbath for magnesium alloy.In our previous study,an electrolessNiP deposition on the AZ91D magnesium alloy was proposed18 from a plating bath containing sulfate nickel after the alloywas pickled in a

16、acid solution of chromium oxide and activatedin HF solution to form a MgF2film.In the present work,a chromium-free solution pretreatment(CHFP)technology on AZ91D magnesium alloy is investigat-ed.Then subsequent NiP plating is realized in the plating bathwhere the nickel ions are provided by sulfate

17、nickel 18.Thedeposit was characterized by its structure,morphology,cor-rosion characteristics and microhardness.For comparison,theNiP alloy plating on the magnesium alloy substrate with thepretreatment in hexavalent chromium solution and HF solution(CH+HFP)was also provided.2.Experimental procedures

18、The substrate was AZ91D die cast magnesium alloy with asize of 30 mm30 mm3 mm.The chemical composition ofthe alloy was given in Table 1.The substrate was ground withNo.2000 SiC paper before CHFP processes.After grinding,thesubstrate was cleaned in alkaline to remove soils or greases onthe surface of

19、 magnesium alloy and rinsed thoroughly indeionized water to remove all the alkali.Then the magnesiumalloy sample was immersed in the pretreatment bath for 2 min,where H3PO4and Mn(H2PO4)2were the main ingredients.After being rinsed in distilled water,the sample was immersedin the electroless solution

20、 for plating NiP deposition layer.Theelectroless solution was taken in a 1000-ml glass beaker,whichwas kept at constant temperature by a thermostat.The bathcomposition and all operation parameters for the pretreatmentand electroless NiP deposition are listed in Table 2.Surface morphologies were obse

21、rved by SEM(JSM-5310,Japan Electronics).The attached EDS(INC250)was used forqualitativeelementalanalysisofthecoating.Thestructureswerestudied by the X-ray diffractometer(XRD,Rigaku Dymax,Japan)with a Cu K radiation(=0.154178 nm)and a mono-chromator at 50 kVand 300 mAwith the scanning rate and stepbe

22、ing 4/min and 0.02,respectively.The harnesses of themagnesium alloy before and after electroless deposition wereevaluated using a HXD-1000 microhardness tester with Vickersindenter,employing a load of 200 g for 15 s.The thickness ofNiP deposition was measured by the cross-section of eachdeposit at d

23、ifferent intervals using SEM.Corrosion resistance tests were measured under the follow-ing conditions:Porosity test.The test was proposed to evaluate the porosityof the NiP coating on magnesium alloy considering thatthe deposition coating of electroless NiP is generally notvery dense,which are micro

24、-holes or gaps between themicrometer clusters(which are consisted of amorphous ornanocrystalline grains)20.That is,a filter paper(area:1 cm2)was soaked in a reagent solution of 10 g/l NaCl,106 g/l ethanol and 0.1 g/l phenolphthalein dissolved indistilled water.The filter paper was then pasted onto t

25、henickel coating for 10 min.After taking the filter paper away,red spots or red areas were noted on the surface of thecoating.The porosity of coating was evaluated relatively bythe ratio of red spot area to the zone area previously pastedby the filter paper.The principles of the method were brieflye

26、xplained in Ref.20.Acid immersion test.The test in 10%HCl solution at roomtemperature was carried out for different thickness coatingson AZ91D magnesium alloy.If there were micro pores in thecoatings,the solution would erode the substrate through thepores.Then the H+in the solution would be reduced

27、by themagnesium and turned into the hydrogen gas bubbles 21.Electrochemical measurements.The polarization curves ofNiP deposits were performed on an Electrochemical Ana-lyzer(CHI800,Shanghai,China)by Linear Sweep Voltam-metry technique at room temperature in a 3wt.%NaClaqueous solution using a class

28、ic three-electrode cell.Theworking electrode was cleaned in acetone agitated ultrason-ically for 10 min before testing.The coated samples weremasked with epoxy resin(EP 651)so that only 1 cm2areawas exposed to the electrolyte.Samples were also degreasedwith acetone,rinsed in deionized water before e

29、lectrochem-ical test.Before the dynamic potential sweep experiments,Table 1The compositions of the AZ91D magnesium alloy(in wt.%)AlZnMnNiFeCuCaSiKMg8.770.740.180.0010.0010.001b0.01b0.01b0.01BalanceTable 2Composition and operating conditions of the NiP plating on AZ91Dmagnesium alloy(the samples were

30、 cleaned thoroughly with deionized wateras quickly as possible between any two steps of the treatments)Process OperationPlating bath composition(g/l)Condition1GrindingNo.2000 SiC sandpaper2AlkalinecleaningNaOH4565 CNa3PO412H2O1020 minMn(H2PO4)20.5H3PO4(85%V/V)15 mlRoom temperature3Pretreatment C2H4O

31、220 ml13 minCH3CH2OH50 mlHNO3(80%V/V)5 ml4ElectrolessNiPNiSO46H2O15NaH2PO2H2O14NaC2H3O213pH 6.40.2HF(40%V/V)12 ml/l Temperature822 CNH4HF28Stabilizer0.0014595W.X.Zhang et al./Surface&Coatings Technology 201(2007)45944600the samples were immersed into electrolyte for about 20 minto stabilize the open

32、-circuit potential(OCP)E0.The scan-ning rate was 50 mV min1for all measurements.Tafel plotwas transformed from the recorded data and the corrosioncurrent density(icorr)was determined by extrapolating thestraight-line section of the anodic and cathodic Tafel lines.3.Results and discussion3.1.Composit

33、ions and morphology of the coatingsSince magnesium is one of the most electrochemically activemetal,when contacts with air or water,an oxide and hydroxidelayer forms quickly on the surface 3,which have a detrimentaleffect on coating adhesion and uniformity.The quasi-passivefilm on magnesium is much

34、less stable than the usual passivefilms,which form on metals such as aluminum and stainlesssteels.This film provides only poor pitting resistance for mag-nesium.Meanwhile,the AZ91D alloy consisted of primary-Mg grains surrounded by a eutectic mixture of and-Mg17Al121.There is internal galvanic corro

35、sion causedby the second phases or impurities.The-phase precipitatedalong the grain boundaries,which exhibited higher cathodicreaction activity and lower corrosion current density than thatof 22.Therefore,a suitable pretreatment is very necessaryto insure that during the successive electroless depos

36、ition thedeposition rate of metal ions is much higher than the corrosionrate of magnesium,especially in the acidic plating bath 18.Once a suitable base coating is in place many desired metalscan be plated on magnesium alloy.The XRD patterns taken on the AZ91D magnesium alloysubstrate,the CHFP surfac

37、e and the electroless NiP depositionFig.1.The XRD patterns of the electroless NiP deposition on the AZ91Dmagnesium alloy at different intervals:(a)AZ91D magnesium alloy substrate,(b)the substrate after pretreatment 2 min,and(c)the electroless plating on thepretreatment layer after 1 h(Ni,Mg,Mg17Al12

38、).Fig.2.SEM images of the AZ91D magnesium alloy substrate surface(a)and the pretreatment 2 min surface(b).Qualitative chemical analysis of the substrate(a)andthe pretreatment surface(b).Scanning along the line labeled in the figures.4596W.X.Zhang et al./Surface&Coatings Technology 201(2007)45944600o

39、n the AZ91D magnesium alloy are shown in Fig.1(a),(b)and(c),respectively.Compared Fig.1(a)with(b),it is noticed thatthe-Mg17Al12phase became distinguished on the surface afterCHFP.The composition of the pretreatment layer is not de-tected,whose thickness may be under the limit of the instru-ment.The

40、 diffraction patterns of the as-plated electroless NiPdeposits shown in Fig.1(c)had only a single very broad peak at2=44.88 and the reflections corresponding to the(111)planeof a face-centered cubic(fcc)phase of nickel could be observed,which consisted with the phosphorus contents 5.6wt.%ana-lyzed b

41、y EDS.This pattern indicates that the structure of the as-deposited NiP coating was a mixture of amorphous and nano-crystalline nickel 23.Although the phases in the pretreatment layer cannot bedetected by XRD,the pale colored surface may imply thepresence of passive film on the substance.The surface

42、 mor-phologies of the AZ91D magnesium alloy substrate and thepretreatment layer were observed using SEM,shown in Fig.2(a)and(b),respectively.A thin film can be clearly shown on thepretreatment surface.Element surface screening analysis(omit-ted figure)on these two surfaces showed that that besides M

43、g,Al and Zn,phosphorus and manganese elements exist on thepretreatment surface;their concentrations in the layer analyzedby EDS were 0.84 and 0.11wt.%,respectively.While as forsubstrate,the contents were 0.07 and 0.02wt.%,respectively,which are the trace elements and could be ignored.Therefore,phosp

44、horus and manganese elements adhered to the substrateduring the CHFP.Fig.2(c)and(d)is the line screening ofelement analysis taken along the lines marked on Fig.2(a)and(b),correspondingly.From Fig.2(c),it can be seen that eachelement uniformly distributed on the magnesium alloy substratefrom the scan

45、ning line.The increase of Al and decrease of Mgspectra(Fig.2(d)on the white phase(Fig.2(b)positionindicate that the white phases on Fig.2(b)should be phase.Much more phases were revealed after CHFP,which isconsistent with the XRD pattern of Fig.1(b).Fig.3.Polarization curves of AZ91D magnesium alloy

46、 substrate and thesubstrate with different layers in a 3wt.%NaCl aqueous solution.(a)Themagnesium alloy substrate,(b)substrate with the CHFP,(c)substrate with theNiP layer deposited on CH+HFP layer,(d)substrate with the NiP layerdeposited on CHFP layer.Table 3Corrosion potential and corrosion curren

47、t density values obtained from theelectrochemical polarization curvesSamplesCorrosion potentialvs.Ag/AgCl,Ecorr/VCorrosion currentdensity,icorr/A cm2(a)Substrate1.502411.8(b)Substrate with the CHFPlayer1.44248.91(c)Substrate with the NiPlayer on CH+HFP layer0.78117.79(d)Substrate with the NiPlayer o

48、n CHFP layer0.59915.98Fig.4.SEM images of the electroless NiP coating after 1 h on AZ91Dmagnesium alloy surface(a),its corresponding cross-section(b)and qualitativechemical analyses(c),scanning from the coating surface to the substrate alongthe line labeled in the figure.4597W.X.Zhang et al./Surface

49、&Coatings Technology 201(2007)45944600Therefore,after the pretreatment,phase(magnesium)onthe substrate was eroded and phase was exposed on thesurface.Furthermore,the spectra of phosphorus and manganeseelements and their variation shown in Fig.2(d)indicate that Pand Mn were prestige to precipitate on

50、-phase compared with-matrix during pretreatment.The dissolution of Mg(-matrix)results in the increase of the volume fraction of phase on thesurface.Both the precipitation of phosphate and manganese andthe increase of the volume fraction of phase may improve thecorrosion resistance of the substrate.T

51、he electrochemical behaviors of the substrate before andafter the pretreatment were carried out by polarization analysis.The electrochemical results were given in Fig.3(a)and(b)andthe corrosion potential and corrosion current density of thesubstrate and the pretreatment sample obtained from the elec

52、-trochemical polarization curves were summarized in Table 3(a)and(b).According to the polarization curve,no defects wereassumed to be present in the tested samples 24.As seen fromFig.3,the cathode reaction in the polarization curves corres-ponded to the evolution of the hydrogen,and the anodic pola-

53、rization curve was the most important features related to thecorrosion resistance 25.For the pretreatment samples,thecorrosion potential Ecorrwas shifted positively about 60 mVcompared with that of the substrate and the corrosion currentdensity icorrwas decreased significantly from 411.8 A/cm2ofthe

54、substrate to about 48.91 A/cm2for the pretreatment layer.A subsequent electroless NiP alloy was deposited on theCHFP surface on the magnesium alloy through the sulfate bath18 shown in Table 2.The surface morphology of NiPdeposit was compact,uniform and shown the typical sphericalnodular structure in

55、 Fig.4(a).The morphology of the cross-section of electroless NiP coating on the AZ91D magnesiumalloy after plating 1 h,detected by SEM was shown in Fig.4(b).Some pores in the coatings in Fig.4(b)may result from theevaluation the hydrogen during the electroless deposition.Thecorresponding EDS analysi

56、s was also shown to give thequalitative chemical analysis of electroless plating in Fig.4(c).It seems that coating produced by this process do not haveobvious interfacial boundary with the substrate,and henceexhibit excellent adhesion.It is concluded that the coating wasconnected closely to the subs

57、trate by the elements distributingfrom the coating surface to the substrate along the line labeledin Fig.4(b).Fig.5 shows the thickness of the electroless deposited NiPlayers obtained from the CH+HFP 18 and the new CHFP atconstant temperature.The as-deposited layers seem to linearlyincrease as a fun

58、ction of the deposition time without consider-ing the inducement time,which of the electroless plating NiPalloys at different deposition parameters is almost the same.Therefore it does not render an influence on the deposition ratebasically.It seems that the deposition rate is increased by theCHFP c

59、ompared with traditional CH+HFP.It is concludedfrom the above experiment results that in the CHFP,the effect ofelectrochemically heterogeneous AZ91D substrate surface wassufficiently low and could restrain the corrosion the magnesiummatrix for the subsequent treatment.Thus,in further electrolessplat

60、ing,the deposition rate could be increased in the newprocess treatment.The hardness of the as-deposited NiP coating was about580 VHN,which is far higher than that of the AZ91D magne-sium alloy substrate(about 100 VHN).Fig.5.The effect of the deposition time on the thickness of the electroless NiPpla

61、ting.donated the NiP deposited on CHFP layer and showed the NiPdeposited on CH+HFP layer.The meaning was the same in hereinafter.Fig.6.Effect of the thickness of the electroless NiP alloy on the red areapercentage by the porosity test.Fig.7.Effect of the thickness of the electroless NiP alloy on the

62、 time in 10%HCl solution.(The time donated the intervals between the start of the acidimmersion test and the first hydrogen bubble arising from the coating surface.)4598W.X.Zhang et al./Surface&Coatings Technology 201(2007)459446003.2.Corrosion characteristics of the coatingsThe corrosion resistance

63、 of the as-deposited electrolessNiP was investigated by porosity test(Fig.6),acid immer-sion test(Fig.7)and polarization curves(Fig.3(c)and(d)measurements.The nickel/Mg system is a classical example of cathodiccoating on an anodic substrate.Hence,the porosity in thecoating might influence the corros

64、ion behaviour and servicelifetime of the electroless nickel-plated magnesium 1.Theporosity(represented by the red area)of the NiP coatings wasestimated by the porosity test proposed in the experiment partand the results were shown in Fig.6 as a function of the depositthickness.As shown in Fig.6,the

65、red area decreased rapidly asthe thickness increased,and relatively compact NiP coatingwere obtained.When the thickness reached 12 m,no red colorspots were found on the tested coatings.This observation isunderstandable that the tested areas are thick enough and poresfree to protect the substrate fro

66、m corrosion.The correspondingtest was carried out on the NiP samples with CH+HFP.As forthe samples,red spots remain found on the tested papers withthe thickness of 22 m.The result is corresponding to themorphologies of the cross-section of the NiP coatings on theAZ91D magnesium alloy in Fig.4(b)and Fig.6 in Ref.18.Itis found that the sample of electroless on the CHFP layer(Fig.6)possesses comparative low porosity to that of electro-less on chromium conversion coating sample.While the CH+HFP laye