高耐腐蚀涂层的纳米镍AZ91D镁合金@中英文翻译@外文翻译

ystallineMaterials, 130025,15paper. The surface morphologies of the coatings were studied by SEM1. Introductionresistance, solderability, electrical conductivity or decorativeappearance. This can be accomplished by coating the partswith a metal that has the desired properties necessary for theelectroless nickel plating and zinc immersion 2. It can benoted that in many previous reports on the electrolessplating on magnesium alloys 36, the nickel ions werethe plating bath.Surface & Coatings Technology 200Magnesium is becoming increasingly significant as alightweight metal structural material (with a density of 1.74g/cm3) in many industriesaircraft construction, spacetechnology, optics, and automobile manufacturing, forexample. However, magnesium is intrinsically highlyreactive and its alloys usually have relatively poor corrosionresistance, which restricts the application of magnesiumalloys in practical environments. So, it is often desirable toalter the surface properties of a magnesium or magnesiumalloy workpiece in order to improve its corrosion and wearspecific application 1.Since magnesium is one of the most electrochemicallyactive metals, any coatings on magnesium alloys should beas uniform, adhered and pore-free as possible. One of themost cost effective and simple techniques for introducing ametallic coating to a substrate is the plating techniques,including electroless plating and electroplating. Further-more, magnesium is classified as a difficult substrate to platemetal due to its high reactivity. As for electroplating on themagnesium alloy, there are currently two processes used forplating on magnesium and magnesium alloys: directand FESEM. The nc Ni coating had an average grain size of about 40 nm and an evident 200 preferred texture revealed by XRD. Thehardness of the nc Ni coating was about 580 VHN, which was far higher than that (about 100 VHN) of the AZ91D magnesium alloysubstrate. The electrochemical measurements showed that the nc Ni coating on the magnesium alloy had the lowest corrosion current densityand most positive corrosion potential among the studied coatings on the magnesium alloy. Furthermore, the nc Ni coating on the AZ91Dmagnesium alloy exhibited very high corrosion resistance in the rapid corrosion test illustrated in the paper. The reasons for an increase in thecorrosion resistance of the nc Ni coating on the magnesium alloy should be attributable to its fine grain structure and the low porosity in thecoating.D 2005 Elsevier B.V. All rights reserved.Keywords: Nanocrystalline nickel; Magnesium alloy; Electroplating; Electroless nickel; Corrosionmagnesium alloy with different thickness were also presented in theAbstractNanocrystalline (nc) Ni coating was direct-current electrodeposited on the AZ91D magnesium alloy substrate aimed to improve itscorrosion resistance using a direct electroless plating of nickel as the protective layer. As comparison, two electroless Ni coatings on theHigh corrosion-resistance nanocrmagnesiumChangdong Gu, Jianshe Lian*, JinguoKey Lab of Automobile Materials, Ministry of Education, College ofChangchunReceived 22 March 2005; acceptedAvailable online0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2005.07.001* Corresponding author. Fax: +86 431 5095876.E-mail address: (J. Lian).Ni coating on AZ91DalloyHe, Zhonghao Jiang, Qing JiangScience and Engineering, Jilin University, Nanling Campus,Chinain revised form 4 July 2005August 2005(2006) 5413 5418/locate/surfcoatprovided by basic nickel carbonate inDifferent from the methods mentioned above, direct electro-less nickel plating on the AZ91D magnesium alloy wasquickly as possible between any two steps of thetreatments. The direct electroless nickel plating with thehardness tester with Vickers indenter, at a load of 100 gand duration of 15 s.Electrochemical measurements were performed on anElectrochemical Analyzer (CHI800, Shanghai, China),which was controlled by a computer and supported bysoftware. Linear Sweep Voltammetry experiments werecarried out in a 3 wt.% NaCl aqueous solution using aclassic three-electrode cell with a platinum plate (Pt) ascounter electrode and an Ag/AgCl electrode (+207 mV vs.SHE) as reference electrode. Before testing, the workingelectrode was cleaned in acetone agitated ultrasonically for10 min. The exposed area for testing was obtained bydoubly coating with epoxy resin (EP 651) leaving anuncovered area of approximately 1 cm2. The reference andplatinum electrodes were fixed near to the workingelectrode (about 0.5 mm), which could minimize the errorsdue to IR drop in the electrolytes. During the potentiody-namic sweep experiments, the samples were first immersedinto 3 wt.% NaCl solution for about 20 min to stabilize theopen-circuit potential. Potentiodynamic curves wererecorded by sweeping the electrode potential from a valueof about 300400 mV lower to a value of 500600 mVupper than the corrosion potential, respectively, at asweeping rate of 5 mV/s. The log(i)E curves wereTime 30 min (about 15 m)Fig. 1. The technical flow chart of the electroplating nc Ni on the AZ91Dmagnesium alloy.Technologythickness of about 10 Am on AZ91D magnesium alloy 7was used as the protective layer for further plating on themagnesium alloy. The electroplating nc Ni coating on themagnesium alloy was direct-current electroplated from abath containing nickel sulfate, nickel chloride, boric acidand saccharin at a pH of 5.0 and a temperature of 50 -C.During the electrodeposition process, the anode was usedan electrolytic nickel plate. The operation of electroplatingnc Ni coating was undertaken for about 30 min whichwould give the coating with the thickness of about 15 Am.A scanning electron microscope (SEM, JEOL JSM-5310,Japan) and a field emission scanning electron microscope(FESEM, JEOL JSM-6700F, Japan) were employed forthe observations of the surface of the coatings and thecross-section morphology and an EDX attachment wasused for qualitative elemental chemical analysis. Crystal-line structure of the sample was studied by the X-raydiffractometer (XRD, Rigaku D/max, Japan) with a Cutarget and a monochronmator at 50 kV and 300 mA withthe scanning rate and step being 4-/min and 0.02-,recently undertaken by using a plating bath containingsulfate nickel 7.In the recent years, there have been considerable interestsin understanding the mechanical properties, the corrosionresistance and the wear resistance of nc metals produced byelectrodeposition, for example 814. According to them,nc materials exhibited many unusual mechanical andelectrochemical properties compared with conventionalpolycrystalline or amorphous materials. So introducing anc coating combined the high corrosion resistance withgood wear resistance on magnesium alloy substrate wouldbe very promising.In the present paper, the electroless Ni plating from anacidic bath 7 was first deposited on AZ91D magnesiumalloy as the protective layer for the further electroplatingoperation, and then a nc nickel coating was direct-currentelectroplated on the protective layer. The microstructuresand the electrochemical properties of the coatings on theAZ91D magnesium alloy substrate were studied by SEM,FESEM, XRD and electrochemical measurement.2. ExperimentalThe substrate material used was AZ91D die castmagnesium alloy with a size of 30C240C25 mm. Thealloy was mainly contained about 9.1% Al, 0.64% Zn,0.17% Mn, 0.001% Fe and Mg balance. The samples wereabraded with no.1500 SiC paper before the pretreatmentprocesses. The technical flow chart of the electroplating onthe AZ91D magnesium alloy is shown in Fig. 1. Thesamples were cleaned thoroughly with de-ionized water asC. Gu et al. / Surface & Coatings5414respectively. The hardness of the magnesium alloy andthe coatings were evaluated using a HXD-1000 micro-Alkaline cleaning NaOH 45 g/lNa3PO412H2O 10 g/lTemperature 65 CAcid pickle CrO3125 g/lHNO3(70% V/V) 100 ml/lTime 40 sRoom temperatureFluoride activationHF (40%V/V) 350 ml/lTime 10 sRoom temperature Electroless plating protective layer ref.7Time 15 min (about 10 m)Electroplating nanocrystalline Ni NiSO46H2O 250-300 g/lNiCl26H2O 30-40 g/lH3BO330-45 g/lC7H5NO3S 0.1-0.2 g/lCurrent density 3 A/dm2200 (2006) 54135418measured and plotted after the above electrochemicalmeasurements. The corrosion potential Ecorrand corrosioncurrent density icorrwere determined directly from theselog(i)E curves by Tafel region extrapolation. Acidimmersion test in 10% HCl solution at room temperaturewas undertaken to test the corrosion resistance of the nc Nicoating on the magnesium alloy. If there were micro poresin the coatings, the corrosion solution would erode themagnesium substrate through the pores. Due to the highchemical activity of the magnesium, the H+in thecorrosion solution would be reduced by the magnesiumand turned into the hydrogen gas bubbles. So the timeinterval between the start of the test and the first hydrogengas bubble arising from the coating surface could be usedto donate the corrosion resistance of the coatings on themagnesium alloy substrate. For comparison, the electrolessplating nickel coatings with different thickness (10 and 25Am) were also tested in the paper.3. Results and discussions(200) peak via the Scherrer equation 16:dXRDkkbhcosh1Where k is the X-ray wavelength, b the FWHM (fullwidth of half maximum) of the (200) diffraction peak, h thediffraction angle and the constant kC2291. From Eq. (1) theaverage grain size of this nc Ni was about 40 nm. Inaddition, the XRD results also showed that the nc Ni coatinghad an evident 200 preferred texture. In fact, Ni electro-deposits are known for giving numerous, well-definedpreferred orientations depending on electrodeposition con-ditions, i.e. electrolyte composition, temperature, pH,current density, stirring and organic additions 17,18. The200 preferred texture of the nc Ni in this study may beattributable to the given electrodeposition conditions whichmay lead to higher electrode overpotential and reducedconcentration of Ni2+at the electrode surface 18.Fig. 3 (a) showed the typical surface morphology of theas-deposited nc Ni coating. It can be seen that the as-deposited surface of the nc Ni coating was very compactand no colony structures, which was totally different fromthe cauliflower-like (in micrometer size) surface mor-phology of the electroless nickel deposition (see Fig. 3(b). Moreover, the as-deposited surface of the nc Nicoating exhibited a flat and mirror-like appearance and the25000NiC. Gu et al. / Surface & Coatings Technology20 30 40 50 60 70 800500010000150002000020 30 40 50 60 70 8005001000150020002500(a)2/ deg.20 30 40 50 60 70 80050010001500(b)Intensity / CPSMg17Al12Mg(c)Fig. 2. The XRD patterns of the electroplating Ni on the AZ91Dmagnesium alloy at different intervals, (a) AZ91D magnesium alloy3.1. Microstructures and hardness of the coatingsFig. 2(a) showed the pattern of XRD of the AZ91Dmagnesium alloy, which indicated that the substrate alloyconsisted of primary a (Mg) grains surrounded by aneutectic mixture of a and b (Mg17Al12) 15. It also can beseen from the XRD pattern of Fig. 2 (b) and the morphologyof Fig. 3 that after electroless Ni plating for about 15 min,the AZ91D magnesium alloy was fully covered by theelectroless nickel deposition. The phosphorus content in theelectroless deposition was very low, because Ni was firstdeposited on the surface of magnesium alloy according tothe deposition mechanism of electroless plating on themagnesium alloy 7. The XRD analysis results of the as-deposited electroplating nc Ni coating was shown in Fig. 2substrate, (b) electroless plating on the substrate for 10 min, (c) theelectroplating nc Ni coating.(c). The grain size d of nc Ni can be determined from theFig. 3. The surface morphology of a) the nc Ni coating and b) theelectroless nickel plating on the AZ91D magnesium alloy for about 10 min.200 (2006) 54135418 5415grain size of the deposits could not be resolved byconventional SEM observations. In order to make a clearobservation of the fine grain structure, specimen of this ncNi was polished and corroded by dilute nitrate/ethanolsolution before the SEM observation. The surface mor-phology of the nc Ni after above pretreatments was shownin Fig. 4. The very uniform nc grain structures can be seenon the surface of nc Ni after slightly corroded. Fig. 5 (a)showed the cross-section morphology of the nc Ni coatingon the AZ91D magnesium alloy. To indicate the substrateand two types of the Ni layers on the cross-section of thecoating, the substrate, the protective layer and the nc Nilayer were marked by the line with two arrows, respec-tively, in the Fig. 5 (a). From this figure, it can be seenthat the acid pickle pretreatment prior to electroless platingprotective layer for the magnesium alloy provides surfacepits to act as sites for mechanical interlocking to improveadhesion 7. Moreover, there were no obvious boundariesbetween the protective layer and the nc Ni layer, whichindicated that the nc Ni layer was tightly attached to theprotective layer. In addition, the nc Ni layer showed verycompact and no pores from its cross-section observation.Fig. 5 (b) gave the qualitative element analysis of the ncNi coating on the AZ91D magnesium alloy by EDXanalysis. For the lower phosphorous content in theprotective layer previous deposited on the AZ91D magne-sium alloy substrate, the distribution of element ofphosphorous was not detected by EDX. From the elementsdistributing from the substrate to the coating surface alongthe line labeled in Fig. 5 (a), it can be seen that the coatingwas connected closely to the magnesium alloy substrate.Fig. 4. The surface morphology of the electroplated nc Ni coating afterpolishing and corroded by dilute nitrate/ethanol solution.C. Gu et al. / Surface & Coatings Technology 200 (2006) 541354185416Fig. 5. (a) The cross-section morphology of the Ni coating on the AZ91D magnesiummagnesium alloy by EDX analysis, scanning from the substrate to the coating surfacealloy, (b) the qualitative element analysis of coating on the AZ91Dalong the line labeled in the figure.Generally, nanocrystalline materials usually haveimproved hardness compared to the conventional polycrys-talline materials 9,10. In this study, the hardness of the ncNi coating was about 580 VHN, which was far higher thanthat of the AZ91D magnesium alloy substrate (about 100VHN). Therefore, the magnesium alloy coated by the nc Niwith high hardness would be expected to greatly improve itssurface properties, for example the wear resistance.3.2. Corrosion properties of the coatingsFig. 6 gavetheelectrochemicalpolarizationcurvesfor theAZ91D magnesium alloy substrate and the various coatingson the magnesium alloy substrate in a 3 wt.% NaCl aqueoussolution at room temperature. The cathodic reaction in theelectrochemical polarization curves corresponded to theevolution of the hydrogen, and the anodic polarization curvencNicoating,itshouldbenoticedthatthecorrosionpotentialEcorrshowed a significant shift to the positive directionC. Gu et al. / Surface & Coatings Technologywas the most important features related to the corrosionresistance. For the magnesium alloy substrate and thesubstrate with 10 Am electroless Ni alloy layer, when theapplied potential increased into the anodic region, anactivation-controlled anodic process was observed. Thepolarization current increased with increasing the appliedanodic potential and no obvious passivation occurred.However, the corrosion potential Ecorrof the substrate with10 Am electroless Ni layer was shifted positively about 80mV compared with that of the substrate and the corrosioncurrent density icorrdecreased from 1.546 mA/cm2of thesubstrate to 0.327 mA/cm2of the 10 Am thickness electrolessNi layer. As for the electrochemical polarization curve of thesubstrate with 25 Am thickness electroless Ni coating, thecorrosion potential Ecorrwas shifted positively to C00.410 Vand the corrosion current density was only 9.8 AA/cm2. Andwhen the applied potential increased to C00.287 V, a thinpassive film was formed on the surface of the coating withthe corrosion current density of about 15.2 AA/cm2.However, at the potential of 0.085 V, nickel dissolution-2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.61E-91E-81E-71E-61E-51E-41E-30.01Potential (V vs. Ag/AgCl)a) Magnesium alloy substrateb) Substrate with 10m electroless Ni coatingc) Substrate with 25 m electroless Nicoatingd) Substrate with nanocrystalline Ni coatingdcbaCurrent density (A/cm2)Fig. 6. The electrochemical polarization curves for the AZ91D magnesiumalloy substrate and the various coatings on the magnesium alloy substrate ina 3 wt.% NaCl aqueous solution at room pared with that of the substrate and the corrosion currentdensity icorrwas largely decreased. Between the potentialvaluesofC00.174and0.162V,theformationofathinpassivefilm occurred resulting in a limited current at approximately4.92 AA/cm2. This film, however, breaks down once theapplied potential was beyond the value of 0.162 V. After thefilm has broken down, the nickel dissolution would haveoccurred through the pores of the nc Ni coating. So it can beseen that the corrosion potential Ecorrof the electroless Nicoating on the AZ91D magnesium alloy increased greatlytowards the positive direction when the coating thicknesswasincreased.Therefore,thencNicoatingpossesseshighestcorrosion potential and lowest corrosion current densityamong all these studied coatings. In fact, the electrochemicaltechniques have been reported to study the relation betweenporosity and corrosion rate measurements for electrolessnickel deposits on steel 19,20. As the porosity of theelectroless nickel coating is reduced, the value of corrosionpotential Ecorrwould become more noble and the corrosioncurrent becomes smalle