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The electroless nickel-plating on magnesium alloy using NiSO4d6H2O as the main salt Jianzhong Lia,*, Zhongcai Shaob, Xin Zhanga, Yanwen Tiana aSchool of materials and metallurgy, Northeastern University, Shenyang 110004, China bFaculty of Environment and Chemical Engineering, Shenyang Institute of Technology, Shenyang 110168, China Received 23 July 2004; accepted in revised form 19 December 2004 Available online 26 January 2005 Abstract In this paper, the electroless nickel-plating on magnesium alloy was studied, using NiSO4d 6H2O as the main salt in the electroless plating alkaline solutions. The effects of the buffer agent and plating parameters on the properties and structures of the plating coatings on magnesium alloy were investigated by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X- ray diffraction (XRD). In addition, the weight loss/gain of the specimens immersed in the test solution and plating bath was measured by using the electro-balance, to evaluate the erosion of the alloy in the plating solutions. The adhesion between the electroless plating coatings and the substrates was also evaluated. The compositions of the non-fluoride and environmentally friendly plating bath were optimized through Latin orthogonal experiment. The buffer agent (Na2CO3) added to the plating bath was found to be useful in increasing the growth rate of the plating coating, adjusting the adhesion between the electroless plating coatings and the substrates, and maintaining the pH value within the range of 8.511.5, which is required for the successful electroless nickel-plating on magnesium alloy with NiSO4d 6H2O as the main salt. Trisodium citrate dihydrate was found to be an essential component of the plating bath to plate magnesium alloy, with an optimum concentration of 30 g L?1. The obtained plating coatings are crystalline with preferential orientation of (111), having advantages such as low- phosphorus content, high density, low-porosity, good corrosion resistance and strengthened adhesion. D 2004 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Electroless plating; Buffer; Corrosion resistance; Adhesion 1. Introduction The use of magnesium alloys in a variety of applications, particularly in aerospace, automobiles, and mechanical and electronic components, has increased steadily in recent years as magnesium alloys exhibit an attractive combination of low density, high strength-to-weight ratio, excellent cast- ability, and good mechanical and damping characteristics. However, magnesium is intrinsically highly reactive and its alloys usually have relatively poor corrosion resistance, which is actually one of the main obstacles to the application of magnesium alloys in practical environments 13. Hence, the application of a surface engineering technique is the most appropriate method to further enhance the corrosion resistance. Among the various surface engineering techniques that are available for this purpose, coating by electroless nickel is of special interest especially in the electronic industry, due to the possession of a combination of properties, such as good corrosion and wear resistance, deposit uniformity, electrical and thermal conductivity, and solderability etc. As far as magnesium alloys are concerned, the main salts of electroless plating solutions mostly focus attentions on basic nickel carbonate or nickel acetate 49, which result in high-cost, low-efficiency, instability of electroless plating solutions and little applications. In addition, the basic nickel carbonate or nickel acetate of plating solutions yet including fluoride, are harmful to the environment, therefore, it is urgently needed to develop new environmentally friendly plating bath. It is difficult to carry 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.12.009 * Corresponding author. Tel.: +86 24 8368 7731; fax: +86 24 2398 1731. E-mail address: mengsuo66 (J. Li). Surface & Coatings Technology 200 (2006) 30103015 out electroless plating on magnesium alloys due to the high- corrosion rate of magnesium alloys in the plating bath with NiSO4d 6H2O or NiCl2d 6H2O as the main salt. It is reported 10 that the corrosion rate of magnesium and its alloys in NaCl solutions solely depends on the pH of the buffered chloride solutions. The objective of this study was to find a buffer agent and determine how the buffer agent affects the dissolution of magnesium alloy in NiSO4d 6H2O alkaline solutions, and the non-fluoride plating solutions for magne- sium alloy with NiSO4d 6H2O as the main salt. The microstructure, compositions and corrosion behavior of the coatings were investigated in detail. 2. Experimental The substrate material used in the research was AZ91D ingot-cast alloy. The chemical composition of the alloy is given in Table 1. Substrates with a size of 50 mm?40 mm?20 mm were used in the research. The substrates were mechanically polished with emery papers up to 1000 grit to ensure similar surface roughness. The polished substrates were thoroughly washed with distilled water before passing through the pre- cleaning procedure as shown in Table 2. The substrates were air-dried after the fluoride activation (the last step in the pre-cleaning procedure). In a typical experiment, the initial weight of a air-dried substrate was measured and then quickly transferred to the plating bath (1000 mL) in a glass container placed in a water bath with a constanttemperatureof808C.Afreshbathwas usedforeach experiment to avoid any change in concentration of bath species. The bath compositions and other parameters used in these experiments are given through Latin orthogonal experiment in Table 3. Final weights of the specimens were determined and the coating rates in micrometer per hour were calculated from the weight gain. At the same time, in order to study the each buffers influence on the substrates and find a buffer appropriate for the electroless plating on magnesium alloy, test solutions with compositions similar to those of the plating bath except that sodium hypophosphite was not added, were prepared to simulate the corrosion rates of magnesium alloy in plating bath and the behaviors of the buffers. Duplicate experiments were conducted in each case, and the coating rate reported is the average of two experiments. The growth rates of the plating coating were measured using the electro-balance made in America, which is the 0.1 mg precision. In the research, the pH value of plating bath was monitored by a pHS-25C model of precision pH/mV meter. Morphology of the coatings was analyzed using a scanning electron microscope. The energy dispersive X-ray spectroscopy analysis was used for determining the content of phosphorus in the coatings. Crystallinity of the coatings was investigated by Rigaku D/ max-rA X-ray diffractometer with Cu K-alpha radiation. The adhesion strength of the electrolessly deposited nickel coatings to the magnesium alloy substrates was determined by scratch test. During the scratch test, the specimen was moved at a constant speed of approximately 11.4 mm/min. Scratches were generated on the specimen using a diamond indenter with a spherical tip of 300 Am in diameter. Corrosion potential measurement in 3.5 wt.% NaCl solution was carried out to comparatively investigate the corrosion behaviors of the bare substrate and the nickel-plated substrates. The electrochemical cell used for corrosion potential measurement consisted of a bare substrate or a nickel-plated substrate as the working electrode (exposed area: 1 cm2), a saturated calomel electrode (SCE), and a platinum-foil counter electrode. Table 1 Chemical composition of the AZ91D alloy (in wt.%) AlMnNiCuZnCaSiKFeMg 9.10.170.0010.0010.64b0.01b0.01b0.01b0.001Bal Table 2 Optimized pre-cleaning procedure Table 3 Optimized bath composition and parameters Bath species and parametersQuantity NiSO4d 6H2O25 g/L NaH2PO2d H2O30 g/L C6H5Na3O7d 2H2O30 g/L Na2CO330 g/L NH3d H2OAdjusting pH pH value11 Temperature80F2 8C J. Li et al. / Surface & Coatings Technology 200 (2006) 301030153011 3. Results and discussion 3.1. The buffers behaviors in the test NiSO4solutions and the choice of an appropriate buffer Fig. 1 shows the variation of weight loss of magnesium alloy as a function of the immersion time with different buffers in the test solutions. The compositions and the controlled temperature of the test solutions were similar to those of the plating bath except that sodium hypophosphite was not included. The pH values of the test solutions were adjusted by NH3d H2O to fix at 11. The weight loss increases linearly with the immersion time increasing of magnesium alloys in the Na2CO3, Na2B4O7, and CH3COONa test solutions. It is revealed in Fig. 1 that the corrosion rates were constant throughout the examined immersion time. As recognized from the slope of each solid line in Fig. 1, corrosion rate in the test solution containing Na2CO3 buffer is the lowest among the three tested buffers. The obtained slopes are 0.015, 0.022 and 0.056 mg cm?2 min?1for Na2CO3, Na2B4O7and CH3COONa buffers, respectively. These results can be explained in terms of dissociation constants of the corresponding acids, which arek2=4.7?10?11(k1=4.4?10?7),k2=1?10?9 (k1=1?10?4), and k=1.75?10?5for H2CO3, H2B4O7and CH3COOH, respectively. The second dissociation constant of a binary acid decides the buffer capability of the buffer. Obviously, the Na2CO3buffer has the lowest cost and best buffer capability among the tested buffers. Fig. 2 shows the weight loss of the substrates versus immersion time in the test solutions with pH values at 9, 10 and 11, using Na2CO3as the buffer. Corrosion of the specimens in non-buffered test solutions with pH values at 9, 10 and 11 was also investigated. The corresponding weight loss curves are shown in Fig. 2. All test solutions used for these experiments had compositions similar to those in the plating bath except that sodium hypophosphite was not included. The weight loss linearly changes with the increase of the immersion time in all cases shown in Fig. 2. Under the same pH value, the corrosion rate of the substrates in the buffer solution is obviously lower than that of the substrates in the non-buffered solution, as shown by the slopes of the curves in Fig. 2. This suggests that the buffer solution has a considerable effect on the corrosion rate of magnesium alloy. In both the Na2CO3buffered and non-buffered test solutions, the corrosion rates of magne- sium alloy decrease with the increase of the pH value. This indicates the weight-loss of the substrates is related to the reaction between the substrate metal and the hydrogen ions. But the corrosion reaction between the substrate metal and the hydrogen ions goes gradually on, because the low concentration of hydrogen ions is presented in the plating alkaline solutions. And then, the concentration of hydrogen ions is weakly decreased during the test progress. This leads to the constant corrosion rates in the short test time, which is shown in Figs. 1 and 2. At the same time, knowing that for Mg(OH)2Ksp at 25 8C=8.9?10?12at pH 9, OH?=10?5M, most Mg2+diffused into plating solution to form up to 10?2 M. At pH 11, OH?=10?3M, the Mg2+ couldnt exceed 10?6M, thus most Mg2+formed Mg(OH)2and stayed near the substrate. Mg(OH)2could increase the adsorption energy barrier and reduce the corrosion rate. Therefore, higher pH resulted in lower corrosion rate. As to the Na2CO3buffered solutions, for MgCO3Ksp at 25 8C=10?15, in test solutions, Na2CO3N0.1 M, thus the possible Mg2+b10?14M. This means that the driving force for Mg to form Mg2+was very low. Instead of dissolving Mg, the CO3 2? ion would bond or be adsorbed to the substrate surface to form local MgCO3 2?. In this case, the substrate surface area exposed to H2O or H+was reduced a lot, 05101520253035 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Na(CH3COO) Na2B4O7 Na2CO3 Weight loss/mg.cm-2 Time/min Fig. 1. The variation of weight loss of magnesium alloy in test solutions with different buffers. 05101520253035 0 1 2 3 4 5 solution pH=9 pH=10 pH=11 pH=9 pH=10 pH=11 Weight loss/mg.cm-2 Time/min in non-buffered solution in Na2CO3 buffered Fig. 2. The variation of weight loss of magnesium alloy in test solutions with different pH values. J. Li et al. / Surface & Coatings Technology 200 (2006) 301030153012 leading to lower corrosion rates. The pKa2for Na2CO3is 10.33, at pH lower than 10.33 some CO3 2? ions formed HCO3 ?. Reaction Mg+2HCO3?=MgCO3+H2 potentially existed. At pH higher than 10.33, HCO3 ? is negligible. Therefore in Fig. 2, we can see that the corrosion rate at pH 11 was not reduced as much, compared the rate at 10. H2B4O7and CH3COOH dont have such advantages. 3.2. The effects of plating parameters on coatings The coating rate, surface appearance, and adhesion of the coatings at different concentrations of Na2CO3buffer are listed in Table 4. The critical load (LC) was measured under progressive loading conditions, which can be used to accurately characterize the adhesion strength of the deposit/ substratesystem13.Theadhesionbetweenthecoatingsand substrates decreases obviously with the increase of the concentration of Na2CO3. Surface appearance of the plating coatings becomes gradually shining with the increase of the Na2CO3concentration. Grave corrosion of the substrates was foundinthenon-bufferedplatingbath.Thegrowth rate ofthe coatingsnoticeablyincreaseswiththeincreaseoftheNa2CO3 concentration. Considering the combination of growth rate, surface appearance, and adhesion of the coatings, the optimumconcentrationoftheNa2CO3bufferwasdetermined tobe30gL?1.Withthisconcentration,thepurposeofadding Na2CO3in plating bath is commendably achieved. In the research, it was found that the pH value of plating bath had a considerable effect on the growth rate and the surface appearance of the coatings. The hydrogen ions in plating bath were not only astricted by the CO3 2? ions dissociated from thebufferNa2CO3,butlinkedwiththeOH? ions. When the pH value of the plating bath was below 8.5, point corrosion or dark gray coatings were obtained and the coating growth rate was low. When the pH value of the plating bath was above 11.5, the adhesion between coatings and substrates were deteriorated, although the growth rate and the surface appearance of the coatings were satisfying. Duringtheelectrolessplating,thepHvalueoftheplatingbath was monitored with a pHS-25C model of precision pH/mV meter. In this research, the preferred pH range of the plating bath for electroless plating on magnesium alloy is 8.511.5. Table 4 Coating rate, surface appearance and adhesion of the coatings obtained from the plating bath with different amounts of Na2CO3 Concentration of Na2CO3(g L?1) Coating rate (Am/h) Surface appearanceLC(N) 0Grave corrosion 1012.32Point corrosion81 2016.41Dark gray76 3018.32Shining73 4018.91Shining61 5019.26Shining51 203040506070 13 14 15 16 17 18 19 20 The coating thickness/m The trisodium citrate dihydrate content/g.L-1 Fig. 3. Relationship between the coating thickness and the trisodium citrate dihydrate concentration. 30405060 1000 2000 3000 4000 5000 6000 7000 8000 Intensity 2 /( ) Fig. 4. XRD patterns of the electroless plating coating. Fig. 5. Surface morphology of a plating coating. J. Li et al. / Surface & Coatings Technology 200 (2006) 301030153013 Fig. 3 shows the variation of coating thickness on magnesium alloy at same plating time as a function of the trisodium citrate dihydrate concentration at constant temper- ature and pH. The coating thickness decreases with the increase of the trisodium citrate dihydrate concentration. According to De Minjer and Brenners explanation 11, at low concentrations the low adsorption of ligand on the catalytic surface of the substrate accelerates the plating reaction. At higher concentration, there is a high adsorption of ligand on the surface, which slows down the plating reaction. But when the concentration was below 20 g L?1, the plating bath became destabilized and nickel precipitate was observed. 3.3. Properties of the plating coatings from nickel sulfate The coating obtained under optimized bath composition wasprobablypreferentiallycrystallized(seeFig.4).Theonly and strong diffraction observed in the XRD spectrum corresponds to the (111) peak of nickel. Fig. 5 shows the surface morphology of the plating coating. The surface is optically smooth and of low porosity. No obvious surface damage was observed. The compositions of the plating coating were determined to be 5.39 wt.% P and 94.61 wt.% Ni byenergy dispersive X-ray spectroscopy. Fig. 6 shows the cross section of an electroless plating coating. The coating has a good adhesion to the substrate and no cracks or holes were observed. Fig. 7 shows the curve of the NiP coating free corrosion potential with time. After the sample was immersed in 3.5 wt.% NaCl solution at room temperature for 2 h, the free corrosion potential of the coated magnesium alloy approached to about ?0.4 V. The steady-state working potential of magnesium electrode is generally about ?1.50 V, although its standard potential is ?2.43 V 14. This indicates the improved corrosion resistance of the plating coatings prepared in this research, compared with the bare alloy. The adhesion between the coatings and the substrates was evaluated by means of quenching and the scratch test. The plated specimens were heated at a temperature of 250 8CF10 8C for 1 h, and then quenched in the cold water. This process was repeated for 20 times on each specimen. No discoloration, cracks, blisters, or peeling was observed 12. For the scratch test, the critical load (LC) of 73 N was found for the coatings obtained in the optimized bath composition and parameters. These results suggest the excellent adhesion of the plating coating to the