Electroless deposition of Ni W P coating on AZ91D magnesium alloy

Transcription

1 Applied Surface Science 253 (2007) Electroless deposition of Ni W P coating on AZ91D magnesium alloy W.X. Zhang, N. Huang, J.G. He, Z.H. Jiang, Q. Jiang, J.S. Lian * Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, Changchun , China Received 26 October 2006; received in revised form 15 November 2006; accepted 15 November 2006 Available online 14 December 2006 Abstract Ternary Ni W P alloy coating was deposited directly on AZ91D magnesium alloy by using an alkaline-citrate-based baths. Nickel sulfate and sodium tungstate were used as metal ion sources, respectively, and sodium hypophosphite was used as a reducing agent. The ph value of the electroless bath was tailored for magnesium alloy. The coating was characterized for its structure, morphology, microhardness and the corrosion properties. SEM observation showed the presence of dense and coarse nodules in the ternary coating. EDS analysis showed that the content of tungsten in the Ni W P alloy was 4.5 wt.%. Both the electrochemical analysis and the immersion test in 10% HCl solution revealed that the ternary Ni W P coating exhibited good corrosion resistance properties in protecting the AZ91D magnesium alloy. # 2006 Elsevier B.V. All rights reserved. Keywords: Electroless Ni W P coating; Magnesium alloy; Corrosion resistance 1. Introduction Magnesium and its alloys have been widely used in a range of structural applications such as aerospace, electronics and automobile fields owing to their unique characteristics of higher strength-to-weight ratio and a relatively high stiffness [1 4]. However, the application of magnesium alloys has been limited due to the undesirable properties, including poor corrosion and wear resistance. Corrosion and wear resistance of magnesium alloys are often enhanced by means of surface coatings or treatments. The corrosion of magnesium alloys depends on their metallurgy and environmental factors. To improve the corrosion resistance of magnesium alloys, many researchers have tried to develop various anticorrosive strategies [4 8]. Among various surface techniques, the electroless Ni P alloy deposits are considered as an effective method to modify the physical and chemical properties of the substrates due to their good corrosion resistance and high hardness (hence high wear resistance). The electroless Ni P alloy deposits with good quality and uniformity can be obtained without special requirements for substrate geometries and with capability of * Corresponding author. Tel.: ; fax: address: lianjs@jlu.edu.cn (J.S. Lian). depositing on either conductive or non-conductive parts, which have been widely applied in many industries. It is thus of interest to assess some ternary nickel-based alloys such as Ni Cu P [9 12], Ni W P [12 19], Ni Mo P [19], Ni Fe P [20,21], Ni Sn P [22] and Ni Re P [23], which have also been developed to further improve these properties of binary systems by adding the second metal salts to nickel solution for meeting some special demands. Among the possible metals, tungsten appears to be a significant importance due to its high hardness, excellent corrosion-resistant and high melting point. According to the available literature, ternary Ni W P deposits had been prepared on mild steel specimens [12 17], tool steel specimens [18] and Cu coupons [19]. However, the electroless ternary Ni W P plating on difficult substrates, such as magnesium alloys, has many challenges in the plating processing and there is no such report on magnesium alloys. In the previous reports on the electroless Ni P plating on the magnesium [4,24], the nickel ions were provided by basic nickel carbonate in the bath, which increased the cost and required careful control of the solutions. The traditional nickelplating baths were acidic, which could easily attack or corrode the magnesium surface. In recently developed electroless Ni P plating process on magnesium alloy [25], although the electroless plating bath used nickel sulfate as nickel sources, the substrates needed be etched first in a solution of chromate /$ see front matter # 2006 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 W.X. Zhang et al. / Applied Surface Science 253 (2007) and nitric acid and then soaked in HF solution to form a conversion film before electroless nickel deposition. The chromium compounds are carcinogenic substances, which were prohibited in metal finishing industries and HF also exhibits strong corrosive that should be used with caution. Generally, the traditional nickel-plating baths are acidic ones with nickel sulfate being the main salt, which are more efficient than the basic nickel carbonate bath. However, magnesium and its alloys are more easily corroded in the acidic bath than that in the basic bath. Therefore, in the present study, we present a modified alkaline-sulfate nickel bath for first depositing Ni W P ternary alloy on the AZ91D alloy. Furthermore, in our previous study, a chromium-free bath [26] was proposed to create an equipotentialized surface before electroless Ni P deposition on the AZ91D magnesium alloy. This pretreatment is also used in the present study. The structure, morphology and corrosion characteristics of the Ni W P alloy coating are studied. 2. Experimental procedures The substrate was AZ91D die cast magnesium alloy with a size of 30 mm 40 mm 3 mm. The chemical composition of the alloy is given in Table 1. The substrate was metallographically ground with no SiC paper before pretreatment processes. After ground, the substrate was cleaned in 10% sodium hydroxide solution for 15 min to remove soils or greases on the surface of magnesium alloy and rinsed thoroughly in running water and deionized water to remove all the alkali. Then, the magnesium alloy sample was treated in the pretreatment bath [26] for 2 min to obtain a conversion film. Magnesium is classified as a difficult to plate metal due to its high chemical reactivity. Moreover, the AZ91D magnesium alloy is especially difficult to plate because the intermetallic species such as b-mg 17 Al 12 are formed at the grain boundaries, resulting in a non-uniform surface potential across the substrate. Thus, these challenges necessitate an appropriate pretreatment of the substrate to introduce an equipotentialized surface. In the present experiment, the pretreatment solution is a chromium-free one, where H 3 PO 4 and Mn(H 2 PO 4 ) 2 are the main ingredients [26]. After the pretreatment, the electrochemical heterogeneity of the AZ91D substrate surface is sufficiently low, which will restrain the corrosion of the anodic phases (a-mg) during the further electroless plating. On the other hand in further electroless Ni W P plating, the plating Table 1 The compositions of the AZ91D magnesium alloy (in wt.%) Al 8.77 Zn 0.74 Mn 0.18 Ni Fe Cu Ca <0.01 Si <0.01 K <0.01 Mg Balance Table 2 Composition and operating conditions of the Ni W P plating bath Concentration (g/l) Plating bath composition NiSO 4 6H 2 O 15 Na 2 H 2 PO 2 H 2 O 20 Na 2 CO 3 20 Na 2 WO 4 10 Na 3 C 6 H 5 O 7 2H 2 O 40 NH 4 HF 2 8 Thiourea Operating conditions ph 9.0 Temperature (8C) 80 2 bath should provide an acceptable deposition rate, relatively low corrosion rate to magnesium alloy substrate and good stability during deposition. The bath composition and operation parameters for the electroless Ni W P deposition are listed in Table 2. The electroless Ni W P bath contains sulfate nickel as the nickel ions provider, sodium hypophosphite acts as a metalreducing agent and sodium citrate as complexing agent. An aqueous ammonia bifluoride is used for restraining the corrosion of magnesium in the baths. Thiourea also acts as a solution stabilizer and brightening agent. The surface observation and qualitative elemental chemical analysis of the electroless Ni W P coating were realized by SEM (JSM-5310, Japan Electronics) and the attached energy dispersive X-ray spectroscopy EDS (INC250), respectively. The structure of the as-deposited Ni W P coating was analyzed by the X-ray diffractometer (XRD, Rigaku Dymax, Japan) with a Cu Ka radiation (g = nm) and a monochromator at 50 kv and 300 ma with the scanning rate and step being 48/min and 0.028, respectively. The hardness was evaluated by using a HXD-1000 microhardness tester with Vickers indenter, employing a load of 200 g for 15 s. Five readings were taken on the deposit, and the values were then averaged. In order to evaluate the porosity of the ternary coating on magnesium alloy, a porosity test was used. The method was briefly explained in Ref. [27]. That is, a filter paper (area: 1cm 2 ) was soaked in a reagent solution of 10 g/l NaCl, 106 g/ L ethanol and 0.1 g/l phenolphthalein dissolved in distilled water. The filter paper was then pasted onto the nickel coating for 10 min. After taking the filter paper away, red spots were noted on the surface of the coating. Thus, the porosity of coating was evaluated relatively by the ratio of red spot area to the zone area previously pasted by the filter paper. The immersion test in the 10% HCl solution at room temperature was carried for the ternary Ni W P coating on AZ91D magnesium alloy with different thickness. The time interval between the start of the acid immersion test and the first hydrogen bubble arising from the coating surface was recorded and used to evaluate the corrosion resistance of the coatings on magnesium alloy [28]. To further evaluate the corrosion resistance and possible passive behavior of the samples, electrochemical measurements were performed on an

3 5118 W.X. Zhang et al. / Applied Surface Science 253 (2007) electrochemical analyzer (LANLIKE, Tianjin, China). Linear sweep voltammetry experiments were carried out in a 3 wt.% NaCl aqueous solution using a classic three-electrode cell with a platinum plate (Pt) as counter electrode and an Ag/AgCl electrode as reference electrode. The experiments were done at room temperature. The working electrode and the samples were cleaned in acetone agitated ultrasonically, rinsed in deionized water before the electrochemical test. The coated samples were masked with lacquer so that only 1 cm 2 area was exposed to the electrolyte. During the potentiodynamic sweep experiments, the samples were first immersed into electrolyte for about 30 min to stabilize the open-circuit potential (OCP) E 0. Tafel plot was transformed from the recorded data and the corrosion current density (i corr ) was determined by extrapolating the straight-line section of the anodic and cathodic Tafel lines. The sweeping rate was 50 mv min 1 for all measurements. 3. Results and discussions One important modification of the present Ni W P electroless bath is the addition of sodium carbonate, which is used as complexing agent, accelerator and buffer to adjust the ph of the bath to be an alkaline one (ph 9.0). The optimum content of sodium carbonate in the baths is assessed by the deposition rate, bath stability and porosity of the coating. The pretreatment layer decreased the potential difference between the matrix and the second phase and thus could restrain the corrosion the magnesium alloy [26]. Meanwhile, the alkaline bath is used in the autocatalytical process since it is the most advantageous solutions for the electroless plating of Ni-based alloys and it could provide deposits with better corrosion resistance [19,30]. Especially for magnesium and its alloys, they are prone to corrosion in acid baths, while are hardly corroded in alkaline baths [1]. Fig. 1 showed the effects of sodium carbonate concentration and deposition time on the thickness of the electroless deposited Ni W P layers obtained from the bath at a constant temperature of 80 8C. The thickness of the asdeposited layer seems to linearly increase as a function of the depositing time. As for the bath without addition of sodium carbonate, although the hydrogen bubbles arose evidently during the electroless process, there was almost no Ni W P layer deposited on the AZ91D magnesium alloys after 1-h deposition. The experimental results in Fig. 1 show that the deposition rate comes to a value of about 7 mm/h in the bath with addition of 20 g/l Na 2 CO 3. But the main disadvantage of even high concentration of Na 2 CO 3 (above 20 g/l) was the substantial decrease of solution stability. As for the sulfamate nickel bath [29], the deposition rate was approximate 12 mm/h on mild steel specimens. But the bath was stable only for ph values ranging from 1 to 4 and at higher temperatures. While the deposition rate of Ni Fe P alloy on copper and carbon steel substrates obtained from the sulfate nickel bath was around 3 mm/h [20]. Therefore, the deposition rate of 7 mm/h at 20 g/l Na 2 CO 3 in the present bath is acceptable as to the ternary Ni W P electroless process on magnesium alloy. Fig. 2(a) shows the pattern of XRD of the AZ91D magnesium alloy. The AZ91D alloy consists of primary a-mg grains surrounded by a eutectic mixture of a and b-mg 17 Al 12 [25]. While the XRD pattern of the substrate pretreated in the chromium-free solution is illustrated in Fig. 2(b). The emerging peak at about 368 signifies that the b-mg 17 Al 12 phase became obvious on the surface after the pretreatment (Fig. 2(b)). After electroless plating, the AZ91D magnesium substrate was fully covered by the electroless Ni W P alloy, shown by the XRD pattern of Fig. 2(c). Apart from high intensity peak one more very low intensity peak is also noticed which is ascribed to Ni(2 0 0) plane. It is believed that the as-deposited electroless Ni W P plating has a mixture microstructure of amorphous and nanocrystalline, like the as-deposited electroless Ni P deposit. The major constituent, nickel, was autocatalytically active for the Fig. 1. The thickness increase of the deposited Ni W P layer with the deposition time from the baths with two different carbonate concentrations (10 and 20 g/l). Fig. 2. The XRD patterns of the electroless Ni W P deposition on the AZ91D magnesium alloy at different stages: (a) AZ91D magnesium alloy substrate, (b) the substrate after the pretreatment, and (c) the electroless coating Ni W P deposited on the pretreatment layer after 1 h (&, Ni;!, Mg; *, Mg 17 Al 12 ).

4 W.X. Zhang et al. / Applied Surface Science 253 (2007) Fig. 3. The surface morphology of the electroless Ni W P coating (a), the cross-section morphology of Ni W P coating (b) and qualitative chemical analysis of the ternary coating on the AZ91D magnesium alloy (c) scanning from the coating surface to the substrate along the line labeled in (b). deposition process, while the reduction of the second metal, tungsten, will be determined by its electrochemical potential as well as its catalytic activity for the reduction process [12]. Codeposition of the tungsten resulted in ternary Ni W P coating with phosphorus content of 4.9 wt.% and tungsten content of 4.5 wt.%, respectively, which were analyzed by EDS. These results are corresponding with the others experimental results on steel specimens [17,18,29]. The ternary Ni W P coating shows the typical spherical nodular structure with good uniformity and dense coverage (Fig. 3(a)). The morphology of the cross-section of the electroless Ni W P coating detected by SEM was shown in Fig. 3(b). Some pores in the coatings may result from the evaluation the hydrogen during the electroless deposition. The corresponding EDS analysis shown in Fig. 3(c) gives the elements distributions in the electroless coating along the line labeled in Fig. 3(b). It seems that the coating is connected closely to the substrate shown by the elements distributions from the coating surface to the substrate, and hence should exhibit good adhesion to the substrate. The hardness of the as-deposited Ni W P coating is about 660 VHN, which is higher than that of the as-deposited Ni P coating with similar phosphorus (5.6 wt.%) content on magnesium alloy (approximately 580 VHN) and far higher than that of the AZ91D magnesium alloy substrate (about 100 VHN). The corrosion resistance of the as-deposited electroless Ni W P was investigated by the porosity test (Fig. 4), the hydrochloric acid immersion test (Fig. 5) and the polarization measurement (Fig. 6). Magnesium alloy substrate is prone to galvanic corrosion because most other metals have a nobler electrochemical Fig. 4. The relation between the red area percentage by the porosity test and the deposition thickness of the electroless Ni W P coating and the compared Ni P coating.

5 5120 W.X. Zhang et al. / Applied Surface Science 253 (2007) Fig. 5. The immersion time in 10% HCl solution vs. the thickness of the electroless Ni W P coating and the compared Ni P coating. (The immersion time donates the intervals between the start of the acid immersion test and the first hydrogen bubble arising from the coating surface.) potential [1]. The nickel/mg system is a classical example of the cathodic coating on an anodic substrate. Therefore, the coating, which only provides a physical barrier against the corrosion attack of magnesium alloy, must be uniform, adherent and pore free otherwise the corrosion rate will increase. The porosity (represented by the red area) of the Ni W P coatings was estimated by the porosity test described in the experiment part and the results were shown in Fig. 4 as a function of the deposit thickness. As shown in Fig. 4, the red area decreases rapidly as the thickness increases. When the thickness reached to about 8 mm, no red spots were found on the tested coatings. This observation is understood as that the tested areas are thick enough and pores free to protect the substrate from corrosion. As for the compared electroless Ni P coatings with the chromium pretreatment [25], no red spots were found on the tested papers only when the layer thickness reaches to about 28 mm. Because the Ni W P coatings on the AZ91D magnesium alloy were deposited in the alkaline bath, the pores due to the hydrogen evolution were fewer than that of the Ni P coating deposited from acidic bath. Thus, the electroless coating with low porosities was effective to protect the substrate from the corrosion of Cl ions. The immersion test results in the 10% HCl solution for the Ni W P with different thickness were summarized in Fig. 5. The corresponding results of the Ni P samples were also shown in Fig. 5 for comparison. The time interval between the start of the test and the first hydrogen gas bubble arising from the coating surface was used to donate the corrosion resistance of the coatings on magnesium or magnesium alloys. In the test, there were no hydrogen gas bubbles arising from the 24-mm Ni W P coating with chromium-free pretreatment on the substrate after immersed in the 10% HCl solution for 187 min. While the 28-mm Ni P coating with the chromium pretreatment endured only 45 min without corrosion of the magnesium substrate. Therefore, it can be further confirmed that the ternary Ni W P coating with chromium-free pretreatment layer on Fig. 6. Polarization curves of AZ91D magnesium alloy substrate (curve a), the substrate with Ni W P coating (curve b), and the substrate with the Ni P coating (curve c) in a 3 wt.% NaCl aqueous solution. magnesium alloy exhibits better anticorrosion performance than the Ni P coating. Fig. 6 showed the electrochemical polarization curves for the AZ91D magnesium alloy substrate (a) and the electroless Ni W P coating (b) in a 3 wt.% NaCl aqueous solution at room temperature. For comparison, the polarization curve of the electroless Ni P [25] on the substrate (c) was also shown in Fig. 6. The cathode reaction in the polarization curves corresponded to the evolution of the hydrogen, and the anodic polarization curve was the most important features related to the corrosion resistance [31,32]. For the magnesium alloy substrate and the substrate with about 24 mm Ni W P layer, when the applied potential increased into the anodic region, an activation-controlled anodic process was observed. The polarization current increased with increasing the applied anodic potential and the polarization curves of Ni W P layer showed passive plateaus that indicated the formation of passive film. The corrosion potential E corr showed a significant positive shift to V compared with that of the magnesium alloy substrate. The corrosion current density i corr decreased evidently from ma/cm 2 of the substrate to only ma/cm 2 of the electroless Ni W P layer. Between potential of and V, the passive film formed on the surface with a much smaller corrosion current density of about ma/cm 2. When the potential was more positive than about V, the passive film broke down and the nickel in the coating was dissolved at the same time [28]. As for the 28 mm Ni P layer on the substrate the corrosion potential E corr was shifted positively to Vand the corrosion current density i corr decreased to about ma/cm 2. Therefore, the 24 mm Ni W P layer on the AZ91D magnesium alloy possesses more positive corrosion potential E corr, and exhibits much lower corrosion current density i corr. The polarization results indicate that the ternary Ni W P coating should exhibit high corrosion resistance that is in accordance with the porosity and immersion corrosion tests. As the porosity of the ternary nickel coating is reduced, the corrosion potential E corr would become positive and the corrosion current i corr become smaller [31].

6 W.X. Zhang et al. / Applied Surface Science 253 (2007) Conclusions The electroless of Ni W P coating on AZ91D magnesium alloy was deposited from a sulfate nickel bath with addition of sodium citrate and sodium carbonate. The bath allows the deposition rate up to 7 mm/h and shows reasonable stability. Compositional analysis by EDS shows that the tungsten content in Ni P matrix is about 4.5 wt.%. The ternary Ni W P deposits shows typical dense nodular structure. The porosity test, acid immersion test and electrochemical measurement reveal that the presence of tungsten and the denseandpore-freemicrostructuremakethecoatingpossessing nobler anticorrosion properties on the AZ91D magnesium alloy substrate. Acknowledgements The authors gratefully acknowledge the foundation of national key basic research and development program no. 2004CB and 985 Project of Jilin University for provided support of this work. References [1] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [2] G.E. Shahin, Minerals, Metals and Materials Society, Seattle, WA, United States, 2002, p [3] Z.M. Liu, W. Gao, Appl. Surf. Sci. 253 (2006) [4] R. Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124. [5] H. Umehara, M. Takaya, S. Terauchi, Surf. Coat. Technol (2003) 666. [6] M. Dabala, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol. 172 (2003) 227. [7] M. Zhao, S. Wu, J.R. Luo, Y. Fukuda, H. Nakae, Surf. Coat. Technol. 200 (2006) [8] L. Kouisni, M. Azzi, M. Zertoubi, F. Dalard, S. Maximovitch, Surf. Coat. Technol. 185 (2004) 58. [9] H.S. Yu, S.F. Luo, Y.R. Wang, Surf. Coat. Technol. 148 (2001) 143. [10] K.L. Lin, Y.L. Chang, C.C. Huang, F.I. Li, J.C. Hsu, Appl. Surf. Sci. 181 (2001) 166. [11] Y. Liu, Q. Zhao, Appl. Surf. Sci. 228 (2004) 57. [12] J.N. Balaraju, K.S. Rajam, Surf. Coat. Technol. 195 (2005) 154. [13] S.K. Tien, J.G. Duh, Y.I. Chen, Surf. Coat. Technol (2004) 532. [14] S.K. Tien, J.G. Duh, Y.I. Chen, Thin Solid Films (2004) 333. [15] S.K. Tien, J.G. Duh, Thin Solid Films (2004) 268. [16] J.N. Balaraju, C. Anandan, K.S. Rajam, Surf. Coat. Technol. 200 (2006) [17] J.N. Balaraju, S.M. Jahan, C. Anandan, K.S. Rajam, Surf. Coat. Technol. 200 (2006) [18] F.B. Wu, S.K. Tien, W.Y. Chen, J.G. Duh, Surf. Coat. Technol (2004) 312. [19] G.J. Lu, G. Zangari, Electrochem. Acta 47 (2002) [20] L.L. Wang, L.H. Zhao, G.F. Huang, X.J. Yuan, B.W. Zhang, J.Y. Zhang, Surf. Coat. Technol. 126 (2000) 272. [21] S.L. Wang, Surf. Coat. Technol. 186 (2004) 372. [22] B.W. Zhang, H.W. Xie, Mater. Sci. Eng. A 281 (2000) 286. [23] D. Mencer, J. Alloys Compd. 306 (2000) 158. [24] A.K. Sharma, M.R. Suresh, H. Bhojraj, H. Narayanamurthy, R.P. Sahu, Met. Finish. 96 (1998) 10. [25] C.D. Gu, J.S. Lian, G.Y. Li, L.Y. Niu, J. Alloys Compd. 391 (2005) 104. [26] W.X. Zhang, J.G. He, Z.H. Jiang, Q. Jiang, J.S. Lian, Surf. Coat. Technol. 201 (2007) [27] J.S. Lian, G.Y. Li, L.Y. Niu, C.D. Gu, Z.H. Jiang, Q. Jiang, Surf. Coat. Technol. 200 (2006) [28] C.D. Gu, J.S. Lian, Z.H. Jiang, Adv. Eng. Mater. 7 (2005) [29] J.N. Balaraju, S.M. Jahan, K.S. Rajam, Surf. Coat. Technol. 201 (2006) 507. [30] S. Lee, H. Liang, Plat. Surf. Fin. 78 (1991) 82. [31] C.D. Gu, J.S. Lian, J.G. He, Z.H. Jiang, Q. Jiang, Surf. Coat. Technol. 200 (2006) [32] H. Dong, Y. Sun, T. Bell, Surf. Coat. Technol. 90 (1997) 91.