More value from nickel

Transcription

1 More value from nickel A thermally evaporated thin zinc film was applied to aluminium alloy sheet as an alternative to zincate pretreatment prior to nickel electroplating. The evaporated layer showed a different surface morphology, and enhanced both the corrosion resistance and adhesion of nickel coatings applied to the aluminium alloy. Thermal deposition process enhances plating adhesion to aluminium Alsayed Abdel Aal* Because aluminium alloy materials are lightweight and possess a good strength, excellent workability and electrical properties, they have found extensive applications as construction materials, automobile wheels and body panels, canning materials and for various metallic containers, and also in the electronics field [1-3]. However, these materials suffer from corrosion in acidic or alkaline environments [4-6]. It is generally believed that effective and durable corrosion resistance can only be achieved for aluminium alloys via a surface barrier/coating using techniques such as microarc discharge oxidation, gas-flame spray plasma thermal spray, physical vapour deposition, high temperature glass enamelling methods, laser welding and electro or electroless deposition [7-11]. Electroplating is convenient but has some problems Electroplating is a commonly used protective process, due to the convenience of processing, low process temperature and low cost of raw materials and equipment [12]. However, it has been found that direct Ni plating on Al alloy substrates produced a non-adhered Ni coating due to the formation of galvanic corrosion cells between Al and Ni [13].In this respect, pre-treatment of the Al substrate by zincate immersion was essential in order to form a Zn layer which prevents direct contact between the Ni and Al substrate during the manufacturing of electronic devices such as large scale integration (LSI) chips [14,15]. However, the application of zincate pretreatment to Al-Si alloys resulted in a non-uniform Zn deposition, probably due to the non-uniformity of the surface condition induced by the Si atoms [16]. Golby and Dennis reported on the preferential deposition of Zn around eutectic Si/Al [17], which led Monteiro and Barbosa to use an etching treatment in order to reduce the effects of such intermetallic compounds on Ni plating [18].The effects of applying a thin Zn film to an Al substrate as a barrier layer between substrate and Ni electroplating as an alternative to the conventional zincate layer are considered below. A thermal evaporation technique can be used to deposit the film, and this is a promising way to obtain fast results. Due to the relatively easy control of the deposition parameters, the evaporation technique permits fabrication of thin films with high layer coverage and better adhesion if the stresses are low [19]. The influence of the evaporated Zn layer on the quality of the final Ni plating is then considered. Application and test procedures summarised An Al A356 alloy with a chemical composition of 6.6 % Si, 0.5 % Fe, 0.34 % Cu, 1 Mg, the balance being aluminium, was used as the matrix alloy. Prior to Zn evaporation, all the Al specimens were cleaned using a solution comprising 97 vol. % methanol and 3 vol. % nitric acid. A Zn thin film was thermally evaporated onto clean and dry Al alloy substrates, using a "VUP-5M" multipurpose vacuum station as coating unit. The vacuum pumping system employed was a combination of diffusion pump and rotary pump capable of producing an ultimate base vacuum of 10-5 torr. The distance between the target and the substrate was 10 cm, the substrate temperature was maintained at 50 C and the electron energy was about 5.5 kev. High purity Zn metal (99.99 % purity) was used as a target and the growth rate was adjusted to be as low as 10 nm/sec.a Watts bath was used for Ni deposition. The formulation contained 300 g/l NiSO4.6H2O, 45 g/l NiCl2.6H2O and 40 g/l H3BO3 at ph 5.5 and a temperature of 55 C, an applied current density of 5 A/dm2 and 100 rpm agitation. The structural properties of the deposited Zn films were analysed by X-ray diffraction (XRD, Bruker "axc- D8 Advance") using Cu-Koc radiation (40 kv/40 ma). The surface morphology of the Zn layer and Ni coating was studied by scanning electron microscope (SEM, Jeol "JSM-5410").The adhesion of Ni/Zn/Al and Ni/Al coatings was investigated qualitatively using an Olympus optical microscope after heating the samples for one hour at 150 C [13]. Potentiodynamic polarisation corrosion tests were used to study the general corrosion resistance of the samples (Ni/zincate/Al and Ni/Zn/Al) at room temperature using an Auto lab "Pgstat 30" with "corrware" software.the electrochemical measurements were made in a conventional three-electrode cell using a saturated calomel electrode (SCE) as a reference electrode and a Pt rod as a counter-electrode. After stabilising so that the electrochemical testing system was stable, the measurements were carried out in 3.5 %NaCl solution. Zinc and zincate treatments compared A thin Zn film was deposited on Al alloy sheets using the thermal evaporation technique and the morphology of the obtained layer was studied using SEM. Figure 1a shows the micrograph of the developed layer, in which the evaporated Zn particles have a uniform nano-rod shape with an average diameter of 50 nm. The good distribution and complete surface coverage of the substrate by evaporated particles confirmed the production of a dense and pore-free Zn film. From the SEM crosssectional image shown in Figure 1b, the thickness of the Zn thin film was determined. It can be seen that a coherent Zn film with 2-3µm thickness is deposited.

2 Previously, zincate immersion films had been successfully applied in the pre-treatment of Ni electroplating onto Al alloys [20]. Zincate bath composition and plating conditions for the preparation of Al/zincate/Ni coatings were optimised and have been published elsewhere [13, 20]. The Al substrate was firstly pre-treated based on both the processes of Zn evaporation and zincate immersion and then Ni electroplating was applied. The SEM surface morphology of the coated Ni/zincate/Al and Ni/Zn/Al samples is shown in Figure 2. It can be observed that the coated Ni/Zn/Al sample is smoother, finer, and more compact than the conventional Ni/zincate/Al sample.the crystal structure of the evaporated Zn layer and Ni/Zn/ Al coated samples was studied using XRD. Figure 3 represents the X-ray diffraction patterns of an evaporated Zn thin film. Four peaks appear on the XRD pattern corresponding to the planes with Miller Indices (0 0 3), (1 1 0), (0 2 1) and (0 1 5) of the Al0.71Zn0.29 rhombohedral crystal structure. In addition, two peaks characterised for the Al substrate were observed having Miller Indices of (1 1 1), (2 0 0). The application of an Ni coating over the Zn layer results in the disappearance of the AlZn peaks, and the entire range of peaks observed matches the Ni cubic structure well, indicating a phase-pure crystalline structure. Adhesion and corrosion tests show positive effects The adhesion strength between the Ni coatings and Al substrate with and without Zn layer was qualitatively evaluated by optical microscopy. Figure 4 shows photomicrographs of samples after heating for one hour at 150 C. It is obvious that the Ni/Al coated sample exhibited poor adhesion and the deposit was peeled off (Figure 4a). On the other hand, the Ni coating observed in Figure 4b for a Ni/Zn/Al sample had good adhesion due to the formation of an Al0.71Zn0.29 intermetallic compound. In order to evaluate the evaporated Zn layer, the corrosion behaviour of Ni/zincate/Al and Ni/Zn/Al coated samples was studied. Figure 5 shows the anodic polarisation curves of the samples studied, in 3.5 % NaCl solution. The corrosion potential of Ni/Zn/Al coating is more positive than that of the Ni/zincate/Al coating. Furthermore, the Ni/Zn/Al gave a larger corrosion current than that of the Ni/zincate/Al films, indicating a better corrosion resistance for Ni/zincate/ Al than Ni/Zn/Al coating. Further, Figure 6 shows SEM micrographs of the corroded surfaces for the Ni/zincate/Al and Ni/Zn/Al coated samples examined. For the Ni/zincate/Al sample, an extensive localised corrosion spot was observed after a corrosion test. On the other hand, the evaporated Zn layer of the Ni/Zn/Al sample acts as a sacrificial anode to protect the Al substrate and suppresses the corrosion of Ni/Zn/Al [21]. Evaporated zinc treatment enhances performance A thin Zn film was deposited by thermal evaporation technique onto the surface of Al alloy sheet following the Ni electroplating from a Watts bath. The evaporated layer demonstrated a uniform and compact microstructure and was coherent with the substrate. The electroplated Ni layer on the Al alloy with evaporated Zn films showed a smooth and fine morphology. The presence of a dense and pore-free Zn layer improves not only the adhesion between Ni plating and substrate but also the corrosion resistance of Ni coatings. REFERENCES [1] Y. Yang et al, Acta Metall. Mater., 38 (1990) 2613 [2] R. J. Arsenault, L. Wang, C. R. Feng, Acta Metall. Mater., 39 (1991) 47[3] A. G. Wang, I. M. Hutchings, Mater. Sci. Technol., 5 (1995) 3 [4] M. S. N. Bhat, M. K. Surappa, H. V. S. Nayak, J. Mater. Sci., 26 (18) (1991) 4991 [5] F. Gnecco, A. M. Beccaria, Br. Corros. J., 34 (1) (1999) 57 [6] L. H. Hihara, R. M. Latanision, Int. Mater. Rev., 39 (6) (1994) 245 [7] X. Nie, A. Leyland, H. W. Song, A. L. Yerokhin, S. J. Dowey, A. Matthews, Surf. Coat. Technol., 116 (1999) 1055 [8] P. Csokan, Electroplat. Met. Finish., 15 (1962) 75 [9] Y. C. Wang, S. C. Tung, Wear, 229 (1999) 1100 [10] W. B. Xue et al, J. Phys. Condens. Matter, 14 (2002) [11] A. Abdel Aal, J. Mater. Phys. Chem., 106 (2007) 317 [12] J. M. Molina, R. A. Saravanan, J. Narciso, E. Louis, Mater. Sci. Eng. A, 383 (2004) 299[13] J. Chen, et al, Surf. Coat. Technol., 201 (2006) 686 [14] C. S. Chang, A. Oscilowski, R. C. Bracken, IEEE Circuit Devices, 14 (1998) 45[15] D. A. Hutt, C. Q. Liu, P. P. Conway, D. C. Whalley, S. H. Mannan, IEEE Trans. CPMT, 25 (2002) 87 [16] L. Nanis, Plat. Surf. Finish., 92 (2005) 27 [17] J. W. Golby, J. K. Dennis, Surf. Coat. Technol., 12 (1981) 141 [18] F. J. Monteiro, M. A. Barbosa, Surf. Coat. Technol., 35 (1988) 321 [19] L. Holland, Vacuum Deposition of Thin Films, Chapman and Hall Ltd, London, 1970 [20] M. T. Abu- El-Khair, A. Abdel Aal, Mater. Sci. Eng. A, (2007) 156[21] Y. G. Kweon, C. Coddet, Corrosion, 48 (1992) 97 Results at a glance» Aluminium alloys are extremely versatile, but in many applications plating with other metals is needed to enhance corrosion protection. In some cases a pretreatment is required to ensure the plating is effective.» A nanocrystalline thermally evaporated thin Zn film was applied to Al alloy sheet as an alternative to standard zincate pretreatment prior to nickel electroplating.» The surface morphology and phase structure of the two coatings were studied. The evaporated Zn particles had a uniform nano-rod shape and the Ni/Zn/Al coating produced a smoother, finer, and more compact microstructure than the conventional Ni/zincate/Al sample.» The adhesion and corrosion properties of Ni/zincate/Al and Ni/Zn/Al coated materials were examined. Initial results show that the evaporated Zn layer gave better performance in both respects. * Corresponding Author: A. Abdel Aal Central Metallurgical Research & Development Institute Cairo, Egypt Tel foralsayed@gmail.com

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