Electroless Nickel Phosphorus Plating on AZ31
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INTRODUCTION
TO improve high-temperature mechanical properties such as the elastic modulus, ductility, and creep resistance and to depress chemical reactivity and improve corrosion resistance, magnesium is commonly alloyed with other elements. These alloys are suitable base materials for a variety of applications in the automotive, electronic, and computer industries, but they need to be improved in surface hardness, corrosion, and wear resistance. One way to achieve these improvements is to apply an electroless nickel coating on the surface.[1–4] Electroless nickel plating is the controlled chemical reduction of aqueous nickel ions onto a suitable catalytic surface. The electroless nickel solution must contain[5] a reducing agent for supplying electrons for the reduction of nickel ions, a suitable complexing agent for controlling the free nickel ions available to the reaction, and a stabilizer/inhibitor for controlling the reduction reaction, so that deposition occurs only on the substrate to be plated. Energy in the form of heat is provided by heating the electroless nickel plating bath. Finally, the source of nickel ions is provided by adding a KH.M. SHARTAL, formerly Graduate Student with Materials Engineering, Department of Process Engineering and Applied Science, Dalhousie University, is with Arabian Consulting & Engineering Services Co., Tripoli, Libya. G.J. KIPOUROS Professor, is with Materials Engineering, Department of Process Engineering and Applied Science, Dalhousie University, Halifax, NS, Canada B3J 2X4. Contact e-mail: [email protected] Manuscript submitted August 19, 2008. Article published online February 21, 2009. 208—VOLUME 40B, APRIL 2009
nickel salt in the form of either a sulfate or a chloride (the sulfate is preferred when the coatings are required for corrosion resistance applications). Electroless techniques for coating aluminum[6–9] and magnesium[10–12] alloys have been developed in our laboratory. For an electroless nickel-phosphorus (EN-P) deposition, the reducing agent is sodium hypophosphite; for electroless nickel-boron plating, sodium borohydride is used. Magnesium is very reactive and, with nickel metal, forms a galvanic couple. For additional protection, an insulator in the form of a conversion coating is produced on the surface of the alloy prior to the nickel-phosphorus or nickel-boron coating. A phosphate-permanganate conversion coating that is free of chromate ions was developed.[10] There is a major difference in the electroless plating processes developed for aluminum and those developed for magnesium alloys. Aluminum alloys are used in the manufacturing of automotive engine blocks, because they are not corroded in glycol-based engine coolants. As a result, glycine is used as an effective complexing agent in electroless plating baths destined to produce nickel-phosphorus coatings for aluminum alloys. In the case of magnesium, glycol-based solvents cannot be used, both because of their corrosiveness toward magnesium alloys and because the electroless plating baths contai
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