A critical review pertaining to surface treatments on magnesium alloys was published in 2002 (Gray & Luan, 2002). Two years later, another review paper on the same subject was published (Natarajan et al., 2004). According to the authors of both reviews, only complex and multilayer coatings are able to produce optimum results in term of corrosion and wear protection of magnesium alloys, and a great deal of research has to be done furthermore to develop technically and economically viable coating systems. One of the main challenges in the plating treatments of magnesium alloys is their susceptibility to corrosion. A way to prevent corrosion is by providing a barrier between the metal substrate and the environment. A coating has to be uniform, adherent, pore and crack free, and self-healing in the applications where damage of the coating can occur. The high reactivity of magnesium alloys causes the formation of a natural oxide when the metal is in contact with air and water. A treatment process for cleaning and removal of these oxides, that are unfavourable for coatings adhesion, is necessary before any electroplating and electroless plating treatment. The electrochemical and electroless depositions of coatings on magnesium alloys demand significant technical requirements. In fact, magnesium is subjected to corrosion in acidic aqueous environments and is only partly protected by an hydroxide layer that forms at pH higher than 9-10. Moreover, the most widely used commercial alloys, i.e. AZ91D and AM60B, contain alloying elements which form intermetallic phases (Fig. 1a). From an electrochemical point of view those phases have different behavior when the alloy is immersed into a solution for plating or chemical treatment. This means that the surface must be made equipotential by modifying the metallurgical structure before the coating process or by a specific surface pretreatment. Lunder reported that the corrosion potential of ┚-Mg17Al12 phase in 5% NaCl saturated with Mg(OH)2 is approximately -1.2 V vs. SCE, whereas the values for pure Mg and AZ91 are -1.66 V and -1.62 V vs SCE, respectively (Lunder et al., 1989). The heterogeneous structure of the alloys affects their corrosion resistance behavior and has a strong influence on the behavior of the alloy in the aqueous treatments processes, such as electroless and electrochemical depositions, and chemical conversions. A successful electroless or

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Electroless and Electrochemical Deposition of Metallic Coatings on Magnesium Alloys Critical Literature Review

This chapter discusses the initial stages involved in the handling of magnesium surfaces and looks at the important issues surrounding the preparation of magnesium alloy surfaces for further treatment, including optimized conditions for finishing processes. It begins with a review of mechanical cleaning procedures such as grinding, chemical cleaning, acid pickling and activation. Later sections go on to explain the method of action of various cleaners and their relationship to common post-processing treatments. The chapter ends with a discussion of current trends in the pre-conditioning of magnesium surfaces in the context of magnesium-based engineering and a brief summary.

Surface cleaning and pre-conditioning surface treatments to improve the corrosion resistance of magnesium (Mg) alloys

By means of X-ray diffractometry (XRD) and transmission electron microscope (TEM), the micro-structure of 5 different Ni–B alloy coatings prepared by electroless plating with Potassium borohydride content ranging from 02 to approximately 14g/L was studied The influence of the KBH4 content in the coatings on their micro-structure was also investigated It has been found that Ni–B alloy coatings showed amorphous sytuctue with XRD while with TEM it showed a combination of amorphous and nanocrystalline structure It was further revealed that the amorphous structure increased with increasing KBH4 content, which means that Ni-B alloy coating prepared with increasing KBH4 content undergoes a transition process as follows: nanocrystalline → crystalline → amorphous structure Results indicated that the micro-structure of Ni-B alloy coatings should be studied by both methods of XRD and TEM

Shu-Sheng Jia

Papers

Electroless nickel-plating on Die Cast Magnesium Alloy AZ91D