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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

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