Magnesium Alloys - Corrosion and Surface Treatments

Industrial deployment of magnesium alloys in most instances requires anti-corrosion coatings. Distinct from conversion coatings, the electro/electroless plating techniques are emerging as the common means of coating magnesium; however, more research is needed. A state-of-the-art review was undertaken with respect to aqueous plating systems (including pretreatment, under-coating, and electroplating), electroless plating (being treated individually, though it is used as undercoating for subsequent plating in some cases), non-aqueous plating systems (including high-temperature molten salts and ionic liquids), and novel plating methods. In addition to the performance assessment of various plated coatings on magnesium alloys, merits and demerits of existing plating techniques are discussed. Based on the literature to date, the practical issues faced in magnesium plating are raised, and possible advances discussed, providing some instructive guidelines for future work.

Corrosion-Resistant Electrochemical Platings on Magnesium Alloys: A State-of-the-Art Review

With the exception of a very few applications, the industrial deployment of magnesium alloys requires anti-corrosion coatings. This chapter looks at aqueous electrochemical plating systems (including pretreatment, undercoating, electroplating and electroless plating) and non-aqueous plating systems (including high temperature molten salts and ionic liquids). The performance assessment of various plated coatings upon magnesium alloys is discussed, along with the general pros and cons of plating techniques.

Corrosion-resistant electrochemical plating of magnesium (Mg) alloys

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

The synergistic fluoropolymer self-lubrication coatings on magnesium alloys were studied. Magnesium alloy was firstly treated by electroless nickel plating. Then micropores of electroless nickel plating layer was enlarged. Then the fluoropolymer particles were infused into micropores, precise vacuum heat treatment ensured thorough infusion with fluoropolymer, finally the synergistic coating was formed. The hardness of the coating reached 600HV, the dynamic coefficient of friction of the coating was less than 0.15, and the low coefficient of friction could be kept during a long duration wear to realize the permanent dry lubrication. The corrosion resistance was also excellent to endure 5% neutral salt spray test for 500 hours. Also the structure and composition of the coating were observed and studied.

Hong Qing Zou

Papers

Synergistic Fluoropolymer Coating on Magnesium Alloys