Corrosion and Protection of Magnesium Alloys - A Review of the Patent Literature

Magnesium (Mg) and its alloys posses low density, high specific stiffness and electromagnetic shielding property, which are attractive to the automobile industry, 3C (computer, communication, consumer electronics), military and aerospace industry. Mg alloys have relatively low open circuit potential and thus highly susceptible to galvanic corrosion when they are in touch with other metal materials. Mg can also easily be oxidized and form a thin layer of loose MgO when exposed to air or humid environment. These disadvantages limited its applications. Therefore, surface coating technologies have been developed to protect Mg and its alloys from corrosion. Patented state-of-the-art technologies for surface treatment of magnesium alloys including chemical conversion, electrochemical plating, surface coating, and multiple-step surface treatment technologies have been reviewed and analyzed. Some new techniques have been introduced in this paper. It was found that conversion coating technologies account for a large amount of proportion among the patents of surface treatment. These technologies are also the main technologies used in industry. Japanese inventions on conversion coating of Mg alloys were patented in multiple countries. As the structures and service conditions of Mg alloy parts are of variety, a single surface-treatment process might not satisfy all requirements. Combined surface-treatment techniques appeared to meet the needs in different applications. More and more new and environmental friendly techniques are invented. Factors such as capital investment, ease manufacturing, and coating performances have to be considered when develop a coating technology for an industrial applications. There are still many good patents have not been used in industry. Hopefully this review of the state-of-art technologies of Mg alloy corrosion and protection can be helpful to the fellows engaged in this area.