A dipping E-coating for Mg alloys

Mg alloys are becoming more and more important structural materials in automotive industry. However, most commercial Mg alloys require coating protection because of their poor corrosion performance. In this paper, a self-deposited “electroless” E-coating process for Mg alloys is presented. By utilizing the unique electrochemistry of Mg, a thin film is rapidly formed on the surface of Mg alloys by simply dipping in an E-coating bath. The new developed dipping technique does not need a current or potential that is essentially required in a normal E-coating process. It is believed that the surface alkalization effect of Mg is responsible for the deposition of the “electroless” E-coating pre-film while the diffusion of hydroxyls in the porous film is governing the film growth. The rapidly formed pre-film can offer sufficient corrosion protection for Mg alloys in a chloride-containing environment or a phosphating bath in a paint line. The stability of the film can be significantly improved by the curing process. It is found that the coating performance is influenced by the type of Mg alloy and the surface pretreatment. The dipping coating can also act as a base for further powder-coating.