“Electroless” deposition of a pre-film of electrophoresis coating and its corrosion resistance on a Mg alloy

Magnesium has unique electrochemical performance, which can be utilized in its coating or surface treatment. In this study, a new self-deposited coating process is explored for magnesium alloys. It is found that a thin film can be rapidly formed on a Mg alloy AZ91D through simply dipping the alloy coupon in an E-coating bath solution without applying a current or potential that is essentially required in a normal E-coating process. The “electroless” deposition mechanism and the film growth kinetics are investigated and the formed pre-film of E-coating is evaluated for its stability and corrosion protection performance in a phosphating acidic electrolyte and a NaCl corrosive solution. It is believed that the surface alkalization effect of magnesium is responsible for the “electroless” deposition of the pre-film. The diffusion of hydroxyls in the porous film is controlling the growth of the pre-film. The rapidly formed pre-film can offer sufficient corrosion protection for the magnesium alloy in a chloride-containing environment and it is also stable enough to enable a magnesium alloy part to go through a phosphating bath in a paint line.