Effects of Chemical Surface Treatments on the Performance of an Automotive Paint System on Die Cast Magnesium

High purity die cast magnesium clutch housings and 4WD transfer cases have demonstrated their ability to function in corrosive road salt environments without the use of protective coatings. The weight reduction opportunities offered by magnesium also extend to highly visible components such as wheels, retractable headlight covers, and exterior mirror housings, where a decorative and durable finish is required. This paper examines the effects of chemical surface preparation on the performance of a typical automotive paint system on die cast AZ91D, in salt and fresh water laboratory exposures. The effects of prior mechanical damage to the coating by Gravelometer testing and other means are examined. Mild etching chrome pickles provided superior paint preparation for nominally clean as-cast surfaces. Commercial iron phosphates were effective when applied over previously pickled surfaces.