Evaluation of Protective Coatings on Magnesium for Phosphate Process Compatibility and Galvanic Corrosion Prevention

Bare magnesium is not compatible with the zinc phosphate process commonly used in automotive manufacturing today. If a magnesium component is assembled onto the vehicle prior to the phosphate process, it must be coated beforehand to prevent magnesium dissolution in the phosphate bath. In addition, for a component that must function in very corrosive environments, galvanic corrosion problems also need to be addressed because the component is most likely to be connected with a more noble metal such as steel fasteners or to another component made of steel or aluminum. In this paper, several coatings were evaluated for their effectiveness in stopping magnesium dissolution during the phosphate process and preventing galvanic corrosion. The results show that a porous anodized coating or a very thin chemical conversion coating does not completely stop magnesium dissolution during the phosphate process. An epoxy E-coat stops magnesium dissolution during the phosphate process but is not very effective in preventing galvanic corrosion. An epoxy powder coat or a properly sealed anodized film not only prevents magnesium dissolution in the phosphate bath but also provides good galvanic corrosion protection.