Galvanic Compatibility of Coated Steel Fasteners with Magnesium

Selected metallic platings and insulating coatings on steel fasteners were evaluated for ability to reduce galvanic corrosion of die cast magnesium in a modified salt spray test. Proprietary electroplate systems based on zinc, zinc-nickel, zinc-cobalt and tin-zinc were tested, along with a commercial 50-50 tin lead alloy electroplate without supplementary coating. A proprietary liquid-applied zinc-rich inorganic coating successfully used on automotive fasteners was also tested for compatibility with magnesium.

Encapsulation of bolt heads with plastic insulating coatings or special molded caps was evaluated.

Interruption of the continuous salt spray by rinse and bake cycles was investigated as a likely exposure condition for magnesium assemblies in powertrain or underhood applications.

Several of the protection schemes were found to effectively eliminate galvanic corrosion of the magnesium.