Ferrate Conversion Coating for Corrosion Protection of High-Temperature Diecast Magnesium Alloys

The use of high temperature, creep-resistant magnesium alloys has been suggested as a way to reduce an automobile¿s weight, with attendant reductions in fuel consumption and emissions. Target components include transmission housings, covers, and engine blocks. However, the scope of these applications is limited by the reactivity of magnesium in the presence of environmental moisture and salts. Currently, there are no commercial conversion coatings for magnesium alloy that protect against salt damage. This project addresses this problem by coating magnesium surfaces with a ferrate-based conversion coating, in order to eliminate galvanic corrosion in the presence of dissimilar metals (such as aluminum and steel) and salt. Ferrate conversion coating has been previously demonstrated to provide superior protection for aircraft aluminum alloys and is expected to perform equally well for magnesium alloys. In Phase I, corrosion protection will be demonstrated by exposing coated magnesium coupons to a standard salt fog spray and by conducting electrochemical tests that use anodic polarization and electrochemical-impedance-spectrometric methods. Spectroscopic methods will be used to verify self-healing, i.e., the replenishment of protective coating after scratching. The ability of automotive paint to adhere to the ferrate base coat will be determined by electrochemical methods. The cost-effectiveness of the ferrate conversion coating will be calculated from information provided by automotive manufacturers. Commercial Applications and other Benefits as described by the awardee: Magnesium with ferrate conversion coatings would be expected to replace any automotive part that is currently fashioned from aluminum or steel. The coatings would be environmentally friendly as they do not contain chemical compounds that are hazardous, toxic, or give rise to public safety or health concerns. When prepared by electrochemical methods, the ferrate coatings would be low cost and involve no expensive chemical additives.