| The mechanism of corrosion and corrosion control of aluminum/graphite metal matrix composites (MMC) have been investigated. Based on the results of salt fog and immersion exposures, anodic and cathodic electrochemical polarizations, and galvanic coupling experiments of the MMC and their constituents, the rapid disintegration of the composites was found to be unambiguously attributable to oxygen reduction on the graphite fibers.; The corrosion mechanism is divided into the three following steps: Stage I--the penetration of the corrosive environment into the interior of the MMC caused by (a) pitting of the foil, (b) mechanical or processing surface defect, or (c) capillary wicking of the solution down exposed fibers; Stage II--galvanic corrosion of the aluminum at the G/Al interface driven by the reduction of oxygen on the graphite fibers; Stage III--corrosion/mechanical rupture of the MMC caused by wedging of the hydrated corrosion products (formed in Stage II) in the confined spaces of the MMC.; The kinetics of oxygen reduction on fibers are decreased by silica coatings and silicon rich surface modified layers and are increased by both copper and nickel coatings. The galvanic corrosion rate of G/Al MMC is significantly reduced by silica coatings and silicon rich surface modified layers on the graphite fibers. |
