CORROSION BEHAVIOR OF ALUMINA-ALUMINUM AND SILICON CARBIDE-ALUMINUM METAL-MATRIX COMPOSITES

Particle-reinforced, aluminum-based metal-matrix composites (MMCs) are -being considered for a range of applications. Their mechanical proper ties have been investigated in detail, but more information about thei r corrosion behavior is needed. The influences of alloy composition; p article characteristics such as composition, size, volume fraction, an d pretreatment: and composite post-treatment on the aqueous corrosion behavior of aluminum-matrix composites prepared by the melt stirring p rocess were studied. Corrosion tests consisted of prolonged immersion and anodic polarization measurements in sodium chloride (NaCl) solutio ns. The difference between the corrosion potential (E(corr)) and the p itting potential (E(p)) was lowered from similar to 500 mV(SCE) in dea erated NaCl to 100 mV(SCE) in aerated NaCl. Particle addition affected E(p) but not E(corr). Immersion test data revealed significant specim en weight loss for the composites resulting from formation of pits or microcrevices in the matrix near the particle-matrix interface and fro m particle dropout. Pits in the silicon carbide (SiC) composites were deeper than those in the alumina (Al2O3) composites, probably because the SiC particles acted as efficient cathodic sites, Pit initiation an d propagation occurred at weak spots in the air-formed film, correspon ding to phase discontinuities and second-phase particles and to oxygen reduction at the particles or precipitates. Anodization and ceria (Ce O2) coatings improved corrosion resistance of the composites.