CORROSION BEHAVIOR OF AS-CAST, SILICON-CARBIDE PARTICULATE-ALUMINUM ALLOY METAL-MATRIX COMPOSITES

Corrosion characteristics of as-cast, silicon carbide particulate-aluminum (Al) alloy A356 (SiC(p)-A356 [UNS A13560]) were examined in chloride (Cl-) media using optical and scanning electron microscopy, energy dispersive x-ray analysis, cyclic potentiodynamic anodic polarization, and pit propagation rate test methods. Breakdown potential (E(b)) increased slightly as the SiC(p) volume fraction increased but decreased as Cl- concentration increased. Severity of attack increased with increasing SiC(p) volume fraction and Cl-concentration. Correlation of pit initiation sites with microstructural features revealed the critical role of second-phase particles in the composite materials and the corresponding unreinforced alloy matrices. At low Cl-concentrations, FeMg3Si6Al8 intermetallic particles were the most susceptible microstructural features in the unreinforced alloy. The particles were light gray and exhibited script and blade-like morphologies. As Cl- concentration increased, numerous geometrical pits nucleated within the primary Al phase of the unreinforced alloy. Pitting corrosion attack in the composite occurred preferentially at the Al-FeSiAl5 interface at low Cl- concentrations. These particles displayed a blade-like morphology As Cl- concentration increased, pits initiated at the SiC-Si and SiC-Al interfaces.