Corrosion resistance of Al-based metal matrix composites

Corrosion characteristics of Al 6061-based metal matrix composites (MMCs) containing carbon fibres, alumina fibres or silicon carbide whiskers (SiCw) and the effects of ageing heat treatment on the stress corrosion cracking resistance of SiCwAl 7075 MMC were studied in chloride solutions. MMCs were prepared by the squeeze casting method in the laboratory. The pitting potentials for Al 6061 and the three Al 6061-based MMCs were similar. The resistance of SiCwAl 6061 MMC to pit initiation is the same as that of the monolithic alloy. Once pits initiate, damage by pitting corrosion would be greater owing to the formation of crevices between the reinforcement and the matrix by the dissolution of the latter in the pits. The cathodic current of the SiCwAl 6061 is slightly larger than that of the matrix alloy, possibly owing to an interfacial layer observed by transmission electron microscopy. Ageing of SiCwAl 7075 MMC at 170 °C for more than 1 h following ageing at 110 °C makes the MMC resistant to stress corrosion cracking in 3.5% NaCl and increases the pitting potential.