Corrosion behaviour of Al-Si/SiC composite in sea water

A study was made of the corrosion behaviour in sea water of a metal matrix composite (MMC) of Al-si alloy reinforced with SiC particles. Free corrosion and electrochemical tests were carried out at 25 degrees C in quiescent sea water at pH 8.2 containing 6.5 ppm dissolved oxygen. The microstructure if as examined by metallographic microscopy and by X-my microdiffractometry; electrochemical properties (pitting resistance, polarisation resistance) were investigated by potentiodynamic polarisation and electrochemical impedance spectroscopy; pitting formation was observed by optical microscopy; and the corrosion layer was characterised by X-ray photoelectron spectroscopy. In sea water the MMC suffered localised corrosion of the metallic matrix where Al-Cu intermetallic compounds ale present. Pit formation was enhanced by copper enrichment in the surface layer. The MMC showed a small passivity domain which increased slowly,with immersion time. No domain of perfect passivity was present. The SiC particles acted as cathodic sites with respect to the metallic matrix, which experienced selective aluminium dissolution.