Galvanic corrosion and localized degradation of aluminum-matrix composites reinforced with silicon particulates

Galvanic corrosion of an Al-based metal-matrix composite (MMC) reinforced with 40 wt % Si particulates was examined by considering the semiconductor/electrolyte junction properties of the Si reinforcements. Scanning capacitance microscopy and photoelectrochemical experiments indicated that the Si reinforcements were slightly p-doped. At the open-circuit condition, a wide depletion layer was expected to be present at the slightly p-doped Si surface. The blocking characteristics of the depletion layer for electrochemical reactions would thus limit the galvanic interactions between the Si reinforcements and the Al matrix. The photoelectrochemical experiments also suggested that solar irradiation may promote galvanic corrosion because of the enhanced cathodic activity of the Si reinforcements under illuminated conditions. Corrosion was found to initiate from Fe-containing intermetallics, which served as effective cathodes. A chemical decoration method indicated that, while the bulk region of the Si reinforcements was not effective for cathodic reactions, the exposed Si surfaces at the Al/Si interfaces were effective cathodic sites. It was hypothesized that (i) during the processing of the Al/Si MMC, interdiffusion caused the Si surfaces at the Al/Si interfaces to be highly p-doped with Al, and (ii) upon propagation of localized corrosion from the sites adjacent to the Fe-containing intermetallics, the highly p-doped Si surfaces were exposed to solution and served as effective cathodic sites to induce galvanic corrosion. A scanning vibrating electrode technique revealed a net cathodic current over the localized corrosion regions, which were many times larger than the intermetallic particle or the Si reinforcement particle. Accordingly, the solution near the localized corrosion regions was alkalinized, as was revealed by the scanning ion-selective electrode technique. (C) 2008 The Electrochemical Society.