Particulate 6092-T6 Al metal matrix composites (MMCs) reinforced with 20 vol % of B4C, SiC, and Al2O3 exhibited localized corrosion when immersed in a 0.5 M Na2SO4 solution exposed to air at room temperature. For the MMCs reinforced with B4C and SiC, the scanning vibrating electrode technique (SVET) revealed that corrosion initiated at localized anodic regions, which transformed into cathodic regions over time. The scanning ion-selective electrode technique (SIET) revealed that the localized anodic regions were acidified, and the localized cathodic regions were alkalinized. The observed anodic- cathodic transformation behavior was attributed to aluminum which has amphoteric oxides, the formation of microcrevices by reinforcement particles left in relief, and the possible galvanic action between the 6092-T6 Al matrix and the B4C and SiC reinforcement particles. The localized anodic and cathodic regions were many times larger than the individual reinforcement particle size. For the Al2O3-reinforced MMC, localized corrosion coincided with regions showing some cathodic activity and alkalinity. The extent of corrosion of these three MMCs was found to increase with decreasing reinforcement resistivities p (i.e., pAl(2)O(3) > pSiC > rB(4)C). (c) 2005 The Electrochemical Society. All rights reserved.