The corrosion rates of particulate 6092-T6 Al metal-matrix composites (MMCs) reinforced with 20 vol % B4C, SiC, and Al2O3 as well as monolithic 6061-T6 At immersed in American Society for Testing and Materials (ASTM) and real seawaters were lower than those immersed in 3.15 wt % NaCl or 0.5 M Na2SO4 solutions. Scanning electron microscopy revealed that precipitates consisting of agglomerated whiskers formed on the surfaces of the MMCs and the monolithic 6061-T6 Al after prolonged immersion in seawater environments. The precipitates were tentatively identified to be Al-Mg hydrotalcite-like compounds based on energy-dispersive X-ray analyses and X-ray diffraction experiments. The MMCs immersed in ASTM seawater exhibited localized corrosion, however, the scanning ion-selective electrode technique revealed smaller pH fluctuations near the localized corrosion regions compared to the MMCs immersed in NaCl and Na2SO4 solutions. It was postulated that the corrosion of the Al-based materials is inhibited both by the precipitates and by the buffering effect of seawater. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3190521] All rights reserved.