Ultrafine grain (UFG) microstructure of Al-30Si alloy formed by friction stir processing (FSP), was correlated with its corrosion property. Electrochemical impedance spectroscopy and anodic polarization studies of FSPed Al-30Si alloy in 3.5. wt.% NaCl solution showed nobler corrosion and pitting potential as well as reduction of corrosion current density with increasing number of FSP passes. The homogenization of UFG microstructure with refined primary silicon particles changed the semi-conducting nature of passive film formed on Al-30Si alloy from n to p type resulting in improved corrosion resistance. Correlation of grain and silicon particle size on corrosion properties reveals Hall-Petch type relationship. © 2014 Elsevier Ltd.