Corrosion behavior of cast, homogenized (T4) and microcystallized (mc) AZ91D alloy was investigated in NaCl aqueous solution by gas collection and electrochemical measurement. The capacitance property of the product films formed on mc alloy and cast alloy was studied by Mott-Schottky approach. The results implied that the grain size have a significant influence on the corrosion resistance of AZ91D alloy as well as P phase. The product films behaved as P-type semiconductor with the increasing of anodic potential. TEM observation indicated that the particle size of the product film on the me alloy (< 10 nm) was far smaller than that on the cast alloy (200 nm), which might be beneficial to the widening of energy band and the decreasing of acceptor concentration in the product film on me alloy. (c) 2005 Elsevier Ltd. All fights reserved.