The corrosion images of LY12CZ aluminum alloy with different corrosion time were obtained from accelerated corrosion experiment. The changing law of pits corrosion ratio at different corrosion time and corresponding corrosion grade were attained through image processing technique based on binary characteristic extraction. A new thought of corrosion damage evolvement law for aluminum alloy was proposed based on Corrosion grade which was regarded as the evaluation standard of corrosion damage for aluminum alloy.