Electrochemical impedance spectroscopy (EIS) and de polarization techniques were used to evaluate the effect of film thickness on the corrosion behavior of Al thin films exposed to an aqueous solution containing chloride ions. In general, the overall resistance to corrosion increased with film thickness. However, the grain size also increased with film thickness, a direct result of the thin-film deposition process. Therefore, the increase in corrosion resistance was attributed to an enlargement of the grain size; this reduced the grain boundary area, or effective cathodic area, and minimized the number of oxide layer defects that can form at or near the grain boundaries. In order to disassociate the effect of grain growth from that of film thickness, 1 mu m Al specimens were vacuum annealed at different temperatures. Yet, these vacuum-annealed films pitted upon immersion in the electrolyte. The increased susceptibility to pitting corrosion was caused by a reduction in oxide layer integrity during postdeposition thermal processes.