The structure of the etch film on aluminum plates etched in nitric acid was systematically studied by varying the ratio of positive to negative charges provided by a rectangular current waveform. When etched using a symmetrical square current, the plates were dotted with the etch pits inside which a layered etch film formed via the stacking of dark-contrast vs. light-contrast layers. The former was presumably aluminum hydroxide, while the latter was the channels that were filled with the electrolyte during etching. The thickness of the aluminum hydroxide layer increased with increasing the charge passed during the cathodic half-cycle. Hydrogen discharge during the cathodic half-cycle led to the formation of the hydroxide layer. Hydrogen evolution at the interface formed microvoids and might induce defects in the hydroxide layer. Based on dissolution-passivation of the same pit, a possible mechanism was proposed to explain the formation of the layered etch film on aluminum etched in nitric acid using the alternating current with emphasis on the effects of hydrogen discharge. (c) 2005 The Electrochemical Society. All rights reserved.