Bipolar pulse anodizing of aluminum: Understanding the fundamental electrochemical mechanisms

Anodic aluminum oxides (AAO) with attractive morphological properties might be obtained by pulse anodizing via controlled hydrogen release during the cathodic step. In order to enhance such processes currently relying on empirical studies, the present paper aims to investigate basically the AAO formation on pure aluminum in sulfuric acid during a bipolar pulse anodizing process, by combining in situ electrochemical measurements and cross-section scanning electron microscopy. Results show that the hydrogen evolution at the metal/oxide interface during the cathodic step can be easily managed by current control. Actually, under cathodic galvanostatic conditions, successive mechanisms occur, depending on both the duration and the current density: a first stage corresponds to the reorganization of the electric charges and the migration of H+ through the AAO barrier layer and is followed by hydrogen evolution at the metal/oxide interface, which induces the formation of local or more extended defects at the metal/oxide interface until the detachment of the AAO layer.