Reverse scan polarization of anodic aluminum oxide until detachment in sulfuric acid: Mechanisms and morphologies

Abstract

Innovative anodic aluminum oxides (AAO) can be created by applying pulsed/oscillated signals during the aluminum anodizing process. However, a deeper understanding of the electrochemical mechanisms that take place in the voltage decrease during the process, from the anodic to the cathodic regime (also named reverse scan polarization), is still required to design and predict the nanometer morphologies. Three potential regions could be distinguished throughout the reverse scan polarization of AAO layers in sulfuric acid, thanks to voltammetry and in situ electrochemical impedance spectroscopy (EIS) studies performed in a three-electrode cell. At low potential scan rates, it was observed a thinning of the barrier layer and its perforation before the detachment of the AAO layer by hydrogen evolution on bare aluminum. On the contrary, at high potential scan rates, the fast migration of H+ through the barrier layer induces hydrogen evolution at the metal/oxide interface and the detachment from the substrate of the AAO layer, including the barrier layer. Briefly, by modulation of the potential or the potential scan rate, it is possible to form either a perforated or a closed AAO membrane and to monitor the barrier layer thickness, as confirmed by SEM and STEM analyses.