Influence of local heat development on film thickness for anodizing aluminum in sulfuric acid

Joule heating of the oxide-covered aluminum electrode is the major heat source during anodizing, and the extent of heat removal is dependent on the electrolyte flow i.e., convection. For various galvanostatic and potentiostatic anodizing conditions, at different bulk electrolyte temperatures it was found that when the convection heat transfer is not uniform along the electrode surface, i.e., in a wall-jet electrode reactor, a local temperature distribution is established. Due to this nonuniform temperature, the anodic film exhibits a thickness distribution. Further, at higher current densities or lower bulk electrolyte temperatures, anomalous anodizing behavior occurs under certain galvanostatic conditions, associated with spots of increased oxide thickness and high-temperature peaks. By associating the local film thickness to the local temperature distribution, it was found that the higher the local temperature, the greater the local film thickness, implying a higher local oxidation current. This relation is explained by considering thermal enhancement of the field-assisted oxide dissolution at the pore bases, which necessitates a local current density rise to maintain the local anodizing equilibrium. Using transmission electron microscopy, cyclic voltammetry, and capacitance measurements, the previous was confirmed from measurement of an unchanged barrier layer thickness on the anodized aluminum specimen. (C) 2003 The Electrochemical Society.