Effects of ethanol on the efficiency of the formation of anodic alumina in sulfuric acid

Abstract

In the interest of enabling practical applications of anodic oxide films, it is important to develop methods to improve the coulombic efficiency of film formation. In this study, ethanol was added to the sulfuric-acid-based electrolyte to examine its effects on the anodization behavior of aluminum, the maximum attainable film thickness, and the resulting etching ability. Anodization was conducted in sulfuric acid solutions containing various concentrations of ethanol under mild anodization conditions (i.e., 20 °C, as opposed to the lower-temperature hard anodization conditions). When a constant current of 100 A·m−2 was applied, the initial voltage and steady-state voltage were strongly dependent on the ethanol concentration that was added. The growth rate of the alumina film was governed by a field-assisted electrochemical process and, thus, was not affected by the addition of ethanol in the sulfuric-acid-based electrolyte. However, the maximum attainable film thickness increased with increasing ethanol concentration while the charge during anodization was constant. Thus, it was concluded the coulombic efficiency of the film formation could be improved by suppressing the chemical dissolution of the anodic film. Furthermore, the degree of acid dissociation in the water-and-ethanol system is thought to play an important role in controlling the formation and resultant structure of the film.