Formation Efficiency of Anodic Porous Alumina in Sulfuric Acid Containing Alcohol: Comparison of the Effects of Monohydric and Polyhydric Alcohols as Additives

Abstract

Focusing on the coulombic efficiency of film formation, we systematically investigated the effects of polyhydric alcohols added to a sulfuric-acid-based electrolyte on the anodization behavior of aluminum, the coating ratio, the porosity, and the maximum attainable film thickness of the anodic oxide film by comparison with the effects of monohydric alcohols. Anodization was conducted in sulfuric acid containing various alcohols with different physical properties at a constant current density of 100 A·m−2 and at 20 °C. The viscosity and conductivity of the mixed electrolyte were affected by the addition of alcohol, resulting in an increase of the steady-state voltage with increasing alcohol concentration because of the high viscosity and low conductivity of the mixed electrolyte. Although the coating ratio, which is an index value used to evaluate the efficiency of the film formation, and the maximum attainable film thickness also increased with the addition of any of the alcohols, the addition of a polyhydric alcohol was more effective than that of a monohydric alcohol. When sulfuric acid with 50 vol% ethylene glycol was used as an electrolyte, an alumina film with less than 10% porosity was formed with high coulombic efficiency of film formation.