Effect of Alcohol Addition to Sulfuric Acid on Efficiency for Formation of Anodic Alumina

Abstract

Anodization of aluminum in electrolytes containing alcohols as an additive has been studied for improvement of coating properties such as corrosion and wear resistance, and hardness. The influence of the addition of alcohols on the pore arrangement in anodic alumina was also investigated. However, the essential understanding of the effect of alcohol addition is still insufficient. In this study, we used the sulfuric acid-based electrolyte containing various alcohols (e.g., methanol, ethanol, glycerol, and ethylene glycol) as an additive and investigated the effects of types and concentrations of alcohols on the efficiency for the oxide formation and the structure of the porous alumina. High purity aluminum sheets were used as a starting material and anodized at constant current of 100 A m-2 in 1.5 mol dm-3 sulfuric acid with alcohol concentration in the range of 0~50 vol%. The formation of porous alumina film on the aluminum substrate was investigated by measuring voltage transient. In the case of ethanol concentration less than 10 vol%, voltage-time curve shows the steady-state voltage of approximately 15.7 V. When ethanol concentration exceeded 20 vol%, the steady-state voltage obviously increased with increasing ethanol concentration. In sulfuric acid without ethanol addition, the surface layer of oxide film was excessively dissolved after anodization for 220 min. The film thickness was approximately 65 μm. While using sulfuric acid with 50 vol% ethanol, chemical dissolution of the outer surface was significantly suppressed and the film thickness increased from 65 μm to 70 μm at the same anodization time, i.e., the same electric charge. Even in other type of alcohols, chemical dissolution of oxide was suppressed, resulting in improvement of coulombic efficiency for the oxide formation. The variation of dissociation constant of the electrolyte by addition of alcohols might affect the chemical dissolution and formation efficiency of anodic film. K. Kuroda, M. Ozawa, J. Met. Finish. Soc. Jpn., 15, 3 (1965).Y. Li, M. Zheng, L. Ma, W. Shen, Nanotechnology, 17, 5101 (2006).