AC-Bipolar Anodization of Aluminum: Effects of Frequency on Thickness of Porous Alumina Films

Abstract

The formation of porous alumina films using bipolar electrochemistry under alternating current (AC) electric fields without a direct electrical connection (“wireless treatment”) was investigated. The effect of frequency on the thickness of the films was evaluated using spectrophotometric analysis of dyed specimens and field-emission scanning electron microscopy. A film formation was simultaneously achieved on both sides of a high-purity aluminum electrode in oxalic acid under an AC electric field. A film formed at 150 Hz was thicker than those formed at other frequencies even in the same reaction time. Although the growth of non-uniform films were observed for AC-bipolar anodization, the morphology of the obtained films was roughly the same as those formed using conventional AC anodization with a direct connection, suggesting a similar oxide formation mechanism. Moreover, the dependence of film thickness on frequency was almost in agreement with the tendency observed for conventional AC anodization reported previously. The main disadvantage of AC-bipolar anodization lies in the decrease of the efficiency of film formation. From the application standpoint, however, the wireless treatment can introduce new possibilities for electrochemical surface treatment of small conductive materials that require a specific manipulation for a direct electrical connection.