Reaction of Porous Anodic Oxide Films on Aluminum with Hot Water

IV Analysis of the Film Structure by Gravimetry and Porefilling Method

Abstract

Porous anodic oxide films formed on Al in an oxalic acid solution were hydrated for various periods by reaction with hot water. As reported earlier, the hydrous oxide is formed on the entire surface of the pore-walls to fill up the pores in about 10min, and then the hydration slowly continues from the outer surface so that the outermost part of the film becomes completely hydrated. Using gravimetric data and assuming the composition and density of the hydrous oxide to be Al₂O₃⋅2H₂O and 2.6, the thickness of the fully hydrated portion of the film, H_hy, the thickness of the film portion containing the unhydrated pore-wall, H_ox, and the thickness of the unhydrated pore-wall, δ, were calculated. In another series of experiments, the hydrated films were re-anodized with constant currents, i_r, of 0.01 and 0.5mA/cm² in a neutral borate solution to examine potential-time characteristics. For the smaller anodizing current, the steady potentials are related to the thicknesses of the barrier layers remaining unhydrated; they agree well with those estimated from capacitance measurements. Re-anodizing with the larger current causes dehydration of the hydrous oxide in the pores at the barrier layer/hydrous oxide layer interface, and voids produced by the dehydration are filled up with new oxide formed by the transport of Al³⁺ ion through the barrier layer. The growth of the barrier layer is reflected in the voltage-time characteristics. The transport number of Al³⁺ ion, T_Al³⁺, was found to be 0.24. Using this value, Hog was estimated from the voltage-time curve. The values of H_ox obtained as a function of the hydration time, t_h, were in good agreement with those estimated by gravimetry.