Distribution of Anions and Protons in Oxide Films Formed Anodically on Aluminum in a Phosphate Solution

Abstract

Pure Al foil specimens were galvanostatically anodized in a neutral phosphate solution , and the thickness and composition profile of the formed oxide films were examined as functions of the anode potential and current density (c.d.). Analysis of the films was made by chemical sectioning with a solution, combined with (i) electron microscopy, (ii) x‐ray photoelectron spectroscopy, (iii) capacitance measurements, (iv) dc polarization measurements, and (v) solution analysis. The films were found to consist of two parts, an outer part containing and OH−ions, and an inner part consisting of pure alumina. The outer part dissolved in the solution at a rate faster than that of the inner part, and thus the two parts were clearly distinguished by following the time variations in the reciprocal of capacitance during the chemical sectioning. For films formed with a constant c.d., the thickness ratio of the two parts, , and the phosphate concentration in the outer part do not change with the anode potential or the total film thickness. With increasing c.d., the two values tend to increase. The mechanism controlling the anion and proton distribution in the oxide film is discussed.