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

Abstract

Pure A1 foil specimens were galvanostatically anodized in a neutral phosphate solution (pH = 7.0, 20~ 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 H2SO4 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 PO43-and OHions, and an inner part consisting of pure alumina. The outer part dissolved in the H~SO4 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, 6out/6~n, 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.Anodic oxide films formed on aluminum contain an appreciable amount of anions brought from the anodizing solution. When anodizing in phosphate solutions, the incorporated anions locate in the outer part of the film, while an inner part next to the metal consists of stoichiometric oxide. This was first reported ]~y Randall and Bernard (1), who used a radiotracer method combined with a film sectioning technique. Using SIMS, Wood and co-workers (2) obtained phosphorus profiles qualitatively similar to those reported by Randall. More recently, we applied XPS combined with chemical sectioning of the film and ascertained that phosphorus in the outer part exists in the form of PO43-and that some protons are also incorporated in the outermost part of the oxide (3).Lanford et al. (4) measured hydrogen profiles for a variety of anodic oxide films using a nuclear reaction technique, and reported that protons exist only in the outermost part, at concentrations equivalent to about 1 mole percent (m/o) water in alumina.The purpose of the present investigation was to obtain quantitative measurements of the phosphate and proton profiles of oxide films formed galvanostatically in a phosphate solution and also to examine the effects of the anode potential and current density (c.d.) for films formed in phosphate solution at conditions similar to those used here. ExperimentalAnodizing procedure.--Pure, 99.99% A1 foil coupons (2 x 3 cm, tag shaped) were electropolished in a 4:1 mixture of glacial acetic acid and 60% perchloric acid at 10~176 by applying an anodic current of 100 mA/cm 2. The polished coupons were then immersed in a 20 g/1 CRO3-35 mYl H3PO4 solution (90~ for 20 min to remove oxide film formed during the electropolishing, washed with methanol, and dried. Anodizing was carried out in a 0.033M NH4H2PO4-0.067M (NH4)2HPO4 so...

Measurement of Stress Changes during Growth and Dissolution of Anodic Oxide Films on Aluminum

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Characterization of lipid bilayer formation in aligned nanoporous aluminum oxide nanotube arrays

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The effects of film thickness and incorporated anions on pitting corrosion of aluminum with barrier-type oxide films formed in neutral borate and phosphate electrolytes

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