Evidence of oxygen bubbles formed within anodic films on aluminium-copper alloys

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Abstract
The composition of the barrier anodic film formed at 50 A m−2 to 200 V on solution-treated Al-l at.% Cu alloy in ammonium pentaborate electrolyte at 293 K has been investigated by Rutherford backscattering spectroscopy and nuclear reaction analysis. The film contains about 14% more oxygen in comparison with that expected for an anodic alumina film as usually formed on high purity aluminium under similar conditions. The excess oxygen is attributed to the electrochemical production of oxygen near the alloy-film interface during growth of the film. The formation of the oxygen is probably linked to the oxidation of copper in a thin alloy layer, highly enriched in copper, immediately beneath the anodic film. Most of the oxygen appears to reside in bubbles, of dimensions from about 5 to 250 nm. within the inner part of the film, which are revealed in the transmission electron microscope. The estimated pressure of the oxygen gas within the bubbles is ≈ 150 MPa.