EVIDENCE OF OXYGEN BUBBLES FORMED WITHIN ANODIC FILMS ON ALUMINUM-COPPER ALLOYS

The composition of the barrier anodic film formed at 50 Am-2 to 200 V on solution-treated Al-1 at.% Cu alloy in ammonium pentaborate electro lyte at 293 K has been investigated by Rutherford backscattering spect roscopy and nuclear reaction analysis. The film contains about 14% mor e oxygen in comparison with that expected for an anodic alumina film a s usually formed on high purity aluminium under similar conditions. Th e excess oxygen is attributed to the electrochemical production of oxy gen near the alloy-film interface during growth of the firm. The forma tion of the oxygen is probably linked to the oxidation of copper in a thin alloy layer, highly enriched in copper, immediately beneath the a nodic him. Most of the oxygen appears to reside in bubbles, of dimensi ons from about 5 to 250 nm, within the inner part of the film, which a re revealed in the transmission electron microscope. The estimated pre ssure of the oxygen gas within the bubbles is approximate to 150 MPa.