FORMATION OF BARRIER ANODIC FILMS ON ALUMINUM IN ANTIMONATE ELECTROLYTE

Barrier anodic films have been formed on aluminium at a constant current density of 50 A m-2 in saturated potassium antimonate aqueous electrolyte at 293 K to voltages up to 462 V. The resultant films were examined by transmission electron microscopy and Rutherford backscattering spectroscopy. Film growth proceeded at high efficiency, with an average nm V-1 ratio of about 1.3. The film composition was close to Al2O3, but antimony was incorporated as an outwardly mobile species (2.8 at.%, expressed with respect to the aluminium and antimony in the film, at 295 V). The distribution of antimony species within the film section was complex, with several discrete regions being observed and the number of regions being dependent of voltage. Specifically at 295 V, an innermost band, a depleted band, a concentrated band, and fingers were revealed. It is suggested that the unusual behaviour results from a variable incorporation rate for the antimony species due to fluctuations in the pH of the electrolyte at the film/electrolyte interface and from the incorporation of a fine layer of Sb2O5 within the thickening film. The subsequent outward migration of these incorporated antimony species developed various pathways for current passage and ionic transport across the film.