ANODIC FILM FORMATION ON AL-AU ALLOYS - ENRICHMENT OF GOLD IN THE ALLOY AND SUBSEQUENT EVOLUTION OF OXYGEN

The anodic oxidation of Al-Au alloys, containing 0.4, 3.8 and 11 at% A u, has been examined by high-resolution transmission electron microsco py and Rutherford backscattering spectroscopy. Oxidation proceeds init ially by the formation of an anodic alumina film containing few, or no , gold species. The alloy layer just beneath the anodic film is then e nriched progressively in gold as a consequence of the initial formatio n of relatively pure alumina. Upon sufficient enrichment of gold in th e alloy, oxygen gas is produced at, or near, the alloy/film interface. At, or about, the same time, gold-rich clusters a few nanometres in s ize are incorporated into the film at the alloy/film interface. The ne cessary enrichment of the alloy for the production of oxygen gas is de pendent upon the alloy composition: > 2 x 10(19), 8.5 x 10(19) and 1.7 x 10(20) gold atoms m(-2) for the 0.4, 3.8 and 11 at% Au alloys respe ctively. The extensive generation of oxygen within the film at, or nea r, the alloy/film interface, leads to the formation of oxygen-filled b ubbles within the anodic alumina, which may subsequently rupture and r elease oxygen gas.