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% Au, has been examined by high-resolution transmission electron microscopy and Rutherford backscattering spectroscopy. Oxidation proceeds initially by the formation of an anodic alumina film containing few, or no, gold species. The alloy layer just beneath the anodic film is then enriched progressively in gold as a consequence of the initial formation of relatively pure alumina. Upon sufficient enrichment of gold in the 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 size are incorporated into the film at the alloy/film interface. The necessary enrichment of the alloy for the production of oxygen gas is dependent 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 respectively. The extensive generation of oxygen within the film at, or near, the alloy/film interface, leads to the formation of oxygen-filled bubbles within the anodic alumina, which may subsequently rupture and release oxygen gas.