Porous Anodic Film Formation on an Al-3.5 wt % Cu Alloy

Abstract

Anodic film growth has been undertaken on an electropolished Al-3.5 wt% Cu alloy to determine the influence of copper in solid solution on the anodizing behavior. At the commencement of anodizing of the electropolished alloy, in the presence of interfacial enrichment of copper, Al 3+ and Cu 2+ ions egress and O 2- ion ingress proceed; film growth occurs at the alloy/film interface though O 2- ion ingress, with outwardly mobile Al 3+ and Cu 2+ ions ejected at the film/electrolyte interface, and field-assisted dissolution proceeding at the bases of pores. Oxidation of copper, in the presence of the enriched layer, is also associated with O 2 gas generation, leading to development of oxygen-filled voids. As a result of significant pressures in the voids, film rupture proceeds, with electrolyte access to the alloy, dissolution of the enriched interfacial layer and re-anodizing. The consequence of such processes is the development of anodic films of increased porosity and reduced efficiency of film formation compared with anodizing of superpure aluminum under similar conditions.