Morphology, composition and structure of anodic films on binary Al-Cu alloys

This chapter examines the morphology, composition, and structure of oxide films grown during anodizing of Al-Cu alloys. Anodizing of Al-Cu alloys in ammonium pentaborate solution proceeds in two stages in which the initial formation of a Cu-free alumina film is followed by the introduction of Cu species into the film when a sufficient enrichment of Cu is achieved at the alloy/film interface. The occurrence of the second step depends on the Cu content in the bulk material and highly influences the electrochemical behavior of the anodized alloys. Oxygen gas at high pressure in generated within the film following incorporation of Cu. The open-circuit and corrosion potentials of alloys increase by about 500 mV with increase of Cu content to 7 at.%, and are further increased by about 200 - 500 mV by thickening of the surface oxide by anodizing. The increased potential with film thickening probably reflects a reduced anodic current, with a significant remaining cathodic activity.