An Al−1 atom % Au gold alloy is employed to investigate the incorporation of a nonoxidized alloying element into porous anodic alumina films formed in phosphoric acid and sulfuric acid electrolytes. Gold is shown to enrich in the alloy in the initial stages of film growth, when a gold-free film is formed. On sufficient enrichment of the alloy, gold nanoparticles are incorporated into the film above an enriched alloy layer containing nanocrystals of Al 2 Au. The formation of the nanoparticles is accompanied by a reduced rate of film growth due to generation of oxygen. The gold is incorporated into the films preferentially at the cell boundary regions of the scalloped alloy/film interface. Elongated and approximately circular nanoparticles are observed in film sections, the former type being particular to films formed in sulfuric acid electrolyte. The incorporation of gold into the film commences earlier than predicted for a planar alloy/film interface. The behavior suggests that transport of gold occurs within the enriched alloy layer, possibly associated with the stresses at the alloy/film interface due to film growth processes.Porous anodic alumina films are of practical interest for protection of aluminum alloys against wear and corrosion and, more recently, as templates containing self-ordering porosity. During growth at a constant rate, usually achieved in practice by anodizing at either a constant voltage or a constant current density, the films comprise a relatively thin barrier layer next to the metal and an outer porous layer. 1-3 The porous layer thickens essentially in proportion to the charge that is passed during anodizing. The roughly cylindrical pores extend from the barrier layer to the film surface. The barrier layer thickness and pore diameter are related to the formation voltage in the period of steady film growth by similar factors of ϳ1.0 nm V −1 ; the proportionality for the interpore spacing is ϳ2.8 nm V −1 . Recent experimental studies have suggested a major role for flow of film material in the formation of porous anodic films on aluminum in certain electrolytes. 4,5 Flow of anodic alumina occurs in the barrier region of the porous film, where the high electric field results in transport of Al 3+ and O 2− ions toward the pore base and the metal, respectively. The Al 3+ ions are ejected to the electrolyte at the pore base. Thus, no film material forms at this location. In contrast, the inward migration of O 2− ions leads to growth of film material at the metal/film interface. The flow model ascribes the development of pores to the displacement of film material in the barrier layer beneath the pores toward the cell wall regions. A combination of high stress, due to formation of film material and electrostriction, and field-assisted plasticity of the film material, due to the ionic migration in the barrier layer, 6 provides conditions for the movement of anodic alumina within the film. The flow allows the thickness of the barrier layer to remain constant during anodizing at either constant cu...

Porous anodic oxides generated on copper-containing aluminium alloys are less regular than anodic oxides generated on pure aluminium. Specifically, a porous oxide morphology comprising layers of embryo pores, generated by a cyclic process of oxide film growth and oxygen evolution, is generally observed. In this work, the relation between the oxidation behaviour of copper during anodising and the specific porous oxide film morphology was investigated by electrochemical techniques, transmission electron microscopy and Rutherford backscattering spectroscopy (RBS). It was found that the anodising potential determines the oxidation behaviour of copper, and the latter determines the porous oxide morphology. At low voltage, relatively straight pores with continuous cell walls were obtained on Al-Cu alloys, but selective oxidation of aluminium atoms resulted in the occlusion of copper-containing metallic nanoparticles in the anodic film. At higher potentials, copper oxidation promoted oxygen evolution within the barrier layer, and generation of a less regular film morphology. RBS, performed on Al-Cu alloy specimens, revealed a high volume fraction of copper atoms in the anodic films generated at low potentials and a reduced amount of copper atoms in the anodic oxide films generated at high potentials.