Influence of copper on the morphology of porous anodic alumina

Sputtering-deposited Al–Cu alloy layers and an Al–Cu/Al bi-layer are used to investigate the influences of copper on the morphology of porous anodic alumina films formed galvanostatically in either sulphuric or phosphoric acid electrolyte. The results reveal development of an irregular morphology of pores during anodizing of the alloy layers, contrasting with the linear porosity of films formed on aluminium. Further, the rates of film growth and alloy consumption are relatively low, since oxygen is generated following enrichment of copper in the alloy and incorporation of copper species into the anodic film. The linear morphology is re-established following depletion of the copper in the bi-layer and at the same time, film growth accelerates as oxygen evolution diminishes. The irregular pore morphology is considered to arise from stress-driven pore development influenced by effects of oxygen bubbles within the anodic alumina.