Porous Anodic Film Formation on Aluminium Substrates in Phosphoric Acid

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Abstract
The initial film growth and pore development sequences during the anodizing of aluminium substrates at constant current density in phosphoric acid have been observed by transmission electron microscopy of stripped anodic films and ultramicrotomed sections. Initial film growth proceeds by immediate film thickening and preferential film formation above spontaneous or carefully preformed ridges of metal on the substrate surface to produce a cellular arrangement of locally thicker material at the outer surface of the film, which is related to the substrate substructure, but the metal/oxide interface becomes relatively flat. Incipient pores then develop within preferred cells, where the efficiency of film formation is reduced compared with that within other cells. This results in localised scalloping at the metal/film interface beneath the developing pores and the effective diameter of the scalloped regions widens with duration of anodizing. The regular morphology of the porous anodic film is established when the scalloped regions merge, with some incipient pores dying out, and more uniform current density conditions for film growth are created over the substrate surface, namely when the major anodic film parameters become more regular and the field across the barrier layer becomes uniform beneath each pore.