The Morphology of Anodic Films Formed on Aluminium in Oxalic Acid

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Abstract
Electron microscopy of carbon replicas taken from outer surfaces, sections and cell base patterns of anodic films, formed in oxalic acid, has enabled relationships between the main film parameters of barrier layer thickness, cell size and pore diameter to be quantified and related to the anodizing variables. In general, the barrier layer, cell and pore dimensions are all proportional to the forming voltage and are inter-related as a result of the forming mechanism involving field-assisted dissolution and growth of the film in the barrier layer region. If the forming voltage is allowed to rise during the anodizing run at constant current density, inversely funnelled pores are produced in which the pore diameter is smaller nearer the film surface than adjacent to the barrier layer. In order to maintain this morphology it is necessaiy to restrain the electrolyte aggressiveness by controlling the acid concentration and temperature below certain limits. Transmission electron micrographs of stripped thin films have provided information on film development and pore initiation and on the transition from these transient processes to the steady-state growth stages.

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