Mechanism of anodic film growth on aluminium

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Abstract
A systematic study of film growth on aluminium, over a wide range of anodizing conditions, encompassing barrier and porous-type films, has been undertaken to develop a more integrated theory of anodizing than previously possible. The broad findings are summarized, via schematic diagrams of film sections and immobile marker layers developed after anodizing and observed by analytical transmission electron microscopy, surface analysis and, where appropriate, electrolyte analysis; computational analysis of the distribution and field strength variation within appropriate locations of the film sections has also been performed. During anodizing of aluminium, to give barrier type films, Al3+ ion egress and O2-/OH− ingress proceed across the pre-existing air-formed under the field to form film at the film/solution and metal/film interfaces respectively. However, it is evident that under certain anodizing conditions, a direct ejection of Al3+ ions into the electrolyte without forming film, may proceed. This proportion of ejected Al3+ ions, compared with those forming film material at the electrolyte interface, generally increases with decrease of current density such that a critical current density may be discerned below which all mobile Al3+ ions are lost to the electrolyte. At applied current densities below the critical level, the outer regions of the initially formed film (by 02-/0H− ingress) are penetrated by reactive electrolyte species, representing the initial stages of pore development. Preferred penetration paths, where a high electrostriction pressure is developed, are sites of pore development which eventually develop into the steady-state film morphology.