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Abstract
The literature on anodic coating formation on aluminium is critically reviewed. Using published data it is shown that the long accepted barrier layer theory is both inaccurate and inadequate as an explanation of coating formation. It is shown that there is no uniform barrier layer and that initially oxide nuclei are formed randomly but spread to form a semi-conducting oxide film whose thickness varies with grain orientation. Pore formation results from an agglomeration of lattice vacancies whose spacing on average is voltage dependent, as is the cell size. However, both these parameters are statistically variable at any given formation voltage due to variations in the lattice defect structure. Other factors affecting anodic coating structure at the macro level are also considered.
Over a period of 2 years the author had the privilege of intermittently exchanging ideas on anodic coating formation with Dr Jude Runge of Compcote International Inc. This was most helpful in preparing this paper which is a shortened version of the two articles previously published in Plating & Surface Finishing (Nov. 2009 and Jan./Feb. 2010 issues). He also expresses his thanks to D. A. J. Bryant and Mr E. Short (formerly of Alcan R&D Ltd., Banbury, UK) for reading previous drafts of this paper and for helpful comments on the above ideas as they developed. The author also gratefully acknowledges the help of Marion Davies whose help with inputting text and checking references has been invaluable.