Abstract
From knowledge of the morphology of the chromate conversion coating developed on high purity aluminium, the locations of anodic and cathodic sites during coating formation are considered. Concerning cathodic sites, where chromate species are reduced to hydrated Cr 2O 3, the important role of electron tunnelling in sufficiently thin alumina films is developed and quantified further. At anodic sites, where the residual alumina film is thicker than at adjacent cathodic sites, hydrated Cr 2O 3 does not develop, through the low probability of electron tunnelling; thus, dissolution of the continuously developing alumina film proceeds, leading to a locally pitted appearance of the conversion coated high purity aluminium.
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