THE ROLE OF ELECTRON-TUNNELING IN THE DEVELOPMENT OF CHEMICAL CONVERSION COATINGS ON HIGH-PURITY ALUMINUM

From knowledge of the morphology of the chromate conversion coating de veloped on high purity aluminium, the locations of anodic and cathodic sites during coating formation are considered. Concerning cathodic si tes, where chromate species are reduced to hydrated Cr2O3, the importa nt role of electron tunnelling in sufficiently thin alumina films is d eveloped and quantified further. At anodic sites, where the residual a lumina film is thicker than at adjacent cathodic sites, hydrated Cr2O3 does not develop, through the low probability of electron tunnelling; thus, dissolution of the continuously developing alumina film proceed s, leading to a locally pitted appearance of the conversion coated hig h purity aluminium.