The growth of chromate conversion coatings on high purity aluminium

Abstract

The manner of chromate conversion coating development on very high purity (99.9996%) aluminium is considerably different from that observed previously on specimens of reduced purity (99.99% Al). This results from the absence of preferential cathodic sites associated with reduced impurity segregation within the high purity aluminium substrate. Consequently, a relatively uniform hydrated chromium oxide is observed on high purity aluminium, with the occasional presence of holes of various sizes. No preferential coating growth is evident, unlike that for 99.99% Al. Furthermore, elemental analysis of ultramicrotomed sections revealed the presence of only chromium and oxygen within the conversion coating. Neither aluminium nor fluorine was detected, suggesting that Al 3+ and F − transport through the developed coating is unlikely during coating growth. The features of conversion coating growth on high purity aluminium can be explained readily on the basis that the deposition of the coating occurs by electron tunnelling through a thin, alumina passive film which is always present on the aluminium surface.