The morphology, structure and mechanism of growth of chemical conversion coatings on aluminium

Abstract

The morphology and structure of the chemical conversion coating developed on aluminium in an acid chromate/fluoride solution have been examined by transmission electron microscopy of stripped films and ultramicrotomed sections. The use of annealed, high purity aluminium specimens which had been electropolished and subsequently etched prior to the conversion coating treatment allowed a clear identification of the anodic and cathodic sites for the coating growth process. Thus, for the first time, it has been found that the morphology of the coating is strongly dependent on the crystallographic orientation of the substrate which, in turn, appears to control the preferential deposition of the hydrated chromium oxide at grain boundaries or cellular boundaries. Such boundaries predominantly contain the presumed flaw sites due to impurity segregation in the substrate. Based on these observations, the mechanism of conversion coating growth has been developed. Deposition of hydrated chromium oxide is observed at cathodic sites, associated with metal ridges developed on the aluminium surface as a result of the initial pretreatment. The anodic sites lie between the metal ridges, where the initial air-formed film is chemically dissolved in the fluoride-containing electrolyte. Dissolution of aluminium proceeds through the chemically thinned film, revealing scalloped regions between the regions of cathodically developed chromium oxide.