CHROMIUM VALENCE STATE IN CHROMATE CONVERSION COATINGS ON ALUMINUM

Fluorescence mode XANES has been used to probe the chromium valence st ate in chromate conversion coatings on aluminium. The ratio of Cr6+ to total chromium yield (Cr3+ and Cr6+) does not change significantly wi th coating thickness. Conversely, the ratio decreases markedly with su bsequent immersion in sodium chloride solution. The leaching is greate r for thinner coatings; additionally, the Cr edge height generally inc reases as a result of such immersion. The role of the leached Cr6+ spe cies on protection of the aluminium substrate is explored.