Formation of Chromate Conversion Coatings on Aluminum and Its Alloys

In situ X-ray adsorption near-edge structure (XANES) has been used to investigate the formation of chromate conversion coatings on pure Al, commercial Al alloys (AA 1100, AA2024, and AA7075), and a series of binary Al–Cu alloys. The method employed cells designed to determine the growth of the total chromium [Cr(total)] and hexavalent chromate [Cr(VI)] in the chromate conversion coating (CCC) as a function of exposure time to a chromate solution. Three sets of data were obtained, where ( i ) the Al was exposed to only a limited amount of solution; ( ii ) the chromate solution was excluded after short periods of repeated exposures to the solution; and ( iii ) the Al was exposed continuously to the chromate solution. All the results showed a very rapid initial growth within the first seconds, followed by a continued increase in thickness for exposures up to . Measurements with Al–Cu binary alloys demonstrated that the difference observed in AA2024 and AA1100 may not be due to Cu alloying. The proportion of Cr(VI) in the coatings becomes approximately constant after of exposure for all the specimens examined, even though the coatings continued to grow.