Surface analytical study of the corrosion behaviour of chromate passivated Al 2014 A T-6 during salt fog exposure

Surface analysis, using X-ray photoelectron spectroscopy XPS., Auger electron spectroscopy AES., and laser induced mass analysis LIMA., was carried out on chromate conversion coated aluminium alloy 2014 A T-6 prior to, and after, exposure to a neutral sodium chloride salt fog environment. From XPS, no chromium could be detected at these surfaces after a 48-h salt fog exposure. AES also indicated that the chromium content decreased after a 24-h salt fog exposure. However, from depth profiling, carried out using AES prior to and after a 24-h salt fog exposure, the chromium concentration was observed to increase within a few nanometres of the surface, and appeared unaffected by the exposure to the corrosive salt fog environment. During exposure, dark stains were observed to develop on these surfaces within 48 h. LIMA, carried out on these stained areas, indicated the presence of Fe, CN and Cu species, as well as aluminium hydroxide species, indicating that this initial coating failure may occur at copper-containing intermetallic sites on these surfaces. It would appear that the appearance of black stains on these surfaces is the first indication of coating failure, rather than the development of white corrosion product WCP., as the appearance of such stains coincided with the loss of chromate species from the surface. q2000 Elsevier Science B.V. All rights reserved