Corrosion-protective chromate coatings on aluminum

• Published in: Applications of surface science 1979-03, Vol.2 (3), p.416-432
• Main Authors: Katzman, H.A., Malouf, G.M., Bauer, R., Stupian, G.W.
• Format: Article
• Language: English
• ISSN: 0378-5963; 0169-4332
• DOI: 10.1016/0378-5963(79)90073-4

Chromate conversion coatings on aluminum were characterized by Auger electron spectroscopy (in conjunction with ion beam sputtering for depth profiling) and X-ray photoelectron spectroscopy both before and after various exposures in a salt-tog chamber. The structure of coatings grown from a chromate-fluoride solution is described, and mechanisms for coating growth and corrosion are proposed. Results indicate that either HF or OH − must be present in the coating solution, in combination with the hexavalent chromium, in order for a corrosion- protective coating to form. These coatings consist of a layer of hydrated chromium oxide over a layer of mixed aluminum chromium oxide. The coating thickness did not increase after 30 s immersion in the solution, but the amount of aluminum detected in the solution continued to increase with longer immersion times. Chromium was found throughout the coating, and both trivalent and hexavalent chromium were detected on the surface at a ratio of about 2 : 1.