Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024-T3 Probed by Vibrational Spectroscopy

Infrared and Raman spectroscopy were used to characterize a chromate conversion coating (CCC) on 2024-T3 alu-minum aircraft alloy with a long range objective of determining the anticorrosion mechanism of the CCC. Spectra were compared to those from synthetic mixed oxides of aluminum, Cr(III), and Cr(VI) made by treating pure reagents with NaOH. The Fourier transform infrared (FTIR) and Raman spectra of the CCC showed similar behavior to the chromium Ill/VI mixed oxide for both the initial materials and after heat-treatment. Analysis of the CCC and chromium mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ratio of Cr(III) to Cr(VI). When the chromium mixed oxide was im-mersed in pH 4 HNO3, the ratio of released H to released Cr(VI) ranged from 0.98 to 1.07. In addition, a compound with a Raman spectrum very similar to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr2O2, CrO2, or the Alodine chromating bath. The results indicate a strong structural similarity between the Cr-mixed oxide and a major component of the CCC. A likely structure for this common material involves covalent bonding between polymeric Cr(III) hydroxide and Cr202 or CrO.This mixed oxide structure may hydrolyze to release H and soluble chromate