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 c onversion coating (CCC) on 2024-T3 aluminum 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 alumin um, 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 III/VI mixed oxide for both the initi al materials and after heat-treatment. Analysis of the CCC and chromiu m mixed oxide by UV-vis spectroscopy indicated that both have a 3:1 ra tio of Cr(III) to Cr(VI). When the chromium mixed oxide was immersed i n 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 simila r to that of a CCC was formed by a reaction of Cr(III) hydroxide with Cr2O7-2, CrO4-2, 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 invo lves covalent bonding between polymeric Cr(III) hydroxide and Cr2O7-2 or CrO4-2. This mixed oxide structure may hydrolyze to release H+ and soluble chromate.