SPECTROSCOPIC ELUCIDATION OF THE REPASSIVATION OF ACTIVE SITES ON ALUMINUM BY CHROMATE CONVERSION COATING

It has long been hypothesized and recently shown that hexavalent chromium ions in the chromate conversion coating (CCC) migrate to active sites. This repassivation behavior of chromate conversion coatings leads to protection from corrosion resulting from mechanical damage. The nature of this repassivation has been explored here using Raman spectroscopy and synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. Raman spectroscopic analysis of a mechanically damaged area on the CCC supports the theory of chromium ion migration and formation of an Al(III)-Cr(VI) complex at the active sites. Results of an investigation performed using XANES corroborate the formation of the complex at active sites such as pits/valleys/scratches even under other chromating conditions. This study therefore provides unambiguous evidence for the theory of migration of chromates.

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