Structure and Function of Ferricyanide in the Formation of Chromate Conversion Coatings on Aluminum Aircraft Alloy

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Abstract
Raman and infrared spectroscopy were used to determine the structure of and its reaction products in chromate conversion coatings (CCCs) on AA 2024-T3 aluminum aircraft alloy. In addition, Raman spectroscopy was used to monitor CCC growth rates and their dependence on coating bath composition. The IR and Raman spectra of the air‐dried CCC corresponded to those of Berlin green, a polymer, and physisorbed on . No other cyano‐containing products were observed. When was excluded from the coating bath, CCC formation rate greatly decreased. In addition, it was observed that could rapidly oxidize AA 2024-T3, and rapidly reduced Cr(VI) in bath conditions. These results indicate a redox mediation action for , which greatly increases the reduction of Cr(VI) to Cr(III) by the alloy. This process is normally quite slow, and redox mediation by is critical to CCC formation. could substitute for to produce a chromate film with properties very similar to a conventional CCC. The results establish redox mediation as the mechanism of acceleration of CCC formation, but provide no evidence for any additional role of in corrosion protection. © 1999 The Electrochemical Society. All rights reserved.

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