Structure and function of ferricyanide in the formation of chromate conversion coatings on aluminum aircraft alloy

Raman and infrared spectroscopy were used to determine the structure of Fe(CN)(6)(-3) 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 Fe+3-CN-Fe+3 polymer, and Fe(CN)(6)(-3) physisorbed on Cr(OH)(3). No other cyano-containing products were observed. When Fe(CN)(6)(-3) was excluded from the coating bath, CCC formation rate greatly decreased. In addition, it was observed that Fe(CN)(6)(-3) could rapidly oxidize AA 2024-T3, and Fe(CN)(6)(-4) rapidly reduced Cr(VI) in bath conditions. These results indicate a redox mediation action for Fe(CN)(6)(-3/-4), which greatly increases the reduction of Cr(VI) to Cr(III) by the alloy. This process is normally quite slow, and redox mediation by Fe(CN)(6)(-3) is critical to CCC formation. IrCl6-3/-2 could substitute for Fe(CN)(6)(-3/-4) 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 fur any additional role of Fe(CN)(6)(-3/-4) in corrosion protection. (C) 1999 The Electrochemical Society. S0013-4651(99)01-025-3. All rights reserved.