Review of Vibrational Spectroscopy Studies of Coatings Based on Hexavalent or Trivalent Chromium Baths

The studies of hexavalent or trivalent chromium coatings via vibrational spectroscopy have been revised. The coatings can be formed via a conversion bath or electrodeposition from aqueous solutions. Most of these studies are related to the process of forming hexavalent chromium conversion coatings. In this case, there is a profound understanding of the system when vibrational spectroscopy is used in combination with other techniques; for example, synchrotron infrared microspectroscopy confirmed that ferricyanide is a mediator that oxidizes aluminum in the acceleration mechanism for coating formation. When pitting corrosion occurs, the zones with high Cr(VI) contents migrate to repair the coating in other zones. However, the toxicity of hexavalent chromium species has restricted their use in surface finishing, and they have slowly been replaced by trivalent chromium species. Vibrational spectroscopy revealed the formation mechanism of trivalent chromium conversion on an aluminum alloy. The undesired presence of Cr(VI) in these coatings is promoted by the presence of hydrogen peroxide in the bath. Raman spectroscopy combined with X-ray photoelectron spectroscopy (XPS) was used to determine the mechanisms of the inhibition of the formation of hexavalent chromium species. Despite the plethora of work on hexavalent or trivalent chromium electrodeposition in aqueous baths, vibrational spectroscopy has rarely been used to answer several questions about the process. Nonetheless, vibrational spectroscopy was able to identify the bonds Cr-O in the outer layer of a coating and the Cr-O-C bonds in the inner layer. Vibrational spectroscopy can differentiate between Cr(VI) and Cr(III) species.