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

Major vibrational spectroscopy studies have focused on the preparation of chromium coatings via chemical processes (conversion coatings), and few studies have focused on electrochemical processes (electrodeposition). Initially, the chemical precursors were hexavalent chromium salts, but these compounds are now replaced by less toxic trivalent ions. There is a profound understanding of the process when vibrational spectroscopy is used in combination with other techniques. This is the case for chromium(VI) conversion coatings, and the results of several techniques, such as synchrotron infrared microspectroscopy, have made it possible to understand the structure of the two-layer coating and the chemical composition of each layer. Vibrational spectroscopy confirmed the mechanism for coating formation, in which ferricyanide was a redox mediator. In addition, vibrational spectroscopy was effective in determining the mechanism of corrosion resistance of the coatings. Conversely, there are very few studies on the electrodeposition of trivalent chromium ions, and the mechanics of electrodeposition are unknown. To simplify the use of spectroscopy, spectra of potassium dichromate and chromium(III) sulfate are presented as references for coating studies, and a compilation of Cr(III)−O and Cr(VI)−O vibrational modes is provided to facilitate band assignment. Our review highlights that spectroscopic techniques have been insufficiently applied in this field; however, the results of vibrational spectroscopy accelerate the transition to safer Cr(III) technology.