Determination of the chromium(III) reduction mechanism during chromium electroplating

The reduction mechanism of Cr(III) depends on the nature of the complexing agent present in electroplating solutions. In pure water solutions and without any other additives, Cr (III) is complexed with water molecules, creating a stable [Cr(H2O)6]3+ complex. [Cr(H2O)6]3+ can be reduced to Cr(0) in two steps via a chromium(II) intermediate species. However, this reduction is not quantitative, and the Cr(0) coating is difficult to grow. The destabilization of the [Cr(H2O)6]3+ complex via the addition of an organic complexing agent allows the formation of a reducible Cr(III) complex. The different complexes obtained with the addition of oxalate, formate or acetate were characterized by UV-visible spectroscopy and HPLC-ICP/AES. The nature of the ligand, the [Cr]/[L] ratio and the pH of the solution influence the chemical mechanism of the Cr(III) reduction. Independently of the complex nature, the reduction of Cr(III) to Cr(0) occurs in one step with the three studied complexing agent. Moreover a more quantitative reduction can be obtained by using an organic complexing agent, low [Cr]/[L] ratios and a pH of approximately 3.5.