Characterization of the Chromium (III)-Glycine Complexes in Acidic Medium by UV-Visible Spectroscopy and Capillary Electrophoresis

Abstract

Since 2017 REACH restrictions on hexavalent chromium, aeronautic industries have been involved in the substitution processes for chromium plating. So far, the use of trivalent chromium-based precursors seems to be the best solution in terms of process adaptation. The development of such process is nested on a better understanding on solution chemistry of the plating bath. More specifically, it is needed to characterize the complexation mechanisms occurring with the chromium (III), since they are responsible for a higher electrodeposition efficiency. Chromium (III) complexation occurs under specific conditions such as acidic solution, where chromium hydrolysis takes place, leading to a stable aqua chromium complex, which is suspected to be detrimental to an even deposit. To tackle this issue, a complexing agent is added to destabilize hexaaquachromium complex. It is reported that ligands as glycine allow the formation of chromium-ligand complexes under specific pH and chromium-ligand ratio. In order to identify, and to understand the formation of the various complexes, two characterization methods have been intended: UV-visible spectroscopy, which allows the identification of chromium complexes through the d-d electronic transition in the UV-Visible range, and capillary electrophoresis, as separation method, coupled to UV-visible detection in order to identify and quantify each species. UV-visible spectroscopy and capillary electrophoresis analyses revealed that no glycine-chromium complex was observed for pH lower than 2, which are the regular condition for electrodeposition. Only a lower Cr:Gly ratio and/or a pH higher than 2 allowed observing glycine-complexes in the solution. As these conditions could be met during electrodeposition nearby the substrate/electrolyte interface, it is highly likely that the chromium depletion together with the pH increase occurring would favor the formation of glycine-complexes at the interface, which were reported to be required to achieve homogeneous chromium deposits.