Characterization of chromium (III)-glycine complexes in an acidic medium by UV-visible spectrophotometry and capillary electrophoresis

Abstract

Since the 2017 REACH restrictions on hexavalent chromium, the aeronautics industries have been seeking substitute processes for chromium plating. So far, use of trivalent chromium-based precursors seems to be the best solution in terms of process adaptation. The development of such processes requires a better understanding of plating bath solution chemistry. More specifically, there is a need to characterize the complexation mechanisms occurring with the chromium (III), since they are responsible for higher electrodeposition efficiency. Chromium (III) complexation occurs under specific conditions, such as in an 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 such as glycine allow the formation of chromium-ligand complexes under specific pH and chromium-ligand ratios. In order to identify and understand the formation of the various complexes, two characterization methods have been tested: UV–visible spectrophotometry, which allows identification of chromium complexes through d-d electronic transition in the UV–Visible range, and capillary electrophoresis as a separation method, coupled with UV–visible detection in order to identify and quantify each species. UV–visible spectrophotometry and capillary electrophoresis analyses revealed that no chromium-glycine complex was observed for pH lower than 2, which is the regular condition for electrodeposition. Only a higher Gly:Cr ratio and/or pH>2.4 (i.e pKa1 of glycine) made it possible to observe glycine complexes in solution. As these conditions could be met during electrodeposition near the substrate/electrolyte interface, it is highly likely that chromium depletion, together with the pH increase that occurs, could favor the formation of glycine complexes at the interface, which have been reported as being required to achieve homogeneous chromium deposits.