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.

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