Chromium(III) in deep eutectic solvents: towards a sustainable Cr(VI)-free steel plating process

Deep eutectic solvents (DESs) based on choline chloride, chromium chloride, and their mixtures with water are emerging as one of the most promising sustainable alternatives to the banned and carcinogenic Cr(VI) electroplating process (REACH Annex XVII). However, the coordination chemistry of these liquids is still largely unresolved and only a limited number of compositions have been studied to date. Hence, an in-depth study using a large range of techniques was carried out on the complete composition range of stable liquids at room temperature. This resulted in detailed knowledge regarding the concentration profiles of all constituting components, and the discovery of an important link between the coordination chemistry and the conductivity of the mixtures. Additionally, a tool was developed to obtain 10 different variables and concentrations by the measurement of a single UV/Vis absorption spectrum. Based on these results, researchers in both industry and academia will be able to select a certain DES–water mixture to obtain specific characteristics, as well as concentrations of the constituting components and complexes. Additionally, these mixtures can be closely monitored before, during and after electroplating, by means of UV/Vis absorption spectroscopy. These results form an important step towards the industrial application of these promising green electrolytes as alternatives to the wide-spread and recently banned carcinogenic Cr(VI) electroplating baths.