The 'complex' chemistry of Cr(III) in deep eutectic solvents : towards sustainable Cr(VI)-free electroplating

Steel parts are being subjected to extreme conditions in many industrial applications. A widely used technology for the protection of steel is the electroplating of chromium from aqueous Cr(VI) elec-trolytes. However, the use of these metal-containing solutions is being banned because of the carcinogenic properties of Cr(VI). For several years, researchers in OCAS (R&D center affiliated with ArcelorMittal Ghent) are looking for sustainable solutions. Some Cr(III)-containing deep eutectic solvents are being regarded as very promising alternatives for these existing electrolytes. Despite the possibility to deposit function hard chrome coatings from these innovating liquids, still less is known about the mechanism of this pro-cess. This PhD thesis focusses on the question which chromium complexes are present in these new solvents and in which way this influences some important physicochemical properties. In the first part, a series of reference absorption spectra were obtained for several possible com-plexes by using the deconvolution technique ‘MCR-ALS’. This was performed in both aqueous solutions and deep eutectic solvents. In the next part, this knowledge was used to investigate ternary xCrCl3·6H2O/yChCl/zH2O mixtures. Besides the previously used techniques (UV/Vis and EXAFS spec-troscopy), we used ATR-FTIR spectroscopy, density, refractive index, and conductivity measurement to study a variety of properties as a function of the composition.