Chromium Electroplating from Cr(III) in Deep Eutectic Solvents

Abstract

Hard chromium coatings are often used to improve the wear and corrosion resistance of different tools and components. Often electrolytes based on hexavalent chromium compounds, which are highly oxidative and carcinogenic, are still used to plate hard chromium layers. A key challenge is the development of an alternative baths free of Cr(VI). Such an alternative could be chromium coatings from a Cr(III)-based electrolyte.Over the last years alternative electrolytes besides water were studied for electroplating. One of these alternatives are the deep eutectic solvents, DES, of which ethaline is a prominent candidate. It is 1:2 molar ratio mixture of choline chloride and ethylene glycol. This DES show good solubility for various metal salts. The electrochemical potential window of most DES is typically in the range of 3 V. Ethaline has excellent physical and chemical properties, such as: it is stable under ambient conditions, easy to prepare and handle, cheap (manufactured on the M tons scale globally) and biodegradable (used as a B-vitamin in animal food), requires no additional registration for process applications, it has good conductivity and low viscosity.This paper will discuss the electroplating of Cr layers from ethaline on steel substrates. Different concentrations of CrCl3 × 6 H2O in the DES were investigated. The influence of LiCl, NaCl, KCl, formic acid and oxalic acid in the electrolyte as well as the influence of temperature were also investigated. Furthermore, the role of deposition technique (direct current or pulsed current) and of the hydrodynamic conditions on the quality of the chromium layers were evaluated.The electrodeposition of some µm thick and adherent chromium films was proved to be possible from ethaline. The hardness of the deposited Cr layers was up to 2500 N/mm2, depending on the process parameters.