Hydrogen Limitation By Background Salts in the Case of Trivalent Chromium Electrolytes
Authors: Marcelet Martin , Gigandet Marie-Pierre , Frayret Jerôme , Tardelli Joffrey , Hihn Jean-Yves 
Published: December 2020

The toxicity of some chemical compounds for humans and the environment encouraged health authorities to adopt new legislations. This is the case for hexavalent chromium salts, CMR known, which are targeted by the REACH European regulation. These salts are used in various surface treatments processes and their use is subjected to authorization. This situation has severely impacted the 'hard' chromium plating industry. Several projects all over the world deal with alternative solution and most of them are based on non toxic trivalent chromium salts, but more difficult to implement. The present work is undertaken within CRONOS 2024 project supported by a consortium of 20 companies and led by the IRT M2P (Institute for Technological Research Metallurgy Materials Processes in Metz).

Trivalent chromium ions form very stable hexa-aqueous complexes in solution, difficult to reduce. It is therefore necessary to introduce complexing agents in order to facilitate their reduction [1]–[3]. These complexing agents, mostly organic, add a difficulty in the realization of trivalent chromium electrolytes since various phenomena must be considered. In addition, the chromium based electrolytes have a low faradic yield which induces permanent competition between its own reduction and the one of protons to dihydrogen. Excessive consumption of protons near the cathode leads to a pH rise which can cause precipitation and incorporation of chromium hydroxides.

In order to improve the performance of trivalent chromium electrolytes, it is possible to adjust different parameters such as the nature and concentration of the complexing agents, the pH, the temperature, the addition of additives, etc. In this study, we focused mainly on the nature and concentration of the support salt. Previous works demonstrated that over a given concentration of Na2SO4 or K2SO4 support salts, a diffusion plateau appears, which can be attributed to the reduction of protons to dihydrogen [4]. Irrespective of the substrate nature, the composition and concentration of the support salts have an impact. It has been shown that a modulation of this plateau towards the most negative potential values would be able to help in decreasing the part of current attributed to the generation of dihydrogen. Moreover, as demonstrated by Phuong et al. [5], a comparison is possible with faradaic yields. Finally, the influence of organic and inorganic complexing agents has also been studied.

[1] S. Imer et T. Varnali, « Modeling chromium sulfate complexes in relation to chromium tannage in leather technology: a computational study », Applied Organometallic Chemistry, vol. 14, no 10, p. 660-669, 2000, doi: 10.1002/1099-0739(200010)14:10<660::AID-AOC55>3.0.CO;2-P.

[2] Z. Zeng, Y. Sun, et J. Zhang, « The electrochemical reduction mechanism of trivalent chromium in the presence of formic acid », Electrochemistry Communications, vol. 11, no 2, p. 331-334, févr. 2009, doi: 10.1016/j.elecom.2008.11.055.

[3] Y. . Song et D.-T. Chin, « Current efficiency and polarization behavior of trivalent chromium electrodeposition process », Electrochimica Acta, vol. 48, no 4, p. 349-356, déc. 2002, doi: 10.1016/S0013-4686(02)00678-3.

[4] Y. Mukouyama, M. Kikuchi, et H. Okamoto, « Appearance of new potential oscillation during hydrogen evolution reaction by addition of Na2SO4 and K2SO4 », Journal of Electroanalytical Chemistry, vol. 617, no 2, p. 179-184, juin 2008, doi: 10.1016/j.jelechem.2008.02.006.

[5] S. De, J. White, T. Brusuelas, C. Patton, A. Koh, et Q. Huang, « Electrochemical behavior of protons and cupric ions in water in salt electrolytes with alkaline metal chloride », Electrochimica Acta, vol. 338, p. 135852, avr. 2020, doi: 10.1016/j.electacta.2020.135852.