Trivalent Chromium Electrolytes with Organic and Inorganic Ligands: Use of Pulsed Currents

Abstract

Hexavalent chromium salts are targeted by health authorities and in particular by the European regulation REACH because of their high toxicity for humans and the environment. Trivalent chromium salts are a promising alternative to hexavalent ones in the perspective of hard chromium coatings and have been extensively studied [1]. However, trivalent chromium ions bind with water molecules to form complexes resistant to electrochemical reduction, requiring ligands addition to evolve toward more favorable chemical equilibrium. This addition leads to a decrease of the coating properties, due to the incorporation of impurities such as carbon and oxygen. If carbon can be removed by the use of inorganic ligand, the presence of oxygen is more difficult to avoid [2], mostly due to hydroxide species incorporation induced by the pH increase at the interface. This phenomenon is the direct consequence of the important current part used to reduce protons during the plating process. It exists different ways to limit the pH elevation [3] such as the temperature, the electrolytes pH modification or the addition of specific chemical compounds, but an attractive way consists in current modulation i.e. using pulsed currents. The relaxation of the diffusion layer is known to reduce the competition between the diffusion of electrochemical species from the bulk and the proton discharge, which will limit the pH increase and restore the chromium concentration near the surface close to the initial one.Parameters of the pulse sequences tested in the present work were chosen from pH measurements [4] undertaken at the surface vicinity (less than 100 µm), and from the behavior of copper solutions working at high potentials where the competition between copper reduction and hydrogen takes place. Potentials were monitored at two-time scales: ms for the responses to current modulation (transient curves) and min for the average potential evolution all along the process. Finally, a comparison was made between inorganic and organic based trivalent chromium electrolytes, with a special care to cations presence, because of their influence on hydrogen discharge.[1] P. Benaben, Plating and Surface Finishing, p. 8, 2011.[2] A. Baral, R. Engelken, J. Electrochem. Soc., vol. 152, no 7, p. C504, 2005[3] J. Ji, W. C. Cooper, D. B. Dreisinger, E. Peters, J Appl Electrochem, vol. 25, no 7, p. 642-650, juill. 1995[4] M. Marcelet, Y-S. Chao, B. Vuillemin, M-P. Gigandet, C. Gleyzes, R. Oltra, J. Tardelli, J-Y. Hihn, ECS Meet. Abstr., vol. MA2019-02, no 21, p. 1046, sept. 2019