Effect of Pulsed Currents on Oxygen and Carbon Contents of Trivalent Chromium Based Deposits

Abstract

Hexavalent chromium salts have been identified as toxic for both humans and the environment by Health authorities, which led the authorities to take measures such as the implementation of strict rules, such as the REACH regulation. While some applications may be substituted by other processes, the thick hard chromium plating used for its functional properties in friction and wear lacks of alternatives. Among them, trivalent chromium salts have been extensively studied [1], but this kind of electrolytes presents typical difficulties, leading to more fragile coatings with through-cracks, and a typical increase of hardness up to very high values after annealing. This change in the chromium coating behavior is often attributed to a significant increase in carbon and oxygen contents [2].If temperature as well as current density have shown a great influence on both carbon and oxygen, their effects are reversed, with for example a decrease in oxygen content with temperature (due to a high solubility of hydroxides) accompanied by an increase in carbon content that leads to amorphous deposits. It is however very difficult to reduce both carbon and oxygen. The present work is built on a parallel study carried-out in the IRT M2P project CRONOS 2024, which discover the possibilities to limit the pH increase at the interface by inserting long-time toff during electrodeposition [3].Various parameters (ton times, toff times, peak current density) have been scanned for chromium deposition on a copper tip (to make the oxygen and carbon analysis easier). It has been showed that a possible control of oxygen and carbon is possible, leading to very low values and changes in deposits properties (hardness, behavior after annealing, roughness ...). The best sequence was studied for various temperatures, and the combination of parameters extensively described.Finally, 15CDV6 steel samples were coated and fully characterized.[1] P. Benaben, « An Overview of Hard Chromium Plating Using Trivalent Chromium Solutions », Plating and Surface Finishing, p. 8, 2011.[2] A. Baral et R. Engelken, « Modeling, Optimization, and Comparative Analysis of Trivalent Chromium Electrodeposition from Aqueous Glycine and Formic Acid Baths », J. Electrochem. Soc., vol. 152, no 7, p. C504, 2005, doi: 10.1149/1.1933688.[3] M. Marcelet et al., « Influence of Pulse-Current Parameters on pH Measured By Local Method », Meet. Abstr., vol. MA2019-02, no 21, p. 1046, sept. 2019, doi: 10.1149/MA2019-02/21/1046.