Characterization of the Reduction Mechanism of Chromium(III) By in Situ Microgravimetry and Volumetric Determination of Hydrogen

Abstract

Processes for chromium electrodeposition based on hexavalent chromium compounds are more and more restricted by governmental laws1 due to the toxic and carcinogenic nature of Cr(VI) compounds. The most promising alternatives to replace hexavalent chromium solutions are trivalent chromium electrolytes considering process management, quality of the deposits and cost issues.2 However, many of these electrolytes suffer from low current efficiencies and reduced process stability compared to hexavalent chromium electrolytes. Organic additives are used to reduce the overvoltage of Cr(III) reduction and, hence, reach an acceptable current efficiency.3 In order to improve performance and reliability of the deposition process, a profound knowledge of the deposition mechanism and the effect of additives is crucial.In this work the stepwise reduction process of Cr(III) to metallic chromium is investigated by gravimetric and volumetric in-situ methods under variation of current density. Electrochemical quartz crystal microbalance (EQCM) was used to quantify the mass of deposited chromium metal.4 The amount of evolved hydrogen gas was determined with a gas burette in combination with a purpose-built setup for collecting the gas. Based on the results, the partial current densities of the two reduction steps of Cr(III) and the hydrogen evolution were calculated. A shift in the ratio of the partial current densities and an increased current efficiency of metal deposition was observed after the addition of saccharin, which is used as additive in chromium electrolytes for decorative applications.5 A mechanism on the mode of action of saccharin during chromium electrodeposition is proposed. European Chemicals Agency, Authorisation List, EC No. 215-607-8, 231-801-5, 231-889-5, 231-906-6, 232-140-5, 232-143-1, 234-190-3, 236-881-5. Z. X. Zeng, A. M. Liang, and J. Y. Zhang, Recent Pat. Mater. Sci., 2 (1), 50-57 (2009). D. Del Pianta, J. Frayret, C. Gleyzes, C. Cugnet, J. C. Dupin, and I. Le Hecho, Electrochim. Acta, 284, 234-241 (2018). M. Leimbach, C. Tschaar, U. Schmidt, and A. Bund, Electrochim. Acta, 270, 104-109 (2018). R. Katirci, Int. J. Surf. Sci. Eng., 10 (1), 73-85 (2016).