Use of Pulsed Currents with Long Off-Times to Increase Proprieties of Trivalent Chromium Based Deposits

Abstract

Health authorities directives, such as the REACH European regulation, are particularly severe with compounds such as hexavalent chromium salts, toxic for both humans and the environment. As a result, trivalent chromium salts have been extensively studied [1] as a promising alternative to the hard chromium process. However, electrolytes containing trivalent chromium ions present challenges, as the proprieties of the deposits are not identical, although they are getting closer. One difference is the high carbon and oxygen contents in trivalent chromium plating [2], mainly caused by the incorporation of organic and hydroxide species, the latest as a result of the increase of the surface pH. This is a direct consequence of the significant current used to reduce protons during the plating process.There are several approaches to limit pH increase [3], such as adjusting the temperature or the bulk pH, or adding specific chemical compounds. This study explores the use of pulsed currents. An initial research did not be able to define pulse parameters (e.g., current density, on and off-times) that could selectively reduce chromium without also reducing hydrogen [4], which can occur with other electrolytes. We consequently decided to focused on near-surface phenomena by measuring local pH [5]. This was possible by implementating a micro-electrode to a few dozen of µm from the interface. Thus, a critical parameter was identified : resting time (toff). It was found that extending off-times maintained optimal pH values at the interface during deposition. The effect of these pulse sequences was evaluated, among other parameters, on internal stresses formed during deposition and the relaxation phase using a device developed at the University of Houston (material science program and electrical and computer engineering) [6]. [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] J. Ji, W. C. Cooper, D. B. Dreisinger, et E. Peters, « Surface pH measurements during nickel electrodeposition », J Appl Electrochem, vol. 25, no 7, p. 642-650, juill. 1995, doi: 10.1007/BF00241925. [4] J. Rolet, « Influence de la forme de l’onde de polarisation sur la microstructure et les propriétés de revêtements électrolytiques élaborés à base de chrome trivalent », PhD thesis, Bourgogne Franche-Comté, 2017. [5] 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. [6] K. Ahmadi et al., « Crack Free Cr Coatings from Cr 3+ Electrolyte », J. Electrochem. Soc., vol. 169, no 1, p. 012504, janv. 2022, doi: 10.1149/1945-7111/ac4bfa.