A Study of the Effect of Ligand-Metal Interactions on the Electrodeposition of Chromium from Trivalent Chromium Electrolytes

Abstract

With electrolytic chromium coated steel (ECCS) technology, a thin layer of chromium and an outer layer of chromium oxide are deposited on packaging steel to provide corrosion protection and a suitable layer for adhesion of subsequent polymer coatings. In an ECCS plating line, the coating is deposited via a hexavalent chromium electrolyte. Hexavalent chromium has been found to be an environmental and health hazard and is therefore subject to restrictions under REACH legislation. A new coating process based on trivalent chromium chemistry has therefore been developed (TCCT).Coatings produced via trivalent chromium electrolytes have been found to provide comparable corrosion resistance to those achieved with the traditional Cr(VI)-based process [1]–[4]. However, there is limited understanding of the mechanism and kinetics of electrodeposition from trivalent chromium based electrolytes. This knowledge is key to optimizing process design of industrial plating lines. Formate ions are incorporated in the Cr3+ electrolyte as complexing agents to make Cr0 deposition possible. It has been shown that coating composition is dependent on the relative quantity of HCOO- ions present. The present study is comprised of two main research goals - to determine the complexes that form in solution as a function of the relative complexing agent (HCOO-) concentration and to determine the effect of this complex speciation on the deposition mechanism.Results of a multitechnique approach addressing the first of our research goals - complex speciation - will be presented. Density functional calculations have been used to establish the energetic feasibility of the reaction mechanisms suggested by results from UV Vis and FTIR spectroscopy and rotating cylinder cyclic voltammetry experiments. UV Vis and FTIR results show that a clear chemical transition occurs once the formate ion is introduced in the electrolyte – starting ratio 1:6 HCOO-:Cr3+. Beyond this point, no significant change in the spectrum apart from increasing absorbance up to a ratio of 12 HCOO-:Cr3+ is observed. These results suggest the formation of a single new complex in the solution upon introduction of the formate ion in the electrolyte solution which is not modified by further increase of the formate ion concentration.[1] M. Inman et al., ‘Green Process for Fuctional Trialent Crhromium Electroplating’, in 223rd ECS Meeting, 2012.[2] J. H. O. J. Wijenberg, M. Steegh, M. P. Aarnts, K. R. Lammers, and J. M. C. Mol, ‘Electrodeposition of mixed chromium metal-carbide-oxide coatings from a trivalent chromium-formate electrolyte without a buffering agent’, Electrochimica Acta, vol. 173, pp. 819–826, Aug. 2015.[3] J. E. Edy, H. N. McMurray, K. R. Lammers, and A. C. A. deVooys, ‘Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel’, Corrosion Science, vol. 157, pp. 51–61, Aug. 2019.[4] S. Hesamedini and A. Bund, ‘Trivalent chromium conversion coatings’, J Coat Technol Res, vol. 16, no. 3, pp. 623–641, May 2019.