(Digital Presentation) Fundamental Insights into Electrodeposition of Mixed Chromium Metal-Carbide-Oxides from Trivalent Chromium – Formate Electrolytes

Recent restrictions on industrial usage of hexavalent chromium under new REACH legislation have further sparked the development of hexavalent chromium-free chromium plating processes. An industrially important development in this field is Trivalent Chromium Coating Technology (TCCT®), a chromium electroplating technology for packaging steel developed at Tata Steel. In this process, aqueous trivalent chromium electrolytes rather than hexavalent chromium electrolytes are employed for the electrodeposition of metallic chromium. However, to deposit metallic chromium from a trivalent chromium electrolyte, it is necessary to incorporate a complexing agent given the kinetic inertness of aqueous trivalent chromium complexes. Formate plays this role in the TCCT® process yielding coatings comparable to the conventional hexavalent chromium-based process [1-4]. However, understanding the mechanism and kinetics of chromium electrodeposition from this system is quite limited. Fundamental knowledge of the deposition process is key for industrial process optimization. Essential to determining the reaction mechanism and kinetics is the identification of the chemical species involved in the reaction. Using a hybrid multiscale experimental and computational approach, insights into chromium complexation in the bulk electrolytes and how this speciation influences the composition of the deposit have been gained. Samples electroplated in electrolytes of varied formate concentrations were characterized using X-ray Fluorescence (XRF) spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Results from these analyses show that metallic chromium is only deposited when the electrolyte contains formate ions. In the absence of formate, only oxide and carbide species are deposited. The characterization results also show a current efficiency of the TCCT process of ~ 40%. From observations from surface characterization as well as spectroscopic analysis and density functional theory (DFT) and ab initio molecular dynamics studies (AIMD) of the bulk electrolyte, the coordination and complexation of formate ion in the chromium complex responsible for metallic chromium deposition have also been identified. Voltammetric studies coupled with ex-situ XPS and scanning electron microscopy (SEM) surface characterization also lead to a clear definition of the reaction mechanism of metallic chromium deposition that the incorporation of formate in trivalent chromium electrolytes makes possible.