Oxidation reactions in chromium(III) formate electrolytes at platinum and at a catalytic mixed metal oxide coating of iridium oxide and tantalum oxide

Abstract

In trivalent chromium plating processes, the breakdown of useful electrolyte components or the formation of hazardous products, in particular Cr(VI), at the anode is obviously undesired. In this study, the oxidation reactions in trivalent chromium electrolytes with formate as complexing agent are investigated at platinum and at a catalytic mixed metal oxide (MMO) coating comprising iridium oxide (IrO2) and tantalum oxide (Ta2O5) using cyclic voltammetry combined with On-Line Electrochemical Mass Spectrometry (OLEMS) measurements.At platinum the following sequence of reaction steps is proposed (all potentials vs. SCE):1. Oxidative adsorption of [Cr(HCOO)(H2O)5]2+, reversible at −0.4 V;2. CO2 evolution from the Cr(III)-formate complex above +0.4 V;3. Further oxidation to HCrO4− with simultaneous CO2 and O2 evolution above +1.2 V.Cr(VI) formation also occurs in the absence of formic acid, i.e. when only Cr(H2O)63+ is present in the electrolyte.At a catalytic MMO coating the adsorption step is absent, as well as any subsequent oxidation of the Cr(III)-formate complex. This suggests that adsorption of the Cr(III) complex is required for Cr(VI) formation.MMO electrodes allow a safe usage of the electrolyte for industrial applications, whereas platinum electrodes do not.