Electrodeposition of copper zinc alloys from an ionic liquid-like choline acetate electrolyte

The ionic liquid choline acetate was used as a cyanide-free electrolyte for the electrodeposition of copper-zinc alloy (alpha-brass) thin films on a steel substrate. In comparison with the more commonly used choline chloride based deep-eutectic solvents, choline acetate enables the electrodeposition in absence of chloride ions, in order to avoid their accelerating effect. With 0.1 mol dm(-3) Cu(OAc)(2)center dot H2O and 0.1 mol dm(-3) Zn(OAc)(2)center dot 2H(2)O dissolved in choline acetate, the reduction potentials of copper and zinc were separated by 500 mV. Using potentiostatic deposition, only coatings of a negligible thickness could be obtained. Upon addition of triethanolamine, the deposition rates of both copper and zinc increased substantially and the reduction potential of copper shifted 300 mV toward more cathodic potentials. Bright layers with a thickness of up to 200 nm could be obtained. With a 1:2 molar ratio of metal-to-triethanolamine, well-adherent mirror-bright deposits with a composition of 90 wt% Cu and 10 wt% Zn were deposited at -1.5 V versus Ag vertical bar AgCl (3M KCI), with a cathodic current efficiency of around 75%. For longer deposition times, morphological instabilities occurred. Addition of polyvinyl alcohol in concentrations between 8 and 20 mg dm(-3) enabled the deposition of mirror-bright brass layers with a thickness up to I mu m. Variation of the deposition potential between -1.3 and -1.5 V had no marked influence on the composition of the alloy. However, the cathodic current efficiency dropped to 25% when potentials more negative than -1.5 V were applied. A decrease in the copper concentration in the electrodeposition bath resulted in dark-red powdery deposits while an increase of the zinc concentration in the bath, resulted in mirror-bright, well-adherent brass layers, with a copper content as high as 90 wt%. SEM images showed a grain size of 150 nm. XRD analyses indicated that the depositions consisted only of alpha-brass. (C) 2013 Elsevier Ltd. All rights reserved.