The anomalous codeposition of tungsten in the presence of nickel

Deposition of tungsten-rich alloys from plating baths that do not contain ammonia is reported. Alloys with nominal composition of NiW and even NiW₂ have been formed for the first time by electroplating. It was shown in this paper, and in our previous work [Electrochem. Solid-State Lett. 3 (2000) 543; Proc. Advanced Metalization Conf. (AMC 2000) p. 337; Langmuir 17 (2001) 8270; J. Electrochem. Soc. 149 (2002) C100; J. Appl. Surf. Sci. 200 (2002) 1], that alloys of tungsten and nickel having any composition between a few a/o and 67 a/o of tungsten can be electroplated, by suitable choice of the experimental conditions. The most important factors among these are the concentrations of citrate, which acts as the complexing agent, the ratio of concentrations of nickel and tungstate ions and the pH of the solution. New indirect evidence for the existence of a mixed metal complex of the type [(Ni)(WO₄)(Cit)(H)]²⁻ was obtained by: (a) the pH dependence of the composition of the alloy and (b) by the linear dependence of the partial current density for deposition of tungsten on the calculated concentration of this complex, over a wide range of solution compositions. Removing ammonia from the plating bath allows the deposition of tungsten-rich alloys, but this is achieved at the cost of a substantial reduction in Faradaic efficiency.