Electroplating of Ni /W Alloys

Abstract

The electroplating of Ni/W alloys in ammoniacal citrate baths was studied, in an effort to evaluate the mechanism of the synergetic effect of Ni2+Ni2+ ions on the deposition of W, commonly referred to as induced codeposition. The extensive review of the field, published by Brenner in 1963, was summarized and updated. The partial current densities for deposition of Ni and W, as a function of the composition of the solution, the current density, and the rate of mass transport, were determined. There is strong evidence to support the hypothesis that the intermediate, which serves as the precursor for deposition of a Ni/W alloy, is a complex such as [(Ni)(WO)(Cit)(H)]2−,[(Ni)(WO)(Cit)(H)]2−, where Cit is the triply deprotonated anion of citric acid. Analysis of the data indicates that the concentration of this intermediate is in the range of 2-4 mM. Most experiments where done at pH 8.0 and in the presence of a large excess of ammonium hydroxide. The concentration of W in the alloy declines sharply when the pH is increased to a value of 9.0. Alloys rich in W can be deposited in the absence of ammonia. The chemistry of the plating bath, in particular, the relative abundance of the different complexes of nickel and tungstate ions with citrate and NH3,NH3, as a function of pH and the concentrations of the ligands, is discussed. © 2002 The Electrochemical Society.JES All rights reserved.