Electrodeposition of Gold: Depolarization Effects Induced by Heavy Metal Ions

Abstract

Addition of trace quantities (ppm) of heavy metal ions such as Pb(II) or Tl(I) to soft gold electroplating baths induces a marked cathode depolarization that extends the current density range in which bright, fine‐grained deposits can be obtained and produces uniform coverage of irregularly shaped substrates. In this study it is shown that the trace foreign metal additives which are most effective as depolarizers are those which tend to deposit uniformly on a gold surface to form an adsorbed monolayer at electrode potentials positive to those where their cathodic deposition as bulk metals begins,i.e., at underpotentials. Of the series of Group B elements (As, Bi, Cd, Cu, Ga, Hg, Pb, Sb, Sn, Tl, and Zn) investigated for depolarization action in alkaline phosphate solutions (pH 8, 70°C), only Pb, Tl, Bi, and Hg were found to induce significant effects. The latter two metals have not previously been reported to act as depolarizers in gold electrodeposition processes. The kinetics of the potential relaxation effects induced by these species under galvanostatic conditions were found to be strongly dependent on mass transport rate, gold deposition rate (i.e., plating current density), and temperature. In addition, it was found that Pb, Bi, Hg, and particularly Tl enhance the anodic dissolution of gold in cyanide‐containing solutions. The ranges of underpotential deposition of the metals Pb, Tl, Bi, and Hg from very dilute (∼5 μM) solutions of their ions in pure phosphate supporting electrolyte at 70°C were determined under steady‐state mass transport conditions at a rotating gold disk electrode. By varying the disk rotational speed and the potential scan rate, it was shown that a significant amount of Pb(II) [the equivalent of ∼20% of a close‐packed monolayer of Pb(0)] is specifically adsorbed in the ionic double layer at the gold surface and is not desorbed even at high anodic potentials. A similar phenomenon was observed for the Bi(III) species. The depolarization induced by the trace heavy metal ions is examined in terms of electrocrystallization and electrocatalytic mechanisms. From the magnitude of the potential relaxation (∼0.2V), its temperature dependence, and the ability of Hg to alloy with Au, it is concluded that screw dislocation generation is not responsible for this effect. Failure of Cs+ ions to enhance the rate of electroreduction of anions and also the ability of Pb(II) to catalyze the electroreduction of indicates that electrostatic repulsion effects are of minor importance. It is concluded that the depolarization phenomenon is caused by an enhancement of nucleation rate produced by the strongly adsorbed foreign metal atoms. In addition, catalytic electrochemical displacement reactions are made possible by the fact that the exchange current densities of the various Mz+/M depolarizer couples are much greater than that of the couple and by the specific adsorption of the Mz+species. In the case of Hg, displacement is made thermodynamically possible by the complexation of Hg2+ by CN− ions released during the electroreduction of .