ELECTRODEPOSITION OF HARD GOLD

The electrodeposition of hard gold in layers of 2 μm was investigated. The electrolyte was an acid citrate bath (pH 3·5) with cobalt as an additive. A flow cell allowed a controlled variation of the hydrodynamic conditions. The following features were examined quantitatively in the experiments: the current efficiency for gold deposition (10-30%), the carbon and cobalt content, as well as the porosity of the deposits, and the morphology [by scanning electron microscope (SEM)]. Above 50 mA cm-2 the deposition of gold and to a minor extent the incorporation of cobalt become mass transport limited (with certain complications resulting from the complex nature of the diffusion layer). The influence observed below 50 mA cm-2 seems to be due to the synergic effect of the transport controlled reduction of dissolved oxygen. A simple qualitative model for the incorporation of carbon is proposed. The substantial decrease in current efficiency observed upon the addition of cobalt to the bath is probably caused both by a decrease of the hydrogen overpotential and by an increase of the overpotential for gold deposition. From the viewpoint of technical application, the most relevant result, is the substantial decrease in porosity at decreasing current density (c.d.) and increasing flow rate. © 1979 Chapman and Hall Ltd.