Gold Electrodeposition on Tungsten Single Crystals and Thin Wires

Gold plated tungsten wires are used in a wide variety of applications ranging from medical devices to high frequency signal transmission. Thin wires place special demands on the plating process and this study aims to understand how surface properties affect the deposition of gold. Tungsten single crystals and wires with different dimensions were therefore studied in alkaline cyanide electrolyte with and without gold cyanide complexes, using electrochemical impedance spectroscopy under stationary conditions. Measurements were made as function of potential and steady-state currents, capacitances and resistances were extracted from the data. The results show that deposition of gold takes place close to the equilibrium potential irrespective of crystal orientation and morphology of tungsten. Concentration dependences show that the deposition rate is first order with respect to the concentration of gold complex and inversely related to the free cyanide concentration. These results confirm that the reaction sequence includes a chemical dissociation step prior to electron transfer. For partially gold covered tungsten wires, the rate of hydrogen evolution was found to be enhanced compared with a bare tungsten wire and pure gold metal, which is explained by the formation of three-phase junctions where water splitting and electron transfer occur at different sites.