The Effect of Pulse Current Waveform on the Thallium Codeposition Behavior in Gold Electrodeposition

The codeposition behavior of thallium, which acts as a grain refiner in gold electrodeposition, was studied in both DC and pulsed current electrolysis. In jet plating, the maximum current density that produced shiny deposits was the same for both methods, but the codeposition behavior of thallium in gold was quite different. In DC plating, the amount of thallium codeposited in gold increased with greater current density and thallium concentration, but decreased with an increase in the concentration of conductivity salts. In pulse current plating, on the other hand, the amount of thallium codeposited was constant at about 300ppm and independent of these factors. Thus the effect of such pulsed current parameters as pulse duration and waveform were studied in relation to thallium codeposition behavior. It was found that at optimum pulsed current conditions the amount of thallium codeposited in gold was suppressed to about 80ppm. It was also found that periodically reversed current was effective in suppressing the amount codeposited to the same level. The differences in the codeposition behavior of thallium in DC and pulsed current conditions were explained satisfactorily using a diffusion layer model of gold cyanide electrolysis and the grain refining mechanism of thallium proposed by McIntyre and Peck.