Decorative gold coatings are deposited mostly from alkaline cyanide solutions containing, in addition to gold, also copper and silver salts. The color of the deposits is thus controlled and varied by the ratio of the three metals. However, these coatings become dendritic as the thickness surpasses 1 µ (1 micron). In deposition from acidic solutions higher thicknesses can be achieved while still maintaining good morphological properties of the deposits. However, the composition and therefore the color of these deposits cannot be varied due to insolubility of silver and copper at lower pH. It is therefore of practical interest to compare other properties of these deposits which are pertinent to decorative applications. Properties such as wear resistance, porosity and composition, were studied. It was found that the advantage of flexibility in coating color, when using alkaline solutions containing alloying elements, is accompanied by higher porosity and lower wear resistance of the coatings. Pulse plating is advocated for gold and silver electrodeposition. Reports in the literature vary as to the degree of enhancement of deposit properties. Therefore, a comparative study of properties of gold electroplates obtained by pulsed current versus direct current was made on thin deposits obtained from alkaline and neutral solutions. No significant differences were found in wear resistance, hardness and contact resistance. However, a significant reduction in thickness variation was obtained by pulse plating. Silver deposits obtained by pulse plating were studied. It was found that in relatively thick deposits, pulse plating is advantageous. Higher plating rates and lower porosity are obtained as compared to the characteristics of direct current electroplates.

Author: L. Gal-Or