Hull Cell Studies of Various 24 Carat Gold Electrodeposits

Summary

Panels plated in a Hull cell with 24 carat gold deposits from phosphate, phosphate-citrate, phosphonate-citrale and sulphite electrolytes have been examined by electron probe microanalysis and X-ray diffraction techniques. Deposits from the arsenic brightened sulphite bath arc preferentially (220) oriented at low current densities, becoming approximately random as current density is increased. This bath appears to deposit auto- catalytically at current densities greater than about 4 mA/cm². The remainder of the deposits exhibit preferential (111) orientation, with increasing (311) contribution as thickness is increased. Addition of small amounts of cobalt to these latter electrolytes reduces the current efficiency and retards the development of the (311) crystal face. The resulting deposits are of fine crystallite size, highly (111) oriented, and subject to residual stress. Aspects of the mechanism of brightening by codeposited cobalt are discussed. Addition of a proprietary organic brightener to phosphonate-citrate baths produces very highly (200) oriented deposits. Deposits of essentially pure (200) orientation have been prepared in this manner. The crystallite orientation exerts a profound effect on both the covering power and the rate of pore closure in gold electrodeposits. Maximum covering power is achieved by deposits which are highly (111) oriented and of minimum grain size, while the rate of pore closure with increasing deposit thickness is greatest in deposits of highest index crystallite orientation.