The Electrodeposition of Bright Tin-Nickel Alloy Plate

SYNOPSISA process for the deposition of a new bright alloy coating of tin and nickel is described. The electrolyte contains stannous chloride, nickel chloride, and fluorides, the preferred composition being stannous chloride, 50 g/1., nickel chloride, 300 g/l., sodium fluoride, 28 g/1., ammonium bifluoride, 35 g/1. At a working temperature of 65° C. and an average cathode current density of 25 amps. per sq. ft., bright plate is obtained over irregular objects at a current efficiency approaching 100%. The composition of the deposit is largely independent of variations in current density and temperature. Variations of 20–30% in the content of either metal in the electrolyte produce insignificant changes in the composition of the alloy deposited. The chief factor controlling the composition of the deposit is the total fluorine content of the electrolyte.A qualitative study of factors affecting the brightness of the deposit has shown that the broadest bright plating range is obtained at high rather than at low temperatures and that the presence of free hydrofluoric acid and of an alkali metal ion (preferably sodium) is necessary for the best results.The replenishment of the bath may be effected either by using alloy anodes or by employing separate tin and nickel anodes supplied at different voltages. With separate anodes precautionary measures are necessary to minimise the effect of chemical reaction between the tin anodes and the electrolyte.The alloy plate is an intermetallic compound and consequently is fairly hard. It has a pleasing appearance and in some lights exhibits a faint purple colour. The plate is extremely resistant to atmospheric tarnishing.