Electrodeposition of Tin-Nickel Alloy Plate from Chloride-Fluoride Electrolytes

Tin and nickel can be codeposited in approximately equi‐atomic proportions from solutions containing stannous chloride, nickel chloride, and fluorides. The alloy deposits thus obtained are almost fully bright and are single phase, having a nickel arsenide type of structure. Below 300°C the deposits are quite stable, but on annealing at higher temperatures recrystallization takes place and a duplex structure of and results. In the absence of fluorides, satisfactory alloy deposition is not possible from chloride solutions. Evidence is adduced that the fluorine complexes both the tin ions and the nickel ions. The constancy of composition of the cathode deposit that is a feature of the recommended electrolyte suggests that deposition is associated with the presence in the electrolyte of a tin‐nickel‐fluorine complex. Complex ion formation is followed through the medium of cathode potential‐current density curves. The effect of the variables of the process on deposit composition is discussed. The conductivity and throwing power of the electrolyte have been determined. The x‐ray structure of tin‐nickel alloy deposits covering a range of composition has been examined, and it is shown that bright deposits are characterized by a preferred orientation of the crystallites. From solutions of this type, tin‐nickel alloys can only be obtained by operating at a temperature above 45°C, the optimum temperature being 65°C. The metastable structure of these deposits is thought to be explained by the high activation energy of the discharging cations resulting from the high temperature of operation. Tin‐nickel alloy deposits as normally obtained contain approximately 65 per cent of tin and 35 per cent of nickel. Such deposits have a pleasing color, showing evidence of a faint rose‐pink tint, and are equally as resistant, or even more resistant, than electrodeposited chromium to atmospheric tarnishing. The progress made to date in the industrial development of the tin‐nickel alloy plating process is briefly reviewed.