Radiotracer Study of Addition Agent Behaviour: 1—Incorporation of Sulphur and Carbon in Nickel Deposited from Solutions Containing Thiourea

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Abstract
Sulphur is incorporated in nickel deposits from solutions containing thiourea at a rate determined by diffusion. Thus the rate is proportional over a very wide range to thiourea concentration, and virtually independent of current density; it increases with temperature, with an activation energy normal for diffusion processes in solution. The rate of incorporation attains a constant value very soon after beginning plating. Varying pH has only a slight effect on incorporation. Much less carbon than sulphur is included in the deposit, showing that thiourea is decomposed at the cathode. Decomposition also occurs at the anode, probably at a rate determined by diffusion. Changes in the solution probably due to complex formation have no effect on the incorporation of sulphur in nickel deposits, but seem to influence carbon incorporation and polarization—the depolarizing effect of low concentrations of thiourea is absent if time is allowed for complexing to occur. The processes occurring at the cathode are discussed in some detail.

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