An Investigation of the Mechanism of Levelling in Electrodeposition

Synopsis

Levelling power, defined as a function of the deposit thickness obtained at the peaks and recesses of a standard rough surface, has been measured in nickel plating solutions. The effects on levelling power and cathode potential of variations in current density and in the concentration of a variety of addition agents have been examined. Cathode efficiency has been determined and the distribution of incorporated addition agents investigated under selected levelling conditions.

The results support the view that levelling is due to variations in cathode potential between peaks and recesses of a rough surface brought about by differences in the rate of diffusion of addition agent to these points once a diffusion layer has been established. An equation has been derived permitting the calculation of levelling power in terms of current density, addition agent concentration and the rates of change of cathode potential with these two factors. Plots of calculated levelling power against concentration for coumarin, thiourea, and saccharin show many points of resemblance to the corresponding experimental curves.

Some conclusions of practical importance are listed.