The capacitance of the double layer at the cathode in a Watts-type nickel-plating solution, containing prhnary brighteners and secondary brighteners (levelling agents), was measured on a rotating disk electrode using a potential-decay method. Both types of additive increase cathodic polarization and reduce difIerential capacitance. The effects of primary brighteners @-toluenesulphonamide, saccharin) do not depend on the rate of rotation, whereas those of secondary brighteners (coumarin, thiourea) are more pronounced at greater rotation rates, due to diffusion control of their adsorption on to the cathode during the electrodeposition process. R&mn~Mesure sur une Electrode B disque toumant de la capacitanoe de lacouchedoublecathodique dans une solution de d&position de nickel de type Watts, contenant des brillanteurs primaires et secondaims (agents nivelants), au moyen d'une m&hode de perte de potentiel. Ces deux types d'additifs augmentent la pohuisation cathodique et reduisent la capacitance diiferentielle. Les effets des brillanteurs primaims @-tolu&+tlfonamide, saccharine) ne dependent pas de la vitesse de rotation, tandis que ceux des brillanteurs secondaims (coumarine, thiouree) sont plus prononc6s aux vitesses de rotation plus 6levceS. Ceci est dQ au fait que leur adsorption sur la cathode, durant le processus d'&ctrodeposition, est contr816e par la diffusion. ZlMllUUd assung-An einer rotierenden Scheibenkathode in einem Nickelbad vom Watts-Typ wurde die Doppelscbichtkapazitit in Gegenwart von primtien und sekundiiren (einebnenden) Glanzzus&zen mit Hrlfe einer Alxhaltmethode gemessen. Beide Arten von Zus%tzen erhohcn die kathodische Polarisation und emiedrigen die Differentialkapazitiit. Die Wirkung der prim%ren Glanzzus%tze (pToluolsolfonamid, Sac&a&) hiingt nicht von der Rotationsgeschwindigkeit der Elektrode ab, w&rend diejenige der sekundiiren Glanzzt&tze (Coumarin, Thioharnstoff) bei hoheren Rotationsgeschwindigkeiten in Folge ihrer diffusionskontrollierten Adsorption auf der Kathode w%hrend der elektrolytischen Metallabscheidung ausgepriigter ist.
MODERN nickel plating baths usually contain at least two brighteners, viz a so-called primary brightener and a secondary brightener, the latter often being a levelling agent.l,s All known level&g agents are inhibitors of the electrodeposition process and their inhibiting action is more pronounced on micropeaks of an irregular surface. Some authorss*4 have explained this phenomenon on the basis of the assumption of preferential adsorption of an inhibitor on the higher points of a microprofile. 0thers6*6 do not relate inhibiting action with adsorption phenomena and ascribe the non-uniform distribution of inhibition over the microprofile to the local variations in the thickness of the diffusion layer. This explanation has been confirmed by the fact that the rate of consumption of some levelling agents at the cathode during the electrodeposition is controlled by diffusion.'*s The important role of diffusion in the inhibition of electrodeposition by levelling agents has been shown recently by means of polarization measurements on a rotating disk electrode.s~10
However, no relation has yet been established between the inhibiting action and the adsorption of levelling agents on an irregular surface. In addition, it is not clear to what an extent the assumption of preferential adsorption is correct or incorrect, since the available experimental data on the adsorption of levelling agents * Prepared for the 17th meeting of CITCE, Tokyo, September