On the mechanism of levelling by addition agents in electrodeposition of metals

Abstract

The levelling of the surface in the electrodeposition of metals occurs when less metal is deposited on the micropeaks than in the microvalleys. This can be effected by means of special additives, which suppress electrodeposition on the peaks to a greater extent than in the valleys and which are incorporated in the cathodic deposit.The effect of the additives, such as coumarin, thiourea, etc, on cathodic polarization and the levelling power of nickel plating solutions have been investigated.The measurements of cathodic polarization and levelling power of the solution under steady conditions of electrolysis on a rotating disk electrode show that the slowing down of the cathodic process is controlled by the diffusion of the additive towards the electrode. The dependences of polarization and of the levelling power on the concentration of the additives, as well as on the stirring of the solution, show that the levelling of the surface is due to a relative difference in the replenishment of the additives at various points of the microprofile of the surface.