Leveling of Nickel Plating

In addition to produce a lustrous surface, the modern nickel baths used for decorative-protective plating have leveling properties which make possible the filling in, that is, the leveling of polishing scratches and microirregulatorities in the surface of the substrate, and thus reduce or eliminate expensive polishing and buffing operations. The peculiarities of metal plate distribution on microirregulatorities in the substrate were first observed by Meyer in 1935.

Acidic nickel plating baths have a low concentration polarization, that is, the baths have a high concentration of dischargeable ions over microirregulatorities in the cathode surface, and as a result have good microthrowing power and comparatively poor macrothrowing power.

Geometrical leveling from plain nickel baths is relatively unimportant compared to the exceptional microthrow or true leveling which is obtained in nickel baths with the use of certain class II addition agents such as coumarine or trimethyl aconitrate in semibright nickel plating, or with the combination of class I and class II addition agents in high-leveling bright nickel plating baths. Leveling, or smoothing out of surface imperfections with the use of addition agents, was mentioned in the patent literature, but it was Gardam who suggested the correct basis for the mechanism. He suggested that as a result of diffusion, micropeaks receive and have deposited on them above average amounts of addition agents which increase the local resistance and thus reduce the local current density on the peaks with respect to microgrooves.

Leveling depends on the relationship of the mass transport of a high concentration of the leveling agents, most class II addition agents, into the diffusion layer which has a greater thickness directly over a scratch or microgroove compared to the edge of the scratch or the flat portion. It is also necessary for the pH of the nickel bath to be in the range of about 3 to about 5.5 for the best leveling action to take place, and the only buffer that usually can be used for maximum leveling is boric acid, though small concentrations of other buffer may also be present.