Monday, February 25, 2008

Work Process on Nickel Plating

The fundamentals of plating discussed in previous article, together with the chemistry of the Watts bath, are essential to understanding the effects on deposit and the relationship of such variables as bath temperature, nickel concentration, current density, pH and agitation. The Watt bath opened the way for rapid plating of ductile nickel at elevated bath temperature with high cathode and anode efficiencies. The modern Watt formula is more concentrated than the original, and it can be represented reasonable well by Nickel Sulfate (NiSO4.6H2O) 240-340 g/l, Nickel Chloride (NiCl2.6H2O) 30-60 g/l, Boric Acid 30-40 g/l.

Functions of Constituents: The Watt Bath

Nickel Sulfate

Most of the nickel ion content is contributed by nickel sulfate. This salt is used because it is the least expensive salt of nickel with a stable anion that is not reduced at the cathode, oxidized at the anode, or volatized. It is also highly soluble and readily available commercially. The limiting cathode current density for sound nickel deposits is a function of the nickel concentration in the cathode film, which in turn depends on the metal ion concentration of the bath itself. The larger amount of nickel sulfate now used in the Watts bath not only raises the limiting cathode current density but also lowers the resistivity, thus improving plate distribution.

Chloride Ion

A principle function of the chloride ion is to improve anode dissolution by reducing polarization. It also increases the conductivity of the bath and has marked effects at the cathode efficiency, electrolyte conductivity, and slope of the cathode potential curve. The interesting effects of chloride ion in the cathode film have been studied though more needs to be done with brighteners present.

Boric Acid

Boric acid served as a weak buffer in a nickel plating solution. Its principle effect is that of controlling the pH in the cathode film. In the absence of a buffer, nickel deposits at ordinary temperatures tend to be hard, cracked and pitted. Boric acid is obtainable in a very pure and inexpensive form, is nonvolatile and stable, produces whiter deposits, is helpful in its smoothing action on the deposit, and is unique in its cooperative effect with leveling addition agents.