Nickel is one of the most important metals applied by
electrodeposition. The plate is used principally as a bright coating underneath a much thinner chromium electroplate to provide a highly lustrous and corrosion-protective finish for articles of steel, brass, zinc die castings, chemically metalized plastics, and, to a much smaller extent, for coatings on aluminum and magnesium alloys. The protection of the underlying metal depends primarily on the nickel plate, with the thin chromium overlay conferring a permanently nontarnishing, hard, wear-resistant surface. To a far lesser extent, and only for mild exposures, thin gold or brass electroplate with a clear lacquer finish is used as a
decorative coating on thin
bright nickel deposits. Nickel coating alone are also used industrially to afford corrosion protection to prevent contamination of a product. Because of favorable mechanical properties, nickel electrodeposits are used for electroforming of printing plates, phonographs record stumpers’, foil, tubes, screens, and many other articles.
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 (NiSO
4.6H
2O) 240-340 g/l, Nickel Chloride (NiCl
2.6H
2O) 30-60 g/l, Boric Acid 30-40 g/l.
The cathode efficiencies for nickel deposition is high in the Watts Bath; nevertheless sufficient hydrogen ions are discharged that pitted plate result from slowly forming hydrogen gas bubbles clinging to the growing cathode deposit. To prevent pitted deposits, certain anionic wetting agents which do not appreciably affect the ductility or stress of the plate are used. Before the advent of bright plating using organic additives to produce bright deposits, hydrogen peroxide was used in the plain baths to prevent pitting, and is still used in some cases. When it is carefully used dull nickel plate of very high ductility is obtained. It function by depolarization of the cathode with respect to hydrogen evolution, and by oxidation of traces of organic contaminant in the bath.
Other factors also influence the nature of the cathode film and thereby the quality of the deposit. They are current density, temperature, pH, and degree of agitation or relative motion of cathode and solution. These are all interrelated; if a variable such as current density is altered considerably without the other factors being altered of the same time; adverse effects on the deposit may be noted. For example, typical modern Watt bath can be operated at 55
oC and pH 2.0 to give sound, ductile, grey deposits at 6 A/dm
2 with a moderate agitation. If the current density is lowered to 0.2 A/dm
2 without reduction of the temperature or increase of the pH, bright, brittle deposits will be obtained.