Zinc Nickel Plating

Zinc-nickel alloys produce the highest corrosion resistance of electroplated zinc alloys. These alloys contain from 5 to 15% Ni. Corrosion resistance improves with nickel content up to 15 to 18%. Beyond this range the alloy becomes more noble than steel and loses its sacrificial protection property. An alloy containing 10 to 13 wt% Ni is electroplated on steel strip and coil as an alternative to zinc-iron or electrogalvanizing. An advantage of this composition is the formability of the steel after coiling. For components, chromatizing is required; however, best results are achieved on alloys containing 5 to 10% Ni. For alloys in this range of nickel content, corrosion resistance to neutral salt spray reaches 1000 h or more before red rust. An advantage of zinc-nickel alloys is their retention of 60 to 80% of their corrosion resistance after forming and after heat treatment of plated components. This attribute makes these alloys suitable for automotive applications such as fasteners, brake and fuel lines, and other under-the-hood components.

Zinc-nickel alloys plated from alkaline baths have shown potential as substitutes for cadmium coatings in aircraft and military applications. Available chromates are clear, iridescent, bronze, and black. Typical zinc-nickel bath compositions and process parameters are given in Table 2. Alkaline formulations are preferred for their ease of operation and because they provide more uniform alloy composition and better overall corrosion resistance, especially on tubing and on internal configurations of parts.

Table 2 Compositions and process parameters for zinc-nickel plating solutions

Acid baths

Zinc chloride, g/L (oz/gal)

120-130 (16-17)

Nickel chloride, g/L (oz/gal)

110-130 (15-17)

Potassium chloride, g/L (oz/gal)

200-250 (27-33)

Ammonium chloride(a), g/L (oz/gal)

100-200 (13-27)

Organic additives, g/L (oz/gal)(a)

5-10 (0.66-1.3)

pH

5-6

Anodes

Zinc and nickel connected to two separate rectifiers

Alkaline baths

Sodium hydroxide, g/L (oz/gal)

100-130 (13-17)

Zinc, g/L (oz/gal)

6-15 (0.8-2.0)

Nickel, g/L (oz/gal)

0.5-1.5 (0.07-0.20)

Organic additives, g/L (oz/gal)

2-5 (0.25-0.66)

Anodes

Zinc and steel, one rectifier

Temperature, °C (°F)

20-35 (70-95)

(a) Used only in some compositions

(a) Used only in some compositions

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