Zinc Cobalt Plating

Zinc-cobalt coatings contain 0.6 to 2% Co. Zinc-cobalt alloys find extensive use for relatively inexpensive components in applications that require improved abrasion resistance and corrosion protection. Typically, an 8 pm film with 1% cobalt will last up to 500 h in a neutral salt spray test before red rust if the proper chromate is applied. Some reduction in corrosion resistance is experienced after exposure to high temperature, but not as much as with zinc-iron alloys. A unique attribute of zinc-cobalt is its corrosion resistance to sulfur dioxide in accelerated corrosion tests. This suggests that these coatings may be suitable for use in sulfur-containing corrosive environments.

There are two types of zinc-cobalt plating baths; acid and alkaline. Alkaline baths are preferred for tubes and other configurations with internal unplated areas. Exposure to acidic electrolyte reduces the corrosion resistance of such parts. Available chromates include clear, yellow, iridescent and black.

Typical zinc-cobalt bath compositions and process parameters are given in Table 1. Table 1 Compositions and process parameters for zinc-cobalt plating solutions

Constituent or parameter

Amount or value

Acid baths

Zinc chloride, g/L (oz/gal)

80-90 (10-12)

Potassium chloride, g/L (oz/gal)

150-200 (20-27)

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

50-70 (7-9)

Boric acid(a), g/L (oz/gal)

20-30 (3-4)

Cobalt chloride, g/L (oz/gal)

1-20 (0.1-2.7)

Organic additive, g/L (oz/gal)

5-20 (0.66-2.7)



Temperature, °C (°F)

20-40 (70-100)



Alkaline baths

Zinc oxide, g/L (oz/gal)

10-20 (1.3-2.7)

Sodium hydroxide, g/L (oz/gal)

80-150 (10-20)

Cobalt salt complex, g/L (oz/gal)

1.0-2.0 (0.1-0.3)

Organic additive, g/L (oz/gal)

5-10 (0.66-1.3)

Temperature, °C (°F)

25-40 (77-100)



(a) Used only in some compositions

(a) Used only in some compositions

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