Similarities Between Cadmium and Zinc Plating

Except for differences in plating baths and in such operational details as current density and rates of deposition, alkaline cadmium and zinc plating are essentially similar processes. See the article "Cadmium Plating" in this Volume for a detailed discussion of plating methods, equipment, and processing. Exceptions with respect to equipment and processing are described below.

Plating Equipment. The equipment requirements for zinc plating are the same as those noted for cadmium plating, except for the following:

• In barrel plating, zinc solutions require higher voltage and current density and therefore must be provided with greater cooling capacity to prevent overheating. Also, because the cyanide zinc bath generates much larger amounts of hydrogen, barrel design should incorporate safety features to prevent explosions.

• Fume hoods should be used on cyanide, low-cyanide, and, especially, alkaline noncyanide baths to exhaust caustic spray and toxic fumes.

• Barrels, tanks, and all superstructures coming into contact with acid chloride zinc plating baths should be coated with material able to resist acid corrosion. Polypropylene, polyethylene, polyvinyl chloride, and fiberglass are commonly used materials. Lead-lined tanks should never be used in these systems. Heating and cooling coils should be built of titanium that is electrically isolated from the tank, or of high-temperature Teflon.

Hydrogen embrittlement of steels is a major problem in all types of cyanide zinc plating. These formulas should not be used for spring tempered parts or other parts susceptible to this type of embrittlement. Spring-tempered parts and other susceptible parts should be plated in acid chloride electrolyte. When no embrittlement whatsoever can be tolerated, mechanically deposited zinc is the preferable alternative.

Processing Steps. Time requirements for various operations involved in still tank, barrel, and automatic methods of plating zinc to a thickness of less than 12.5 (rm (0.5 mil) are given in Table 7.

Table 7 Process steps and time requirements for zinc plating operations

Times listed are for plating zinc to a thickness of less than 12.5 ^m (0.5 mil).

Processing cycle

Time for each operation

Hand- or hoist-operated still tank

Electrolytic cleaning

1-3 min

Cold water rinse

10-20 s

Acid pickle

30 s-2 min

Cold water rinse

10-20 s

Cold water rinse

10-20 s

Zinc plate

6-8 min

Cold water rinse

10-20 s

Cold water rinse

10-20 s

Chromate conversion coat

15-30 s

Cold water rinse

10-20 s

Hot water rinse

20-30 s

Air dry

1 min

Hand- or hoist-operated barrel line

Soak clean

4 min

Electroclean

4 min

Cold water rinse

1-2 min

Acid pickle

2-3 min

Zinc plate

20-30 mm

Cold water rinse

1-2 min

Cold water rinse

1-2 min

Chromate conversion coat

30 s-1 min

Cold water rinse

1-2 min

Hot water rinse

2-3 min

Centrifugal dry

3-5 min

Automatic barrel line

Soak clean

6 min

Electroclean

3 min

Cold water rinse

2 min

Cold water rinse

2 min

Acid pickle

1 min

Neutralize dip

3 min

Cold water rinse

2 min

Zinc plate

30-40 min

Dragout rinse

2 min

Neutralize rinse

2 min

Cold water rinse

2 min

Nitric acid dip

30 s

Cold water rinse

2 min

Chromate dip

30 s

Cold water rinse

2 min

Hot water rinse

2 min

Centrifugal dry

3 min

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