Cadmium Plating

Plating Baths. Cadmium is primarily plated from a cyanide electrolyte. Acid cadmium baths are also used to a limited extent, and are fluoborate, sulfate, or chloride in nature. The acid-type baths are more desirable if hydrogen embrittlement is a problem, and their waste treatment is simplified. However, the cyanide baths are easier to control than the acid baths. Typical cadmium plating compositions are described in the article "Cadmium Plating" in this Volume.

Applications and Properties. Cadmium provides galvanic protection when deposited on steel. The government is by far the largest specifier of cadmium, for military applications. As shown in Table 28, deposit thicknesses range from 5 to 25 pm (0.2 to 1 mil), depending on the degree of exposure to corrosion, and wear. As with zinc electroplates, chromate conversion coatings improve the corrosion resistance of cadmium electrodeposits. Cadmium is generally preferred for the protection of steel in marine environments whereas zinc is preferred in industrial environments. The coefficient of friction of cadmium is less than that of zinc; therefore, cadmium is preferred for fastening hardware and connectors that have to be taken on and off repeatedly. Typical applications include springs, lock washers, fasteners, electronic and electrical parts, washing machine parts, and military hardware. Because it is toxic, cadmium plating should never be used on parts that will come in contact with food or beverages.

Avoiding Hydrogen Embrittlement. Cadmium deposited from a cyanide solution is more likely to produce hydrogen embrittlement than any other commonly plated metal. Heat treated high-strength steels, particularly those plated and used at 35 HRC and above, are susceptible to hydrogen embrittlement.

Although the thickness of the plated deposit appears to have no direct bearing on hydrogen embrittlement, it is always more difficult to release the hydrogen (by baking) from heavy deposits. By adhering to the following procedures, hydrogen embrittlement can be minimized:

• Use mechanical cleaning methods, such as brushing, blasting, and tumbling

• Wherever possible, avoid the use of strong acid pickling solutions and extended exposure to acid pickling

• If pickling is essential to the preparation of medium-strength and high-strength steel parts, bake the parts at 175 to 205 °C (350 to 400 °F) for 3 h after pickling and before plating

• In plating, use the higher current densities to produce a more porous deposit; 7 A/dm2 (70 A/ft2) in a cyanide bath without brighteners has been satisfactory for steel at 46 HRC

• After plating, bake parts at 175 to 205 °C (350 to 400 °F) for 3 to 24 h. The shorter baking periods are generally adequate for parts with a tensile strength below about 1520 MPa (220 ksi); longer baking periods are recommended for steel of tensile strength above about 1520 MPa (220 ksi) or lower strength parts if sharp notches or threads exist. Parts greater than 25 mm (1 in.) thickness should also be baked for 24 h. The elapsed time between plating and baking must never exceed 8 h, and baking should be carried out as soon as possible, preferably within 4 h

• Plate parts to a thickness of about 5 pm (0.2 mil), bake for 3 h at 195 °C (385 °F), activate in cyanide, and then complete the plating to required final thickness

0 0

Post a comment