Variations in Plate Thickness

For adequate protection of steel, the thicknesses of cadmium in Table 8 are recommended. The shape of a part can markedly influence uniformity of the electrodeposit. Parts of simple design, such as socket wrenches and bathroom hardware, can be plated with a high degree of uniformity of plate thickness. On such parts, about 90% uniformity would be anticipated.

Table 8 Recommended thicknesses of cadmium

Environmental exposure

Description

Thickness

Uses

^m

^in.

Mild

Exposure to indoor atmospheres with rare condensation. Minimum wear and abrasion

5

200

Springs, lock washers, fasteners

Moderate

Exposure mostly to dry indoor atmospheres. Subject to occasional condensation, wear, or abrasion

8

320

Television and radio chassis, threaded parts, screws, bolts, radio parts instruments

Severe

Exposure to condensation, infrequent wetting by rain, cleaners

13

520

Washing machine parts, military hardware, electronic parts for tropical service

Very severe

Frequent exposure to moisture, saline solutions, and

25

1000

Threaded fasteners present a special problem, because of variations in contour and because of tolerance requirements. These items ordinarily are barrel plated, and thicknesses of 3 to 4 ^m (120 to 160 ^in.) are usually specified.

Throwing Power. The effect of shape on uniformity of deposit thickness is exemplified by the open-ended box (100 mm, or 4 in., cube) of Fig. 5. The open end of the box is pointed toward one of the anodes, to produce the most desirable condition for this shape without auxiliary thief rings, shields, bipolar anodes, insoluble anodes, or other devices. Results of plating such boxes with cadmium, silver, and copper, all deposited from cyanide baths, are shown in Fig. 5. These diagrams illustrate two facts: thickness of plate varies significantly from place to place on the simplest shape; and various plating baths have different throwing powers or abilities to plate uniformly over the surface, regardless of shape.

Plating bath

Thickness ratio(a)

Side

Bottom

Cadmium

1:4.25

1:12

Copper

1:3.0

1:6

Silver

1:2.5

1:5

(a) Ratio of average plate thickness of inside of average plate thickness on outside

Fig. 5 Plate thickness deposited on the cross section of a cube-shape workpiece to show throwing power of cadmium relative to that of silver or copper in a cyanide bath. Open ends of the 100 mm (4 in.) cubes were pointed toward ball anodes during plating.

The data on cyanide baths tabulated in Fig. 5 show that cadmium has appreciably less throwing power than silver or copper. However, cyanide cadmium has greater throwing power than nickel, chromium, iron, cyanide zinc, acid tin, acid cadmium, acid copper, or acid zinc. Normally, metals plated from cyanide or alkaline baths are more uniformly distributed than metals from acid baths. As design becomes more complex, uniform thickness of plate is more difficult to achieve without the use of special conforming anodes.

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