Anodes

The anode system for cadmium plating from a cyanide solution consists of ball-shape cadmium anodes in a spiral cage of bare steel (Fig. 1). The spherical shape provides a large surface area in relation to weight, without a large investment in cadmium. Ball anodes also make it possible to maintain an approximately constant anode area, and little or no anode scrap is produced. Cadmium balls are usually 50 mm (2 in.) in diameter and weigh 0.6 kg (14 lb) per ball.

Fig. 1 Unagitated plating tank with spiral steel holders to secure cadmium ball anodes

If a cadmium cyanide solution is to be left idle for an extended period of time (a week or more), the steel anode cages should be removed from the solution, because the galvanic cell set up between the steel and the cadmium anodes will accelerate chemical dissolution of the anodes when the current is off.

When cadmium is plated from an acid solution, such as the fluoborate bath, ball anodes in uncoated steel cages cannot be used, because the steel would dissolve. Rather, bar anodes of elliptical or oval cross section, 460 to 2440 mm (18 to 96 in.) long, are used.

The use of bar anodes in a cyanide solution results in a high percentage of waste, because they must be removed and replaced when the cross-sectional area decreases, or they will dissolve preferentially at the solution level and drop to the bottom of the plating tank.

Purity of the anode is of great importance, especially if a bright deposit is to be produced. The typical composition range for cadmium anodes is as follows:

Element

Composition, %

Cadmium

99.95-99.97

Lead

0.008-0.03

Iron

0.005-0.008

Copper

0.002-0.01

Arsenic

0-0.001

Zinc

0-0.001

Anode composition complying with Federal Specification QQ-A-671 is:

Element(s)

Composition

Cadmium

99.9% min

Silver, lead, tin

0.05% max (total)

Arsenic, antimony, thallium

0.005% max

Insoluble anodes, which are made of low-carbon steel strip or wire, offer no particular advantage except where inside anodes are necessary or for special applications in which they are required because of a need to reduce metal concentration in the plating bath. When insoluble anodes are used, their total area should be 10 to 15% of the total anode area. Insoluble anodes accelerate the formation of carbonate.

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