Current Density

Cyanide cadmium baths may be operated over a wide range of cathode current densities, as indicated in Table 1(b). In a properly formulated bath operated within its intended current-density range, the cathode efficiency is 90% ± about 5%. Thus, to apply a 25 ^(1000 ^in.) deposit of cadmium requires 120 A • h/m2 (11 A • h/ft2).

The ranges of current density given in Table 1(b) are suggested limiting values. Choice of current density is governed mainly by the type of work being plated; for example, low current densities are suitable for small lightweight parts, current densities up to 430 A/m2 (40 A/ft2) for medium-weight parts of fairly uniform shape, and high current densities for uniformly shaped heavy parts such as cylinders and shafts.

Baths containing 19 g/L (2.5 oz/gal) of cadmium are suitable for general use at current densities up to 270 A/m2 (25 A/ft2); higher concentrations of cadmium, up to 38 g/L (5 oz/gal), permit operation at higher current density.

A bath containing 19 g/L (2.5 oz/gal) of cadmium is suitable for barrel plating, where average current density may be about 55 A/m2 (25 A/ft2). Such a bath is also suitable for many still tank or automatic plating applications in which current densities do not exceed 270 A/m2 (25 A/ft2). At higher current densities, burning may result, with attendant dull, rough deposits that lack decorative and protective qualities. Where higher current densities are required, baths of higher metal content should be used.

Too low a current density (less than 55 A/m2, or 5 A/ft2) particularly in still tank or automatic plating, can result in excessively long plating times and inferior appearance of deposits.

The recommended range of current densities for plating with a fluoborate bath is 320 to 650 A/m2 (30 to 60 A/ft2). Even near 650 A/m2 (60 A/ft2), however, the bath has poor throwing power.

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