Operating Condition of Electroplating Process
Bright Plate can be reliably obtained by properly coordinating temperature and current density within definite limits, taking into account the chromic acid concentration of the bath. The operating condition for bright plate should be on the temperature range of 25– - 55 C degree with current density range between 5 -– 25 A/dm2 for solution containing 250 g/l CrO3. For example to produce a bright plate deposit from a solution containing 250 g/l chromic acid and 2.5 g/l sulfate at a temperature of 40 oC, use cathode with current density between 3.1 and 15.5 A/dm2 but at 45 oC use current density about 50 % higher.
Plating at higher temperature and current densities has been proposed for hard chromium plating and porous chromium plating. This idea has been applied successfully to plate a small articles, but the power required for plating larger objects may be prohibited or unavoidable.
Chromium plating speed was vary depend on the chromium concentration on the bath. Billfinger suggests the use of higher temperature and current densities, combined with lower ratios of chromic acid to sulfate, to obtain higher plating speeds for hard chromium plating.
Throwing Power
Both the throwing power and the covering power of chromium plating bath are poor compare with copper or nickel plating bath. The conductivity is high but reduced by such impurities as iron and copper. The cathode polarization during chromium deposition is relatively constant and substantially the same as that obtained during electrolysis of the same concentration of pure chromic acid without chromium deposition. The major variable connected with throwing power in bright chromium plating are the current efficiency and the bright plating range. If a given set of condition gives the widest possible bright plate range and the plating is done at an average cathode current density near the upper limit of current density for this bright plate range, the optimum throwing power will be attained.
High Throw Bath
High-ratio baths containing relatively high concentration of organic acids have been developed which exhibit remarkable throwing power or covering power over bright nickel, almost as good as that of bright nickel bath themselves. These solution operate at lower current efficiencies to produce thin bright plates of good decorative value. It is a characteristic that the coverage increases with the time and the plate continuous to spread into the low-current-density area with longer plating. Thus plating times as long as 5 to 10 minutes are recommended instead of the 2 to 3 minutes often used for decorative plating.
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