Sunday, February 8, 2009

Semi Automatic Plating

Experience has shown that chromium plating can be satisfactorily achieved from chromic acid solutions of any concentration provided than an amount of sulfuric acid (or equivalent sulfate) is present equivalent to about one hundredth part of the of chromic acid concentration. Beyond the limits of about 50:1 and 200:1 the solution just will not work at all. Hydrofluoric acid or flourosilicic acid can be substituted for sulfuric acid, but with no particular advantage.

Chromic acid is in fact difficult to isolate as a single chemical substance, although it undoubtedly exists in aqueous solution. The substance commercially sold as chromic acid is the anhydride (CrO3), i.e. chromic acid less water.

CrO3 + H2O → H2CrO4 ←→ 2 H+ + CrO4-

The unexpected type of composition is not the only unusual feature of chromium plating. Unless the current density exceeds a substantial figure (60 to 80 A/ft2), no electroplate whatever is obtained. Even above this minimum the electroplating process is grossly inefficient, i.e. only a mino, proportion of the current density is employed usefully in depositing chromium; the remainder uselessly generates hydrogen gas. Thus very high current densities need to be used and even then the rate of growth of the deposit is very slow, at 200 A/ft2 it is only 0,0005 in/h. Furthermore the appearance and mechanical properties of the electroplate are very sharply and critically dependent on the temperature of the solution. Below the temperature 45oC the deposit is dark grey, rather hard and brittle, and is considered industrially useless, between 45 and 55oC it is lustrous and bright, glass-hard, intensely brittle and microscopically cracked; above 55oC it is milky lustrous and unsatisfactory. Finally the solution cannot be operated satisfactorily with chromium metal anodes, even if sufficiently pure metal were available, which it is not. The bath has therefore to be run with ‘insoluble’ anodes of lead or of lead-antimony alloy. These soon become coated with an electrically conducting film of lead peroxide, at which the current generates oxygen gas. The oxidizing conditions at the anode maintain the chromic acid in the oxidized state. The mechanism of chromium plating will not be discussed here, except to say that it is exceedingly complex and to some extent not yet fully elucidated.

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