Throwing power in chromium plating

Abstract

During recent years chromium plating has come into extensive use in spite of the fact that it is very difficult to deposit chromium in recesses of irregularly shaped articles. General principles show that in the chromic acid baths used for plating there is little hope of radically improving the "throwing power." The purpose of this investigation was to define those operating conditions which yield the highest throwing power, which at best is poor. The ratio of the weights of metal deposited on two cathodes, one of which is twice as far from a gauze anode as the other, gives a quantitative measure of throwing power. If, under these conditions, as is invariably true in chromium plating, this metal ratio is greater than 2:1, the throwing power is negative. The best throwing power obtained was-13 per cent. Under less favorable conditions it was-100 per cent or still poorer. The conditions found to yield the best throwing power are (a) a high temperature such as 55° C. (131° F.); (b) a high current density such as 35 amp. /dm. 2 (325 amp. /ft. 2) ; (c) a low concentration of chromic acid such as 150 to 250 g/L (20 to 33 oz./gal.); and (d) a low sulphate content, for example, or. 3 =20Q. These conditions usually require a potential of over 6 volts. If this is not available, fair throwing power can be obtained in a more concentrated solution with a lower temperature and current density.