Chromium metal is also appreciably more expensive. The chromium to be deposited is introduced into the bath as chromium trioxide, CrO3 (Chromic anhydride), commonly called chromic acid. It is reddish brown, hygroscopic material, easily soluble in water to give solution containing mainly H2Cr2O7. Many manufacturers, now aware of the effect of even small amount of catalyst acid radicals, furnish a pure grade of chromic axid especially suited for chromium plating. The chromic acid is made to meet sprcification which require that it contain not more than a small of percent of sulfate and be free from other catalyst such as chloride.
To convert a pure chromic acid solution to a chromic plating bath a catalyst is necessary. With given set of conditions of bath temperature, current density, and chromic acid concentration, too small amount of catalyst will result in either no plate or in brown anode stains. Too high catalyst contain will result in either partial plating with poor throwing power or no plate at all, owing to depolarizing action or easy formation of chromium (III) at the cathode. The essential criterion of bath composition in chromium plating from the conventional chromic acid-sulfate solution is the ratio, by weigth, of chromic acid to sulfate. The ratio should be kept within the limit of 50:1 and 250:1; and preferebly at about 100:1. A ratio of 90:1 is common; ratio of 70:1 to 80:1 are common in hard chromium baths especially at higher temperatures.
A typical formula for a chromium plating bath using sulfate as the catalyst acid radical is
- chromic acid (CrO3), 250 gr/lt.
- sulfate (SO4 =), 2.5 gr/lt.