Thursday, December 13, 2007

Bath Function on Chromium Plating (2)

Continue From Bath Function (1). Although concentrations of chromic acid from about 50 g/l up to saturation (about 1000 g/l) can be used, most commercial baths are operated between 150 and 400 g/l. Still higher concentration give very low current efficiencies. The important requirement is the ratio already mentioned. Usually chromic acid solution will down after use and on the critical concentration chromium metal will bad stick to anode.

Baths containing 250 g/l chromic acid have a slightly higher current efficiency than more concentrated solution. They also have a lower conductivity and therefore require a higher voltage for a given current density. The more dilute bath are also more sensitive to addition or removal of catalyst acid radicals from drag in and drag out. Hence they more concentrated solution are favored for decorative applications and the more dilute baths for heavy hard chromium plating, but there are many exceptions.

Siliconflouride has had considerable use as a catalyst in chromic acid baths since it was first proposed. Such solution was difficult to analyze and maintain.