Abstract
The effects of anions in chromic acid bath have been investigated on the appearance of deposits and current efficiency. Effective anion species in chromium plating were found to have an ability either to penetrate into the cathodic film or to dissolve it. From the values of pH in the vicinity of the cathode and of pK of anions, neutral anions such as SO42- and F- are supposed to penetrate into the cathodic film and acidic (pK<2) anions such as HSO4-, HMF6X-1 and HF2- to dissolve the film. These anions, therefore, play an important role as catalytic anions in chromium plating. However, if the bath does not contain enough amount of these anions or pH value of the bath is low because of the low concentration of CrO3, the neutral anions cannot readily penetrate into the film, while the number of acidic anions decreases. Excess of anions or low pH values of the bath, on the other hand, accelerate unnecessarily the penetration and dissolution actions of the anions. In these two cases, good deposits of chromium cannot be obtained. Empirically known facts on the effective anions in chromium plating can be thus explained in terms of the penetration and dissolution actions of the anions on the cathode.
