Abstract
Distribution of chromium complexes in various chromium baths (pH 2-4) was examined by zone electrophoresis in the viewpoint of their charges and studied in relation to bright chromium plating. To obtain bright chromium deposits, charge number q (per one chromium atom) of cationic chromium (III) complexes is affected by the presence of Cr2O72- and [Cr (III) complex] 1-. In the presence of Cr2O72-, 0<q≤1 is necessary. In the presence of both Cr2O7 2- and [Cr (III) complex]1-, q is allowed to cover a wide range, whereas in the absence of both anions, 0<q<1 is necessary. The charge number q generally decreases as the increase of pH and (NH4)2SO4 concentration of the bath. The reason why chromium deposition can occur in both Cr2O72--NH4+ and [Cr(C2O4)3]3--NH4+ baths is deduced to the formation of cationic polynuclear ammine complex with a low charge number in the cathode film.
