Corrosion Resistance of Chromium Plating from Low Concentration CrO3 Bath

Studies were made on the corrosion resistance of chromium plating from low concentration CrO3 bath (CrO3 50g/l, H2SO4 0.5g/l, and Na2SiF6 0.5g/l), and the following results were obtained. The corrosion resistance of chromium plating from a low concentration CrO3 bath was not different from that obtained from a high concentration CrO3 bath; when the latter plating after its further treatment was in the same kind and the same thickness as that of the former plating. When a sample of the former chromium plating was taken out of the bath immediately after plating, a chromate film remained on the surface of sample. This plating with chromate, as such, showed a very high resistance, which was almost in the same degree as that of the latter chromium plating with a chromate film produced by electrolysis of the primary chromium plating from a high concentration CrO3 bath. In general, one of the causes for giving high corrosion resistance of any metal with a chromate film would be due to the following reasons: The potential of that metal in a corrosive solution is several tens or hundreds times mV nobler than its spontaneous potential, and this potential is kept for a long time even though the chromate film is in very small thickness.