Electrolytic coloring of anodized aluminum has been carried out in a CoSO
4-H
3BO
3 bath. The optimal condition of electrolytic coloring to be duskish (brown-black), which was comparable to the color prepared in a NiSO
4-H
3BO
3 bath, was searched.
The best electrolytic coloring condition for the efficiency of electrolytic deposition was as follows : bath temperature 20°C; pH of the bath 4.0; bath voltage 25V. The black film was successfully prepared in 5 min or longer electrolytic coloring.
pH of the bath decreased with a decrease of Co
2+ content in the bath, which made the reproducibility of the film color worse. However, the reproducibility was regained when agar gels containing hardly soluble cobalt compounds were added to the bath for the pH restoration. The best cobalt compound used for the agar gel was found to be Co (OH)
2 for the control of film color.
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