Abstract
Chromate technique in CrO3-halogen bath has been investigated on zinc and zinc alloy plated steel sheet. The colorless chromate film is formed stably by electrolysis in the bath that CrO3, Cl- and Co2+ content are controlled. Deposition efficiency of chromate film in CrO3-Co2+-Cl- bath is six times as much as in CrO3-SO42- bath. The amount of chromate film increases almost linearly in response to quantity of electricity on Zn or Zn-Ni plated steel sheet, and thus can be controlled by quantity of electricity. The chromate film formed on zinc plated steel sheet in CrO3-Co2+-Cl- bath dissolves in 5%NaOH aqueous solution a little more easily than that formed in CrO3-SO42- bath, and the film formed on Zn-Ni plated steel sheet in CrO3-Co2+-Cl- bath dissolves more easily in 5%NaOH. However, chromate materials formed in CrO3-Co2+-Cl- bath show very excellent corrosion resistance and fairly low extension velocity of rust. Furthermore, adhesion property of painted film on the chromate materials is excellent. Meanwhile, improvement effect on corrosion resistance of Ni plated EG by chromate film is more excellent than that of EG, and that of Zn-Ni plated steel sheet is the most excellent among them.
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