亜鉛系めっき鋼板における CrO_3-Co^<2+>-Cl^- 系浴による電解型クロメート皮膜の構造と特性

概要

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Relation between the structures of chromate films, formed by electrolytic reduction in CrO_3-Co^<2+>-Cl^- and CrO_3-SO_4^<2-> bath on Zn and Zn-Ni alloy plated steel, and characteristics has been investigated. The structure of chromate film formed in CrO_3-Co^<2+>-Cl^- bath is amorphous and a little Cl is uniformly distributed in the film. The film is mainly composed of Cr(OH)_2, Cr_2O_3, a lot of Cr^<6+>, and only a little Zn^<2+>, not containing Cr. When the film is formed, Zn^<2+> dissolution rate in chromate bath is affected by Zn and Zn-Ni alloy plated steel sheet, and then dissolved Zn^<2+> reacts with Cr^<6+> in the film to reduce Cr^<6+> and simultaneously produce Cr^<3+> in the film. Co^<2+> in CrO_3-Co^<2+>-Cl^- bath suppresses the dissolution of Zn^<2+>, and so component of the film formed in the bath is stabilized. Furthermore, materials chromated in the bath show excellent corrosion resistance, because much Cr^<6+> in the film repairs defects of surface. Cr(OH)_3 in outer layer of film affects paint adhesion. The more Cr(OH)_3 is, the better paint adhesion becomes. It is necessary that chromated materials secure stabilized properties as the results of controling Zn^<2+> dissolved from materials. In the case of chromating in CrO_3-Co^<2+>-Cl^- bath, Zn^<2+> hardly dissolves in chromarte film.
1992-03-01