亜鉛系めっき鋼板におけるCrO₃-Co²⁺-Cl^-系浴による電解型クロメート皮膜の構造と特性

抄録

Relation between the structures of chromate films, formed by electrolytic reduction in CrO₃-Co²⁺-Cl^- and CrO₃-SO₄^2- bath on Zn and Zn-Ni alloy plated steel, and characteristics has been investigated. The structure of chromate film formed in CrO₃-Co²⁺-Cl^- bath is amorphous and a little Cl is uniformly distributed in the film.
The film is mainly composed of Cr(OH)₂, Cr₂O₃, a lot of Cr⁶⁺, and only a little Zn²⁺, not containing Cr. When the film is formed, Zn²⁺ dissolution rate in chromate bath is affected by Zn and Zn-Ni alloy plated steel sheet, and then dissolved Zn²⁺reacts with Cr⁶⁺in the film to reduce Cr⁶⁺and simultaneously produce Cr³⁺ in the film. Co²⁺ CrO₃-Co²⁺-Cl^-bath suppresses the dissolution of Zn²⁺, and so component of the film formed in the bath is stabilized. Furthermore, materials chromated in the bath show excellent corrosion resistance, because much Cr⁶⁺ 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 Zn²⁺ dissolved from materials. In the case of chromating in CrO₃-Co²⁺-Cl^-bath, Zn²⁺hardly dissolves in chromarte film.