THE STRUCTURE AND PROPERTIES OF ELECTROLYTIC CHROMATE FILM FORMED IN CRO3-CO2+-CL- BATH ON ZN AND ZN-NI ALLOY PLATED STEEL SHEET

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+ in 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 tbe bath show excellent corrosion resistance, because much Cr6+ in tbe 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.