The Ni-Zn electroplated steel sheet was studied in order to improve the corrosion-resistance of the Zn electroplated steel sheet.
In a sulfate bath, Ni-Zn alloy was plated on steel sheets under the high current density (20A/dm²). The relationships among Ni content in deposited film, deposited phase, and corrosion resistance were investigated, and the corrosion behavior of the Ni-Zn plated steel sheet was discussed.
The results are summarized as follows:
(1) Under high current density as well as under low current density, γ-phase of Ni-Zn alloy deposits preferentially, and the deposited film is of dual phase consisting of η+γ for low nickel content, or of α+γ for high Ni content.
(2) The film of γ-phase consisting of 10%16% nickel content shows the best corrosion resistance with a good lustrous appearance, and protects the steel well. The film of dual phase of γ+α shows the less corrosion resistance than one of dual phase of γ+η.
(3) It is supposed that good protection of γ-phase film is based on the corrosion potential of this film being less noble than the steel, and changing to more noble site by concentrating of nickel on the corroded surface during the corrosion advancing.