| Hot-dip galvanizing is widely used all over the world. As we know, the technique of hot-dip Zn-Ni alloy coating is an effective method for solving the reactive steel problem in hot-dip galvanizing. However, with the development of technology, a growing demand has been applied to hot-dip galvanized quality, and it is involved not only the proper coating thickness, but also excellent corrosion resistance. Besides, the coating is apt to suffer from oxidation corrosion at the high temperature, which requests it to have good high temperature oxidation resistance. Therefore, in this thesis, in order to improve the quality and property of hot-dip galvanized coating, the hot-dip Zn-Ni, Zn-V and Zn-0.05wt%Ni-V coatings were gained by adding a small amount of Ni, V in the zinc bath. The microstructure and corrosion resistance of Zn-Ni, Zn-V and Zn-0.05wt%Ni-V coatings were studied. The corrosion resistance and corrosion process of Zn-0.05wt%Ni-0.05wt%V coating were mainly investigated. The isothermal oxidation behaviors and oxidation mechanism of hot-dipped Zn and Zn-0.05wt%Ni-0.05wt%V coatings at 350℃were analyzed.The study on the microstructure of Zn-Ni, Zn-V and Zn-0.05wt%Ni-V coatings reveals that the thickness ofζphase in the Zn-Ni coating reduces gradually with the increasing of Ni content. The Zn-V coating becomes thicker with the increasing of V content. The addition of more than 0.03wt% V to the Zn-0.05wt%Ni bath can effectively restrain the Fe-Zn reaction and obviously inhibit the growth ofζphase in the coating.The results on the corrosion resistance of Zn-Ni, Zn-V and Zn-0.05wt%Ni-V coatings show that the corrosion resistance Zn-Ni and Zn-V coatings can be improved with the increasing of Ni and V content. Compared with pure Zn coating and Zn-0.05wt%Ni , Zn-0.05wt%Ni-V coatings in 5wt% NaCl solution are of larger electrochemical impedance, lower corrosion current, larger polarization resistance, less spontaneous corrosion tendency and better corrosion resistance. Zn-0.05wt%Ni-0.05wt%V coating shows the best corrosion resistance. To some extent, adding a small amount of Ni and V in the zinc bath can suppress the anodic and cathodic processes of the corrosion of coatings, delay the formation of white rust and red rust, enhance the corrosion resistance of coatings.The research on corrosion electrochemical behavior of Zn-0.05wt%Ni-0.05wt%V coating in 5%NaCl aqueous solution indicates that the impedance of the Zn-0.05wt%Ni-0.05wt%V coating sharply decreases in the first two days of immersion because of the oxide layer breaking down. As the surface of the coating covers with corrosion products with the immersion time up to 8 days, the impedance spectra for the coating consists of two capacitance loops and the Warburg diffusion impedance at the low frequency. Moreover, the impedance increases gradually with increasing the immersion time. According to the characteristic of EIS in the corrosion process, EIS of the coating at different immersion time are simulated using appropriate equivalent circuit. The results show that, after immersing 8 days, the Y1, the n1 and the diffusion resistance W-R decreases, the R1 of corrosion products increases gradually.The study on isothermal oxidation behaviors of hot-dipped Zn and Zn-0.05wt%Ni-0.05wt%V coatings at 350℃reveals that the oxidation scales of coatings are composed of ZnO and FeZn8.87 after oxidized for 150h. The isothermal oxidation kinetic curves of coatings follow parabolic rate law. As a large number of voids form in the pure Zn coating during the oxidation process, the oxidation resistance of the coating become worse. The Zn-0.05wt%Ni-0.05wt%V coating shows better oxidation resistance because of the formation of dense microstructure, doping V2O5 in the ZnO and decreasing defect concentration. |
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