Microstructure and corrosion electrochemical behavior of hot-dipped Zn-Ni-V coating

The hot-dipped Zn-0.05%Ni-0.05%V coating was gained by adding a small amount of Ni and V in the zinc bath. The microstructure of Zn-0.05%Ni-0.05%V coating and corrosion process of the coating in 5%NaCl aqueous solution were investigated by SEM and electrochemical impedance spectroscopy (EIS). The results show that, the addition of 0.05% V to the Zn-0.05%Ni bath can effectively restrain the Fe-Zn reaction and obviously inhibit the growth of ζ phase in the coating. Compared with Zn-0.05%Ni coating, Zn-0.05%Ni-0.05%V coating following immersion for 20 min was of larger electrochemical impedance. The impedance of the Zn-0.05%Ni-0.05%V coating was found to sharply decrease in the first two days of immersion in relation with the break down of the oxide layer. As the sample surface covered with corrosion products with the immersion time up to 8 days, the impedance spectra for the coating consisted of two capacitance loops and the Warburg diffusion impedance at the low frequency. Moreover, the impedance increased gradually with increasing the immersion time. According to the characteristic of EIS in the corrosion process, EIS of the coating at different immersion time was simulated using appropriate equivalent circuit. The results show that, after immersing 8 days, the Y1, the n1 and the diffusion resistance WR decreased, the R1 of corrosion products increased gradually. The double-layer capacitance C2 and the polarization resistance R2 of the corrosion reaction have no substantial regularity with prolonging the immersion time.