| Hot-dip galvanizing is an effective and economic method widely used for protecting steel and iron substrates from atmospheric corrosion. However, galvanizing Si-containing steels is a technical challenge for researchers all the time. Dull grey coatings with abnormally thickness will be formed when Si-containing steel is hot-dip galvanized in pure zinc bath. They are brittle and poorly adherent to the steel substrates. It's found that the addition of Ni into zinc bath can control the Si reactivity and improve the quality of coatings. Environmental and healthy concerns are the main reasons that the lead-bearing baths are under way to be phased out of use and Bi is an alternative of Pb used to improve Zinc bath fluidity and reduce Zinc consumption, and the'quality-cost'ratio improved. However, there remain exist disputes about synergistic effect of Ni and Bi on the microstructure of galvanized coating. The information of phase equilibria and the thermodynamic describing of the Zn-Bi-Fe-Ni quaternary system are very important to understand the combined effect of Ni and Bi in the hot-dip. In present work, phase equilibria of Zn-Bi-Ni ternary system and Zn-Bi-Fe ternary system were determined experimentally and thus a reliable basis for fine-tuned thermodynamic model of phase relations in the Zn-Bi-Fe-Ni quaternary system can be presented. Then, a method of obtaining high quality Zinc alloy coatings can be found.In present work, phase equilibria of the Zn-Bi-Ni ternary system at 600°C and 750°C were determined by equilibrated alloys, using scanning electron microscopy coupled with energy dispersive x-ray spectroscopy (SEM-EDS) and x-ray diffraction (XRD). The results indicate that there are three three-phase regions in the isothermal sections at each temperature, namelyγ+β1+L,β1+α-Ni+L and NiBi+α-Ni+L exist in the section at 600°C,β+γ+L,β+β1+L andβ1+α-Ni+L exist in the section at 750°C. The phases of Ni-Zn binary system are all in equilibrium with the liquid phase in these two isothermal sections. Bi is almost insoluble in any Ni-Zn compounds andα-Ni; the solid solubility of Zn in the NiBi phase is also limited. Because of the miscibility gap in Bi-Zn binary system, in these two sections, there is a phenomenon that the L phase inγ+L two-phase region transforms to the L(Zn) and L(Bi) phases. From the isothermal sections of Zn-Bi-Ni ternary system at 750°C, 600°C and 450°C, it can be seen that the equilibrium between the liquid phase andα-Ni is changed to NiBi andβ1 coexistence state as temperature decreased. From this information, it can deduce that there is a four-phase equilibrated reaction between 450°C and 600°C, namelyβ1+NiBi?L+α-Ni. The DTA results indicate that the accurate reaction temperature is 537.3°C.Additionally, the isothermal section of the Zn-Bi-Fe ternary system at 700°C is determined as well. It's simple and there is only one three-phase region L+Γ+α-Fe, 2 two-phase regions.
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