Investigation On Controlling Of Silicon Reactivity By Bath Additives And Their Related Phase Equilibria

Hot dip galvanized coatings are widely used in industry for corrosion protection of steels. Although it has long history, galvanizing Si-containing steels still is a technical challenge for batch galvanizers. Galvanizing the steels in alloyed baths is one of the effective ways to control of silicon reactivity.The present work systematically investigated the effect of one element and contemporary use of two or three elements on galvanizing three kinds of Si-containing steels: Sandelin steel, Q235 steel and Q345 steel. Effect of single element of V, Ti, Ni, Zr or Mo, and synergistic effect of Ni+V, Co+Ni, Ti+V, Co+Zr, Ni+Ti, Mn+Ti, Ni+V+Mg, Co+Ti+V, Ni+Ti+V or Mn+Ti+V were investigated. Based on these investigations, Zn-0.2 wt.%Ni-0.03 wt.%Ti-0.02 wt.%V and Zn-0.5 wt.%Mn -0.03 wt.%Ti-0.02 wt.%V were proved to be two kinds of zinc alloys with low cost and could effective control of silicon reactivity when galvanizing general structural steels. The effect of vanadium on controlling of silicon reactivity was highlight. Experimental indicated that V additions could adequately suppress the Si reactivity of all three steels. The potency increased with the content of V in the bath. Crystallites of a ternary compound, T, were found at theζ/liquid interfaceWhen the steel galvanized in alloyed baths, the reaction between steel and bath should relate to some high order systems. To investigate the mechanical of element on controlling of Si reactivity and develop the Zn-based database, the present work experimentally investigated the Zn-V binary system, the 450℃isothermal section of Zn-Si-V ternary system, and the 450℃and 600℃isothermal sections of Zn-Fe-V, Zn-Sn-V and Zn-Ti-V ternary systems. The phase relationships in Zn-rich corner of the Zn-Fe-Zr system below 800℃were also experimentally investigated. Three kinds of methods, equilibrated alloy, diffusion couple and differential thermal analysis (DTA), were used. And the samples were examined using optical microscope, scanning electron microscopy coupled with energy/wave dispersive spectroscopy (SEM-EDS/WDS) and X-ray diffraction (XRD).The experimental results suggested that VZn₁₆ was indeed an equilibrium phase below 427℃in the Zn-V system. But it is difficult to form. Two new metastable compounds, VZn₉ and V₃Zn₂, were discovered. It was proposed for the first time thatα-V could contain about 2⁴ at.%Zn. Beside, the system was thermodynamically assessed. A ternary compound T with wide composition range was found in the 450℃isothermal section of the Zn-Fe-V system. But it can't stably exist at 600℃. No ternary compound was observed in the 450℃isothermal section of Zn-Si-V system. While all the Si-V binary intermetallics can equilibrate with liquid phase. In the Zn-Sn-V ternary system, moreover, there exist two ternary compounds. The one with Mo₇Sn₁₂Zn₄₀-type crystal structure with lattice parameter of a=25.4919(?) was stable below 510℃. Another one denoted asÏ„was stable below 501℃. In the Zn-Ti-V system, however, TiZn₃ and VZn₃ formed a continuous solid solution (Ti,V)Zn₃, which was coexist with all phases in the system except Ti₂Zn andα-Ti. The lattice parameter of (Ti,V)Zn₃ increased with the Ti content in the compound. Besides, two ternary compounds, ZrFe₂Zn₂₀ and Zr₂Fe₃Zn₅, were found in the Zn-Fe-Zr system. ZrFe₂Zn₂₀, which has narrow composition range, has CeCr₂Al₂₀-type structure with lattice parameter of a=13.941(?). It undergo congruent melting at 878.6℃and can coexist with all phase in the Zn-rich corner of the system. The experimental results indicated that four Zn-Fe intermetallics can tolerant no Zr but little amount of V. VZn₃ and V₄Zn₅ can tolerant any of Fe, Si or Sn. And Ti also can't dissolve into V₄Zn₅. Furthermore, little amount of Zn was detected in all the binary compounds in the Fe-V, the Si-V and the Ti-V systems.Based on the experimental information of hot dipping Zn-V alloys and the phase diagram of the Zn-Fe-V system at 450℃, a diffusion path model was proposed to interpret the effect of V on galvanizing pure Fe or Si-containing steels. V in the bath introduced a new equilibrium state between the liquid and a ternary phase T and made the equilibrium state between the liquid-ζphases metastable. At the same time, the bonding energy between V and Si is much larger that between Zn and Si. And the ternary phase can tolerant some Si. A normal coating was obtained once the diffusion path through theζphase or theζ+T two phase region. All the ternary compounds in the Zn-Fe-X(X=Ni, Ti, V, Zr) systems can tolerant some Si. The present work also proposed that the crystal structure of the ternary compounds in the Zn-Fe-X(X=Ni, Ti, V, Zr) systems have no contribution to the control of Si reactivity.