| Hot-dip galvanizing is a widespread used industrial technology in protecting steel from corrosion. During hot-dip galvanizing,coatings' microstructure affects by the alloying elements in the steel substrate and Zinc bath. Galvanizing the steels in alloyed baths, such as Sn, Ti, Mn and Ce, is one of the effective ways for controlling interface reaction. However, the excess tin in zinc bath can result in the rapid corrosion of zinc-pot. Therefore, it is necessary to understand the interaction between the steel substrate and Zn-Sn bath. For alloying element, there is no universal mechanism of Mn on Fe/Zn interface reaction. The research on the synergy of Ti and Ce on the structure of coatings is limited. This work studied the basic problems occurred during hot-dip galvanizing.The phase equilibria of the Zn-Fe-Sn, Fe-Si-Sn, Zn-Fe-Si-Sn, Zn-Fe-Si-Ti, Zn-Fe-Al-Mn, Zn-Fe-Al-Bi and Al-Fe-Si-Sn systems have been determined using equilibrated alloys approach. The specimens were investigated by means of scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction.The effects of Sn, Ti, Mn and Ce on interface reaction during the hot-dip galvanizing have been studied by hot-dip galvanizing and hot-dip 55%Zn-Al.The 600? and 700? isothermal section of the Zn-Fe-Sn ternary system wereexperimentally determined. No new ternary compound was found in the system. Three three-phase regions exist in the 600? isothermal section. Three three-phase regions have been identified in the Zn-Fe-Sn ternary system at 700?. The FeSn phase can coexist with all phases except for ? phase.Five three-phase regions have been identified in the Fe-Si-Sn ternary system at 450?. No new ternary compound was found in the system. The solubility of Fe in the L phase is no more than 0.7 at.%.The 450 ? isothermal sections of the Zn-Fe-Si-Sn quaternary system with the Zn composition being fixed at 70 and 93 at.% were experimentally determined. Five four-phase regions were identified in the 70 at.% Zn section, whereas no four-phase region was found in 93 at.% Zn section. Liquid phase(Liq.) was found to be in equilibrium with almost all phases in the two sections, including the FeSn, FeSi, FeSi2, ?1, ?, ? and a-Fe phases. Sn solubilities in the ?, FeSi and FeSi2 phases were rather limited. The maximum solubility of Si in the FeSn phase was 0.5 at.%. No quaternary compound was found in this study.The 450? isothermal section of the Zn-Fe-Si-Ti quaternary system with Zn being fixed at 93 at.% was experimentally determined. 14 four-phase regions were experimentally confirmed in this isothermal section. The Fe-Zn-Ti ternary phase T-TiFe2Zn22 was found to be in equilibrium with the liquid, ?-FeZn13, -?2-FeTiSi and TiZn16 phases. The maximum solubility of Zn in ?2-FeTiSi is 2.74 at.%,but less than 1.08 at.% in ?1-FeTiSi2. Si solubility in T-TiFe2Zn22 is 0.31 at.%,but in ?-FeZn,3 is negligible. The solubility of Ti in the liquid phase is limited. The results of present work are consistent with the relevant ternary systems. No true quaternary compound was found in the isothermal section.The 450? isothermal section of the Zn-Fe-Al-Mn quaternary system with Zn being fixed at 93 at.% was experimentally determined. Four four-phase regions were experimentally confirmed in this isothermal section. The Liquid and Fe2Al5 phase are in equilibrium with T,FeAl3,Al11Mn4 and MnZn9. No true quaternary compound was found in the isothermal section.Isothermal sections of the Zn-Fe-Al-Bi quaternary system at 600? with A1 fixed at 75 at.% and Zn fixed at 50 at.% have been determined. One four-phase region and four four-phase regions have been identified in the 75 at.% Al and 50 at.% Zn sections,respectively. The Bi-poor L1 phase and Bi-rich L2 phase are both in equilibrium with(Al), FeAl3, Fe2Als, a-Fe and 8, respectively. No new quaternary phase is found in these two sections. Bi is almost insoluble in all Fe-Al and Zn-Fe compounds.Two four-phase regions and three three-phase regions were determined in the 650? isothermal section of Al-Fe-Si-Sn quaternary system. It was found that liquid phase is in equilibrium with almost all phases in the section.The interface reactions between the Silicon-containing steels and different baths containing Sn or Sn&Ti have been studied. An appropriate amount of Sn in Zinc bath led to a columnar ? phase, and avoided the formation of the diffused-? phase and outbreak microstructure. Adding l.Owt% Sn or 0.8wt%Sn+0.15Ti wt% to zinc bath can effectively restrain Fe/Zn interface reaction of Q235 steel.The effect of Mn in steel and Zn bath on the structure of coatings were systematically studied. By hot-dip galvanizing pure iron and Mn-containing steel, the growth kinetics of the coating were analyzed. Mn has no effect on the growth kinetics but change the thickness of coating. This is due to the fact that the Mn solubility in the? is up to 5wt%?The interface reaction process between the iron-base and different baths containing Sn was studied. The controlled mechanism of alloy layer formation based on the diffusion path change in galvanizing was proposed. In the beginning of the alloy layer formation in galvanizing (diffusion layer), the diffusion path crosses the two-phase region of melting bath and ? compound, the path follows a tie-line. A corresponding continuous lamellar Zn-Fe compound forms firstly on the iron-based surface. With the content of Sn increases, the diffusion path trends to move gradually to the line connecting with two diffusion component points. Once the diffusion path cuts the tie-line of the two-phase region of melting bath and the ? compound, the ?compound layer is continuous losing and breaking. At the same time,the liquid channels form in the alloy layer (diffusion layer) as the diffusion path cut the tie-line,which make Zn-Sn liquid directly connect with iron-base. The reaction is controlled by interfacial reaction, which leads to the rapid increase of the thickness of coating alloy layer.Ce and Ti are effective bath additive for controlling the Fe-Al reactivity in galvalume process. Adding less than 0.2 wt% Ti to Zn-Al bath can effectively restrain Fe/Al reaction. The synergy of Ce and Ti on the structure of coatings were studied systematically by hot-dip Zn-Al bath. The best effect is achieved when Ce&Ti content reaches 0.4wt%. |
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