Enhanced Interfacial Reaction of Fe-Si Alloy Sheets Hot-Dipped in Zn Melt at 460°C

In order to understand the effect of solute Si (in the steel sheet) on the interfacial reaction between liquid Zn and solid Fe (alpha-Fe phase) during the hot-dip galvanizing process, a change in the interfacial microstructure between Zn coating and Fe substrate in Fe-Si alloy sheets hot-dipped in Zn melt with dipping time at 460 degrees C was examined. In pure Fe sheet, the Fe-Zn intermetallic layers form at the interface between solid Fe and liquid Zn at an early stage of dipping and subsequently grow to approximately 60 mu m in thickness after 600 s. In Fe-1Si (wt%) alloy, the thickness of Zn coating substantially increases to beyond 500 mu m after 600 s and the coarse zeta-FeZn13 phase with several facet planes was often observed in the Zn coating. The thickness of the Fe-1Si alloy sheets continuously decreases till 60 s and then is reduced significantly after 600 s. The thickness loss in the later stage of dipping is more significant in the Fe-Si alloy with higher Si content. These results indicate a significant Fe dissolution into liquid Zn could occur at the later stage of dipping the Fe-1Si alloy in Zn melt, which is distinguished from the interfacial reaction between pure Fe and liquid Zn. The enhanced interfacial reaction would be responsible for the decomposition of the initially formed zeta-FeZn13 phase layer to liquid and FeSi phases, which can be proposed based on thermodynamic calculations of the Fe-Zn-Si ternary system.