Role of Si Addition in Interfacial Reactions of Steel Sheets Hot-dipped in Zn–55%Al Alloy Melt

This study was set to fundamentally understand the effect of Si addition on the interfacial reaction between Zn–55%Al alloy liquid (corresponding to a nominal composition of Al–25Zn (at%)) and Fe solid in the production process of GALVALUME steel sheets. The pure Fe sheets were hot-dipped in Al–25Zn and Al–25Zn–2Si (at%) alloy melts at 600, 650, and 700°C for 2~3600 s. Significantly thick coatings were formed on Fe sheets hot-dipped in the Al–25Zn binary alloy melt for a longer time than 10 s. The coating thickness became several millimeters after 30 s, resulting in a delamination of the coating. The significant Fe dissolution occurred in the Al–Zn binary alloy melt, accompanied by a significant growth of η phase (Fe2Al5) toward the solid Fe. The growth could be promoted by the Zn-rich liquid phase with a lower melting temperature. However, in the case of hot-dipping in the Al–25Zn–2Si ternary alloy melt, uniform coatings were formed on the hot-dipped Fe sheets due to the suppressed interfacial reactions. The Fe dissolution slightly occurred, and a continuous layer of Si-rich T5 (Fe2Al7.4Si) phase was formed at the interface of solid Fe with the Al–25Zn–2Si alloy melt. The continuous T5 phase layer would play a role in a diffusion barrier at the interface of solid Fe with liquid Al–Zn alloy, resulting in the suppressed interfacial reaction. These interfacial reaction processes are discussed based on thermodynamic calculations of the Fe–Al–Zn ternary and Fe–Al–Zn–Si quaternary systems.