Formation of periodic layered structure during hot-dip galvanizing in Al-Zn-Mg bath

This study investigated the effect of Al and Mg content and immersion time on the formation of periodic layered structure during hot-dip galvanizing in Al-Zn-Mg bath. The steel substrates were immersed in Al-Zn bath for 10 s to 120 s, with five levels of Al content in the range of 15 to 55 wt.%, and four levels of Mg additions, in the range of 0.5 to 3.0 wt.%. The microstructure and constituent phases of the periodic layered structure were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The coating with periodic layered structure contains a thin light gray Fe2Al5Znx layer near substrate and outer periodic layered structure. The periodic layered structure consists of alternating light gray layer (FeAl3Znx) and black gray layer (Al dendrites and Zn-rich interdendritic regions). The formation of periodic layered structure requires an incubation period t1, while if the immersion time is over t2, the periodic layered structure is destroyed. In addition, t1 and t2 varies with the Al content and Mg addition.