Effect of Mg on Fe-Al interface structure of hot-dip galvanized Zn-Al-Mg alloy coatings

Zn-5 wt% Al-x Mg (x = 0, 1, 2, 2.5, 3, and 5 wt%) alloy coatings were prepared by the flux process. The morphology, phases, and microstructures of the interface reaction layer of the alloy coatings prepared at different Mg contents were analyzed by field emission-scanning electron microscopy, X-ray diffraction, and transmission electron microscopy studies. The results show that the intermetallic layers formed on the steel plate surfaces in the Zn-Al-Mg bath consist of Fe4Al13 and Fe2Al5Zn0.4 phases. With increase in the Mg content, the thickness of the Fe-Al interface reaction layer decreased. Magnesium dissolved in the crystal lattice of the Fe-Al intermetallic compounds, resulting in a change in the lattice constants of the Fe-Al intermetallic compounds. This leads to lattice distortion and inhibition of the Fe2Al5Zn0.4 phase growth. In addition, the grain size decreased, and the interface between the coating and the steel matrix tends to be straight with increase in the Mg content. The Mg element segregates at the grain boundaries of the Fe-Al intermetallic, resulting in grain refinement.