Effect of Mg on the solidification structure and growth of the intermetallic layer of a Zn-11% Al alloy coating

Zn-11%Al, Zn-11%Al-1.5%Mg, Zn-11%Al-3%Mg and Zn-11%Al-4.5%Mg alloy coatings were obtained by dipping pure iron in the alloy bath at 510℃ for different immersion time. The effects of Mg content on the solidification structure of the Zn-11% Al coating and the growth of the Fe-AI intermetallic layer in the alloy coating were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. The results show that the solidification microstructure of the Zn-11% Al alloy coating is composed of Al-rich phase and Zn/Al ternary eutectic. With the increase of Mg content in the Zn-11% Al-x% Mg alloy, Zn/Al/MgZn₂ ternary eutectic, blocky MgZn₂ phase and AL/MgZn₂ binary eutectic occur gradually. The intermetallic layers in the Zn-11% Al alloy coatings with different amounts of Mg consist of Fe₂Al₅Zn_x phase and FeAl₃Zn_x phase, and the thickness of the inter-metallic layer increases with the prolongation of immersion time. The increase of Mg content in the hot dip bath reduces the growth-rate index and the growth rate of the intermetallic layer. At the beginning of the hot dip process, a compact and stable Fe-Al alloy layer in the Zn-11% Al alloy coating form on the iron surface. After immersing for 120 s, the Fe-Al intermetallic layer is instable and broken because of diffusion path movement. Mg element in the Zn-11% Al-x% Mg alloy can significantly delay the time of liquid zinc into the Fe-Al alloy layer, which makes the Fe-Al alloy layer be more stable and compact.