Microstructures and concentration profiles formed between solid Fe and liquid Zn-0.2wt%Al alloy at 723K

Microstructure and reactive diffusion between solid Fe and liquid Zn-0.2wt%Al was experimentally examined using Fe/Zn-Al diffusion couples prepared by the immersion technique, where a pure iron sheet was immersed in a molten Zn-Al bath at 723K (450°C) for various times up to r=2.4ks. The microstructures on the cross section in the Fe/Zn-Al diffusion couples were observed by scanning electron microscopy, and the chemical composition of each phase was determined by electron probe microanalysis. While in the initial stage the formation reaction of the Fe-Zn layer is suppressed, the outburst reaction drastically occurs at around 180 s and the Fe-Zn layer grows in proportion to immersion time. It was found that in the early stage before the outburst the interface concentration of Fe in the liquid-Zn (L) phase at the Solid- Fe/L interface is about 3 wt% being extremely higher than the equilibrium value and that the difference from the equilibrium condition drastically decreases by the formation of the Fe-Zn layer. Furthermore, the concentration change of Fe in the Zn-Al melt induces an up-hill diffusion of Al, and some amount of Al is always supplied from the melt to the Fe/L interface. These results seem to be important to understand the formation mechanism of the Fe-Al inhibition layer.