Further Insight into Interfacial Interactions in Iron/Liquid Zn-Al System

Abstract

In the galvannealing process, steel strips are immersed in molten zinc containing 0.100 to 0.135 wt pct Al at 450 °C. The coating obtained is composed of a thin intermetallic compounds’ layer called the inhibition layer (200 nm) covered with a thick zinc layer (10 µm). The nature of this inhibition layer has been investigated here for a galvanizing bath with a low Al content. The inhibition layer formed on industrial low-alloyed steels was characterized by transmission electron microscopy and atom probe tomography. The inhibition layer is composed of a thin Fe2Al5Znx layer (20 nm), covered with a thicker δ layer (200 nm). The Fe2Al5Znx layer is discontinuous at the lowest bath Al content. Small precipitates (20 nm in diameter) with a stoichiometry corresponding to Fe3Al-containing Zn were also found for the first time in the δ phase. The microstructure of the inhibition layer can be described with diffusion paths drawn in the Al-Fe-Zn ternary section at 450 °C. This means that all interfaces of the inhibition layer are at thermodynamic equilibrium. The Fe2Al5Znx layer is formed on the steel surface before the δ layer. The nucleation and growth of the Fe3Al-Zn particles probably occur in the liquid metal at the same time as δ.