Evolution of the Microstructure of Intermetallic Compounds Formed on Mild Steel During Hot Dipping in Molten Aluminum Alloy Baths

Hot-dip aluminum coating technique has been applied to improve the high-temperature oxidation resistance of steels. This method is adopted widely due to the low cost and good performance. The purpose of this paper is to study the formation of intermetallic layers during the hot dipping of mild steel into a molten aluminum bath. Mild steel specimens were immersed into molten aluminum alloy baths at 750 °C for 1 h, 2 h and 3 h. Intermetallic compounds were analyzed by optical microscope, scanning electron microscope coupled with energy-dispersive spectroscopy and X-ray diffraction analysis. This study was completed by microhardness testing. The results showed that the hot-dip aluminized layer was divided into an outer pure aluminum or aluminum–silicon topcoat and an intermetallic layer. In the bath of pure aluminum, the intermetallic layer was the thickest and consisted of an outer FeAl3 layer and an inner Fe2Al5 layer adjacent to the steel substrate with tongue-/finger-like morphology, while in the Al–Si alloy bath, the thickness of the intermetallic layer decreased substantially and the interface intermetallic/steel substrate becomes flat. The ternary phases Al8Fe2Si and Fe3Al2Si3 are identified in addition to FeAl3 and Fe2Al5. The microhardness testing recorded high values of the intermetallic layers, up to 800Hv01, which confirms the high brittleness of the intermetallic compounds.