Microstructural evolution of intermetallic layer in hot-dipped aluminide mild steel with silicon addition

Mild steel was coated by hot-dipping into molten pure aluminum, Al–0.5 Si, Al–2.5 Si, Al–5 Si and Al–10 Si (wt.%) baths at 700 °C for 180 seconds. The microstructure and phase constitution of the aluminide layers were characterized by means of optical microscope, scanning electron microscope with energy dispersive X-ray spectroscopy, X-ray diffraction and electron backscatter diffraction. Also, the thicknesses of the intermetallic layers and the metal losses of the steel substrate were measured to investigate the interaction between mild steel and aluminum baths. The results revealed that the additions of silicon to the aluminum baths caused Al7Fe2Si and Al2Fe3Si3 phases to form above the FeAl3 layer and in the Fe2Al5 layer, respectively. As the silicon content in the aluminum bath increased, the thickness of the intermetallic layer decreased, and the intermetallic layer/steel substrate interface transformed from an irregular morphology into a flat morphology. The decrease of the thickness of the intermetallic layer was principally attributed to the detachment of the Al7Fe2Si layer from the intermetallic layer into the aluminum bath. The flattened intermetallic layer/mild steel substrate interface was due to the formation of Al2Fe3Si3 precipitates in the Fe2Al5 layer by the serration-like steel substrate reacting with the Fe2Al5 layer containing solid-solute silicon.