Effect of silicon on the formation of intermetallic phases in aluminide coating on mild steel

Mild steel was coated by hot-dipping into molten baths containing pure aluminum, Al–0.5Si, Al–2.5Si, Al–5Si and Al–10Si (wt.%) at 700 °C for 180 s. Silicon’s effect on the formation of the intermetallic phase in the aluminide layer was investigated by using a combination of scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). The micrograph observation showed that the aluminide layer resulting from hot-dipping in pure aluminum possesses the thickest intermetallic layer and a rough interface between intermetallic layer and steel substrate. As the silicon content in the molten bath increased, the thickness of the intermetallic layer decreased substantially and the interface between the intermetallic layer and the steel substrate became flat. On the other hand, EDS and EBSD observation revealed the aluminide layer resulting from hot-dipping in Al–2.5Si not only possessed FeAl3 and Fe2Al5, but also formed cubic τ5(C)-Al7(Fe,M)2Si (M = Mn, Cr or Cu) above the FeAl3 and scattered τ1-(Al,Si)5Fe3 in the Fe2Al5. However, as the content of silicon in the molten aluminum bath increased, τ5(C)-Al7(Fe,M)2Si began to be replaced by hexagonal τ5(H)-Al7Fe2Si and τ6-Al4FeSi.