Effect of chromium on the formation of intermetallic phases in hot-dipped aluminide Cr–Mo steels

Abstract

Cr–Mo steels with different chromium contents were coated by hot-dipping into molten baths containing pure aluminum and Al–10wt.% Si for 180s. The effect of chromium content in the steels on the formation of the intermetallic phases in the aluminide coatings was studied. The results show that all the aluminide coatings can be distinguished into an outer pure aluminum or Al–Si topcoat and an inner intermetallic layer. The intermetallic layers, resulting from the steels hot-dipped in pure aluminum, have the same phase constitution, an outer minor FeAl3 and an inner major Fe2Al5. In the aluminide coatings on the steels with 0 and 2.25wt.% chromium after hot-dipping in Al–10wt.% Si, the intermetallic layers were composed of an outer layer of τ5(H)-Al7(Fe,Cr)2Si and an inner one of FeAl3/τ1-(Al,Si)5Fe3/Fe2Al5, while a small amount of polyhedral τ5(H)-Al7(Fe,Cr)2Si and plate-shaped τ6-Al4FeSi were observed in the Al–Si topcoats. In the aluminide coatings on the steels with 5 and 9wt.% chromium after hot-dipping in Al–10wt.% Si, the intermetallic layers were composed of only a τ5(H)-Al7(Fe,Cr)2Si phase. A large amount of scattered granular τ5(C)-Al7(Fe,Cr)2Si and a small amount of plate-shaped τ4-Al3FeSi2 and τ6-Al4FeSi were also found in the Al–Si topcoats. When the chromium content reached 5wt.%, the amount of steel, which dissolved when samples were hot-dipped in Al–10wt.% Si, increased. Also, the rate of dissolving went up as chromium content went up. The increase of dissolution is because the interdiffusion between steels and Al–10wt.% Si bath was enhanced by the formation of scattered granular τ5(C)-Al7(Fe,Cr)2Si, which was stabilized by chromium.