Characterization of intermetallic layer formation in aluminide/nickel duplex coating on mild steel

Abstract

Nickel pre-plated mild steel was coated by hot-dipping in a molten bath containing pure aluminum at 670 °C with various immersion times. The formation of the intermetallic layer in the aluminide coating was investigated. The results show that an intermetallic layer, composed of (Fe,Ni) 2 Al 9 , NiAl 3 and Ni 2 Al 3 phases formed above the nickel layer, while a small amount of NiAl 3 phase scatted in the aluminum topcoat. In the coating, after hot-dipping for 60 s, a portion of the nickel layer was consumed, leading to the NiAl 3 and Ni 2 Al 3 beginning to transform into (Fe,Ni) 2 Al 9 and FeAl 3 . As the dipping time increased to 120 s, the nickel layer disappeared. The phase constitution of the intermetallic layer changed, as well. The base of the layer became mostly Fe 2 Al 5 with a small amount of FeAl 3 on the top. The kinetic study reveals that the growth of the Ni 2 Al 3 , FeAl 3 and Fe 2 Al 5 followed the parabolic law. Among all of the phases formed in the intermetallic layers, the Fe 2 Al 5 phase possessed the fastest growth rate. This caused the intermetallic layer to grow significantly and form a rough interface with the steel substrate when the nickel layer was consumed. The microstructure and phase constitution of the coating with nickel pre-plating are the same as those of the coating without nickel pre-plating after long-term hot-dipping. ► Aluminide/nickel duplex coating was created by hot-dipping nickel pre-plated steel. ► The effect of nickel pre-plating on the formation of intermetallic layer was studied. ► Nickel pre-plating caused a thin intermetallic layer in the aluminide coating.