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

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)2Al9, NiAl3 and Ni2Al3 phases formed above the nickel layer, while a small amount of NiAl3 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 NiAl3 and Ni2Al3 beginning to transform into (Fe,Ni)2Al9 and FeAl3. 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 Fe2Al5 with a small amount of FeAl3 on the top. The kinetic study reveals that the growth of the Ni2Al3, FeAl3 and Fe2Al5 followed the parabolic law. Among all of the phases formed in the intermetallic layers, the Fe2Al5 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.