Long term oxidation resistance and thermal stability of Ni-aluminide/Fe-aluminide duplex diffusion coatings formed on ferritic steels at low temperatures

A coating with a duplex structure consisting of an outer Ni2Al3 layer and an inner Fe2Al5 layer was formed on a commercial type of ferritic steel P92 using a two step process of electroless Ni/B plating followed by pack aluminising at 650 °C. Nearly 11,000 h oxidation test in air and more than 13,000 h isothermal annealing test in argon atmosphere were carried out to assess the long term oxidation resistance and thermal stability at 650 °C. The amount of oxidation was only about 0.66 mg/cm2 for the coating as compared to 10.3 mg/cm2 for the uncoated steel after nearly 11,000 h oxidation test. Inward Al diffusion took place during oxidation test, which led to the transformation of the outer Ni2Al3 layer to NiAl and increase in the Al diffusion depth. However, once the outer Ni2Al3 layer was completely transformed to NiAl, it stayed stable during the remaining period of oxidation test, providing long term oxidation resistance. Kirkendall voids formed and grew and then finally disappeared in the coating layers due to interdiffusion processes taking place at the oxidizing temperature at the interfaces between different layers of the duplex coating. No spallation was observed in the coating during the entire period of the oxidation or isothermal annealing tests.