Kinetics of formation of pack aluminized coating on 9Cr1Mo steel and interdiffusional behaviour of iron aluminides at intermediate temperatures

Abstract Development of a suitable iron aluminide coating layer on 9Cr 1Mo (P91 grade) steel at or below 750 °C finds importance for improving its oxidation and corrosion behaviour while retaining the tempered ferritic-martensitic microstructure. A theoretical model to predict the kinetics has been developed for correlating growth of the aluminide coating on P91 steel substrate with process parameters of pack aluminizing. Experiments were conducted by varying the pack Al (2–10 wt%) and activator NH4Cl (2–8 wt%) content, temperature (600–700 °C), and time (2-16 h). The coating formation results obtained from the experiments authenticated the validity of the proposed model. The coating mainly consisted of single Fe2Al5 phase. The activation energy for Fe2Al5 formation during pack aluminizing was calculated to be 95 ± 7 kJ mol−1. Interdiffusion studies between Fe2Al5/steel in 550–750 °C with varying times of 8–40 h revealed the formations of FeAl2, FeAl, and Fe(Al) layers of different thicknesses. The diffusional growth of the most desired FeAl aluminide found to follow the parabolic kinetics and the activation energy for the growth of FeAl was calculated to be 237.5 kJ mol−1. Scanning electron microscopic and electron back scattered diffraction analysis revealed the formation of columnar grains in B2-FeAl layer.