Micro-structural evolution of oxide scales formed on a Nb-Stabilizing heat-resistant steel at the initial stage in high-temperature water vapor

Early oxidation behavior of a Nb-Stabilizing heat-resistant steel in water vapor at 700 degrees C for 336 h was investigated. Micro-characterization methods including in-depth XPS, SEM/EDS, FIB were employed to acquire the composition and distribution of oxidation products. The results showed that the oxidation kinetics of this heat-resistant steel almost followed a parabolic law at 700 degrees C, indicating the oxidation was controlled by the ion diffusion. Surfaces of oxidation products on TP347HFG were gradually from compact to loose with a longer exposure. Oxide scales were mainly composed of layered (Mn, Fe)Cr2O4 and Cr2O3 and scattered Nb2O5. Contents of Mn at the surface were much higher than other positions, confirming that Mn diffuse much faster than Fe and Cr. During the initial oxidation stage, NbC was transformed into Nb2O5, which was covered by Fe-Cr oxide scales. The growth of oxide scales on TP347HFG is suggested.