Understanding of selective oxidation of Fe-Mn binary alloys during continuous annealing through X-ray photoelectron spectroscopy

Selective oxidation of Fe-Mn alloys was characterizied through X-ray photoelectron spectroscopy and scanning/transmission electron microscopy during industrial continuous annealing (in an atmosphere of N2 –5 vol.% H2 with traces of water at 800 °C). After annealing, only MnO oxides are formed on and below the surface and few iron oxides appear on the top surface due to oxidation of ambient air or the formation of FeO-MnO solid solutions. Mn concentration profiles exhibit typical selective oxidation and show similar features. Mn concentra- tion first increases to a peak value at a depth of 5–10 nm from surface, and then decreases to the minimum at the oxidation front, following with a floating up and down to bulk composition. According to XPS spectra and Mn concentration profiles as a function of depth, the annealed alloy surfaces can be divided into four zones: ambient air contaminated zone, MnO enrichment zone (external and internal oxidation coexisting here), Mn depletion zone and bulk composition zone. Mn concentration reaches a minimum value at the oxidation front, whose position is deeper with annealing temperature and time increasing. The value of Mn diffusion coefficient in ferrite estimated using diffusion flux at the oxidation front is 2.9 ×10−15 m2 s−1 at 800 °C, which is slightly greater than that in literature.