New insights into the early-stage formation and evolution of the external oxide scale on a low-alloy steel

High-temperature oxidation is one of the major causes of environmental degradation in steels. However, the mechanisms governing oxide scale formation and evolution remain not fully understood. Here, we present new microstructural insights into external oxidation of a low-alloy ferritic steel using transmission Kikuchi diffraction and transmission electron microscopy. The external oxide scales contain Fe 2 O 3 not only in the surface skin layer but also around internal pores, which are generated by the phase transformation stresses associated with Fe 3 O 4 formation. The Fe 2 O 3 exhibits diverse multiscale morphologies, including sub-micron structures, super-fine nano grains near pore edges, and lath-like platelets along the ( 11 1 ¯ ) and ( 311 ) crystallographic planes within the Fe 3 O 4 scale. A spinel-type oxide, Mn(FeO 2 ) 2 , coherent with the Fe 3 O 4 lattice, was also identified in Mn-enriched regions. Thermodynamic computation and in-situ high-temperature microscopy observations further corroborate these findings, providing new insights into the mechanisms of high-temperature oxidation in low-alloy steels.