High temperature oxidation behaviour of AISI 321 stainless steel with an ultrafine-grained surface at 800 °C in Ar–20 vol.% O2

•Early oxidation behaviour of a surface nanocrystallized 321 AISI is investigated.•SNC postpones the onset of breakaway oxidation of 321ASS.•Scale on the surface nanocrystallized 321ASS is dense and protective for 150 h oxidation.•The thickness and uniformity of Cr-rich oxide layers are significantly affected by SNC.•Schematic models are developed to describe the oxidation mechanisms. Oxidation behaviour of surface nanocrystallized 321 austenitic stainless steel (SN–321ASS) and its conventional polycrystalline counterpart (CP–321ASS) in Ar + 20% O2 at 800 °C is comparatively investigated. Severe breakaway oxidation sets in for the CP–321ASS after 100 h, while the SN–321ASS shows a higher oxidation resistance. Surface nanocrystallization increases the diffusion and flux of Cr towards the oxidation front, and can consequently promote the kinetics of early oxidation and accelerate the formation of a thicker Cr-rich inner oxide layer, which aids in prohibiting the enhanced formation of Fe-rich oxides.