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.