•Scale growth on Fe-Cr alloys obeys the diffusion-controlled parabolic kinetics.•Cross-sectional EBSD, Raman spectroscopy and XRD reveal the type of oxides with Cr.•Chromic oxide displays higher hardness and increased compressive (Raman) stresses.•The oxide at metal-oxide interface exhibits an increase in fracture toughness with Cr. The present study involved a range of iron-chromium (0–20 wt% Cr) alloys of similar crystallographic texture. The oxidation kinetics reduced remarkably when Cr content was ≥12 wt%. This coincided with the reduction in the number (from three to two) of clearly distinguishable oxide layers. Though the outer hematite (Fe2O3) layer remained, the inner chromite (FeCr2O4, spinel structure) transformed to chromic oxide ((Fe,Cr)2O3, corundum structure) and the columnar coarse grained magnetite (Fe3O4) middle layer disappeared. The chromic oxide displayed higher hardness and increased compressive residual (Raman) stresses. It also showed significantly higher fracture toughness in micro-cantilever bending tests. A clear picture of oxidation resistance, in high-Cr alloys, through phase transformation and associated mechanical integrity of the inner oxide layer thus emerged.