Unraveling the degradation of protective chromia layer in S-CO2 environment: Carburization-triggered lattice distortion and breakaway oxidation

The long-term corrosion behavior of 316LN austenitic stainless steel in S-CO2 at 500 °C and 25 MPa was investigated up to 6400 h. Revealing three distinct kinetic stages: the initial protection via a thin Cr2O3 layer (≤1600 h, n ≈ 4.06),breakaway oxidation with a 22-fold weight gain surge (1600–3200 h, n ≈ 0.23),and partial re-passivation (3200–6400 h, n ≈ 4.70).HRTEM reveals that breakaway is triggered by carbon-induced Cr2O3 lattice distortion: +3.7% c-axis expansion, interfacial amorphization, and Cr23C6 precipitation. This atomic-scale collapse destroys the diffusion barrier, enabling rapid ion transport. Late-stage Cr2O3 re-formation under low pO2 restores partial protectiveness. Carbon-driven lattice distortion is identified as the root cause of Cr2O3 failure.