Oxidation After Breakdown of the Chromium-Rich Scale on Stainless Steels at High Temperature: Internal Oxidation

Breakaway oxidation of alloy 304L at 600 °C was studied in four environments (O2 + H2O + KCl, O2 + H2O + SO2 + KCl, H2 + H2O + Ar, O2 + K2CO3) for up to 168 h. The resulting scales were investigated by FIB/SEM, SEM/EDX, STEM/EELS, STEM/EDS and oxidation was elucidated by thermodynamic calculations (Thermo-Calc). The initial thin protective scale broke down in all cases. After breakaway, the scale consisted of two layers, i.e. an inward growing spinel/reaction zone and an outward growing iron-rich layer. The general features and microstructure of the scales after breakaway were similar in all environments and were explained in terms of: (1) Different diffusivities of Cr3+ and Fe2+ in the spinel oxide. (2) The appearance of a miscibility gap in the FeCr and FeCrNi spinel oxides. (3) The equilibrium composition of the spinel (at low pO2 Ni is not present in the spinel).