Oxidation After Breakdown of the Chromium Rich Scale on Stainless Steels at High Temperature: Internal Oxidation
Breakaway oxidation of alloy 304L at 600 degC 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)
Oxidation of Metals