High-Temperature Oxidation of FeCr(Ni) Alloys: The Behaviour After Breakaway

The oxidation of an austenitic FeCr(Ni) steel at 600 °C in H2O + KCl + O2/H2 + H2O + Ar environments is studied up to 168 h. The oxidation behaviour after “breakaway” is investigated by microstructural investigations, and thermodynamic calculations (Thermo-Calc) on the FeCr(Ni)–O system are used to interpret the results. The characteristic “breakaway” scale morphology exhibits an outward-growing iron oxide and a bottom layer consisting of inward-growing spinel oxide and reaction zones containing Cr-rich oxide precipitates and Cr-depleted metal. The equilibrium calculations show that a miscibility gap appears in the FeCr spinel oxide below 665 °C influencing the microstructure of the inward-growing scale. Equilibrium calculations on the FeCrNi–O system show that Ni is not expected to enter the spinel oxide in low oxygen activity conditions, in accordance with observations.