Thermodynamics of oxygen in liquid Fe-Cr alloy saturated with FeO · Cr₂O₃ solid solution

The equilibrium relation between dissolved Cr and O in liquid high Cr steel has been assessed on the condition of pure solid Cr₂O₃ saturation in our previous paper. Following this, the present work deals with the equilibrium between Cr and O in liquid iron saturated with FeO · Cr₂O₃ solid solution and phase equilibria among liquid Fe-Cr alloy and Cr₂O₃ containing oxides for the full understanding of the thermodynamic behavior of oxygen in liquid Fe-Cr alloy. The free energy of formation of FeO · Cr₂O₃ and activities of the constituents in FeO · Cr₂O₃ solid solution have also been measured by the chemical equilibrium technique at 1 823 to 1 923 K. The free energy of formation of FeO · Cr₂O₃ was given by the following equation. Fe(s) + 1/2 O₂(g) + Cr₂O₃(s) = FeO · Cr₂O₃(s) ΔG° = -307 600 + 66.82 T (± 6 800) (J/mol) (1 423 K < T < 1 923 K) The activities of Fe_tO(/) and Cr₂O₃(s) in FeO · Cr₂O₃ solid solution exhibit negative deviation from ideality. The oxide phase in equilibrium with liquid Fe-Cr alloy is not Cr₂O₃ but FeO · Cr₂O₃ solid solution when Cr content in metal is less than the critical Cr content of approximately 7 mass%. The present experimental results are in good accord with the thermodynamic relation between dissolved Cr and O calculated by the parameters which have been proposed in our previous work, if appropriate correction for Cr₂O₃ activity is applied.