The formation of solid solution oxides during internal oxidation

The diffusion processes occurring when binary alloys react with oxygen to form an oxide that contains both alloy components in solid solution, either exclusively as internal oxide or in combination with a surface scale, have been analyzed and compared with experimental results for Fe-Mn and Ni-Co alloys. The experimental results available for the Fe-Mn system were obtained under conditions of exclusive internal oxidation, and good agreement was obtained between calculated and experimental results. In the Ni-Co system, a surface scale and a zone of internal oxidation develop. Agreement between calculated and experimental depths of internal penetration is acceptable if the diffusivity of oxygen in the alloy is 3.8×10^–6 cm²/sec at 1100°C. Agreement between calculated and experimental concentration profiles is not very good.List of Principal Symbols B alloy component with higher affinity for oxygen - BO more stable scale component - a _O activity of oxygen - D _O diffusivity of oxygen in the alloy - D _O ^eff effective diffusion coefficient of oxygen in alloy - f volume or mass fraction of internal oxide - f _max maximum volume or mass fraction of internal oxide - G _AO , G _BO free energies of formation of oxides AO and BO, respectively - N _B mole fraction of component B in the alloy - N _B ^O bulk mole fraction of component B in the alloy - N _BO mole fraction of oxide BO in oxide phase - N _O atomic fraction of oxygen dissolved in alloy - N _O ^I ,N _O ^II atomic fraction of oxygen dissolved in alloy at the internal oxide-surface scale and alloy-internal oxide interfaces, respectively - R gas constant - r ratio of number of moles of precipitated oxide to total number of moles of metallic constituents in the alloy - T temperature - t time - X ₁,X ₂ positions of internal oxide-surface scale and internal oxide-alloy interfaces, respectively - x position coordinate - defined as –1/RTG _BO ] - ₁, ₂ dimensionless rate constants describing rate of displacement of the internal oxide-surface scale and internal oxide/alloy interfaces, respectively - _O Henry's law activity coefficient for oxygen dissolved in alloy - defined as –1/RT(G _BO –G _AO )]