Calculation of Boundary Conditions between Internal and External Oxidation of Silicon or Chromium Containing Steels

The boundary constants between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850 degrees C were calculated in order to clarify the formation mechanism of fayalite scale (Fe2SiO4) or chromite scale (FeCr2O4), which can form as a "sub-scale" in Si or Cr containing steels. The diffusion coefficient of oxygen in the alloy, Do, and the oxygen concentration at the specimen surface, N-O((s)), which are constituents of the internal oxidation rate constant, (2D(O)N(O)((s))/NB((O))n), were calculated for various oxidation conditions, and the rate equation for internal oxidation was derived. By comparing the calculated and measured values of (2D(O)N(O)((s))/NB((O))n), we confirmed that the rate equation determined for internal oxidation was reasonable. The boundary condition between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850 degrees C were also calculated by substituting the calculated values of D-O and N-O((s)) into the rate equation.