Calculations of Internal Oxidation Rate Equations and Boundary Conditions between Internal and External Oxidation in Silicon Containing Steels

The rate constants for internal oxidation of Si containing steels (Fe-Si alloys) at 850 degrees C were calculated in order to clarify the formation mechanism of fayalite scale, which can form as a "sub-scale" in Si containing steels. The diffusion coefficient of oxygen in the oxide layer. D-O, and the oxygen concentration at specimen surface, N-O((s)), which are constituents of the internal oxidation rate constant, (2D(O)N(O)((s))/N(B)((O))n). were calculated under various oxidation conditions, and the rate equation for the internal oxide layer was derived. Comparing the calculated and the measured values of (2D(O)N(O)((s))/NB((O))n), we confirmed that the rate equation determined for the internal oxide layer was reasonable. The conditions at the boundary between internal to external oxidation of Si containing steels (Fe-Si alloys) at 850 degrees C were also calculated by substituting the calculated values of D-O and N-O((s)) at the boundary into the rate equation. [doi: 10.2320/matertrans.M2009256]