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

Abstract

The rate constants for internal oxidation of Si containing steels (Fe-Si alloys) at 850°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_ON_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_ON_O^(s)⁄N_B^(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°C were also calculated by substituting the calculated values of D_O and N_O^(s) at the boundary into the rate equation.