Quantitative Approach for Determining the Critical Volume Fraction for the Transition from Internal to External Oxidation

A theoretical approach is presented to calculate the critical volume fraction fv* for the transition from internal to external oxidation. The focus was on the evolution of internal-oxide volume fraction during the initial stage of oxidation. An equation to calculate fv* was developed by considering an effective diffusion coefficient of oxygen in the alloy (DOeff) which takes into account the evolving blocking effect by forming internal oxide precipitates. Based on this physical process, three types of oxidation behavior can result, depending on the properties of the alloy and exposure atmosphere. Further discussions on the remaining discrepancy between predicted and experimentally-determined fv* values are provided for refinement on the theoretical prediction of this parameter.