Another Way to Think About the Critical Oxide Volume Fraction for the Internal-to-External Oxidation Transition?

Simple concepts derived from percolation theory are used to interpret the transition from internal-to-external selective oxidation in alloys. The analysis is most appropriate for relatively low temperatures, where lattice diffusion of the solute (more reactive element) cannot assist in the formation of an external oxide over any reasonable time scale. It is suggested that over a range of pO2 values, the transition will occur at a roughly constant alloy composition corresponding to a continuum percolation requirement for a nanoscale skeletal oxide formed just below the alloy surface. When this oxide has a sufficiently connected morphology, it provides a short-circuit transport path for the solute. Removal of compressive stress by movement of solvent to the surface is discussed. Predicted values of the critical solute concentration range from 10 at% to 16 at%. Experimental data on Alloy 600 (UNS N06600) oxidation are used to support aspects of the analysis, and to identify unresolved issues.