Computational modeling of mixed oxidation-carburization processes: Part 1

Heat-resistant alloys used in mixed-oxidant environments rely on the formation of a chromia, alumina, or silica surface film for corrosion resistance and the presence of second-phase precipitates in the matrix often for their strength properties. The growth of the oxide film on such alloys is often accompanied by the dissolution of precipitates in the alloy subsurface region. Continued oxidation combined with oxide-scale spallation tends to decrease the content of the oxide-forming constituent to such a level that protective scaling can no longer occur and severe degradation can develop. In the present work, the initial corrosion processes involving the complex coupling between oxide scale growth and precipitate dissolution is simulated computationally. As an example, a Ni-Cr alloy containing Cr₂₃C₆precipitates was exposed to an oxidizing-carburizing environment. An approach combining finite difference and Newton-Raphson methodologies is developed to model this diffusion/ dissolution process, incorporating the point-defect-chemistry aspects of the oxide scale. The model is able to predict the chemical and microstructural evolution of high-chromium austenitic alloys during the initial stages of oxidation-carburization.