COMPUTATIONAL MODELING OF MIXED OXIDATION-CARBURIZATION PROCESSES .1.

Heat-resistant alloys used in mixed-oxidant environments rely on the f ormation of a chromia, alumina, or silica surface film for corrosion r esistance and the presence of second-phase precipitates in the matrix often for their strength properties. The growth of the oxide film on s uch alloys is often accompanied by the dissolution of precipitates in the alloy subsurface region. Continued oxidation combined with oxide-s cale spallation tends to decrease the content of the oxide-forming con stituent to such a level that protective scaling can no longer occur a nd severe degradation can develop. In the present work, the initial co rrosion processes involving the complex coupling between oxide scale g rowth and precipitate dissolution is simulated computationally. As an example, a Ni-Cr alloy containing Cr23C6 precipitates was exposed to a n oxidizing-carburizing environment. An approach combining finite diff erence and Newton-Raphson methodologies is developed to model this dif fusion/dissolution process, incorporating the point-defect-chemistry a spects of the oxide scale. The model is able to pre&ct the chemical an d microstructural evolution of high-chromium austenitic alloys during the initial stages of oxidation-carburization.