Performance of Graphite Oxidation with Environment and Specimen Geometry Variations

For oxidation data from diverse studies to be compared and applied to nuclear graphite degradation behavior, all testing must quantify the oxidation rate as a material-specific property under tightly controlled test conditions. Graphite oxidation behavior is affected by a wide range of parameter variations beyond graphite grade and oxidation temperature. ASTM D7542-15 attempts to control the graphite oxidation with specific furnace dimensions and careful test parameters designed to establish surplus oxygen conditions over a limited temperature range. Practical constraints for relating the observed oxidation of the standardized test sample size to smaller sample geometries must be considered along with the testing conditions to enforce an intrinsic, or material-specific, response. Similarly, these same test parameters must be examined when altering the oxidizing furnace (i.e., oxidizing within a thermogravimetric analyzer [TGA]) to ensure consistent intrinsic response rather than responding to the extrinsic test conditions. Here, data are presented from both vertical furnace and TGA experiments. Substantial differences in sample dimensions and geometric proportions require substantial verification of the test conditions. Because these variations can influence oxygen availability, the presence of excess oxygen must be confirmed to validate the range of conditions over which the data from disparate oxidizing systems can be compared. Statistical reproduction of testing over a suitable range of parameters appears to be more important than accommodating larger specimens, competing priorities that are particularly challenging for irradiated materials. The major considerations addressed are oxygen supply (partial pressure, gas flow, availability at the interface), specimen dimensions and purity, grade-specific microstructural effects (mass loss progression, oxidation penetration, and density gradient evolution), concurrent with temperature effects for tested conditions.