Note on Graphite Oxidation by Oxygen and Moisture

Abstract

Simplified equations of graphite oxidation are reviewed for semi-infinite slab, finite slab, and cylinder geometries, using the principal assumptions of linearized oxidation kinetics and quasi-steady state oxidation profile. All equations are coupled to a general surface mass transfer boundary condition. The equations include those for oxidant concentration distribution, surface oxidation rate, burnoff profile, and oxidation efficiency. This review also covers some areas that may not be well recognized. The key role of the effective diffusivity is highlighted, with a brief review of measured values. The temperature-dependence of the surface oxidation rate is shown to be more complex than usually shown for the diffusion-affected zone. Assumption of linear kinetics permits ready estimation of equilibration time for development of the quasi-steady burnoff profile. In addition, approximations for the time-steady hydrogen concentration profiles are developed for the case of oxidation by H2O. All cited methods can be readily evaluated by spreadsheet calculation.