Countercurrent gaseous diffusion model of oxidation through a porous coating

A countercurrent gaseous diffusion model was developed to describe oxidation through porous coatings and scales. The specific system modeled involved graphite oxidized through a porous alumina (Al2O3) overcoat between 570 degrees C (1,058 degrees F) and 975 degrees C (1,787 degrees F). The model separated the porous Al2O3 coating into two gas diffusion regions separated by aflame front, where oxygen (O-2) and carbon monoxide (CO) react to form carbon dioxide (CO2). In the outer region, O-2 and CO2 counterdiffused. In the inner region, CO2 and CO counterdiffused. Concentration gradients of each gaseous specie in the pores of the Al2O3 were determined, and the oxidation rate was calculated. The model was verified by oxidation experiments using graphite through various porous Al2O3 overcoats. The Al2O3 overcoats ranged in fractional porosity (phi) and in average pore radius (r) from 0.077 mu m (3.0 x 10(-6) in., Knudsen diffusion) to 10.0 mu m (3.9 x 10(-4) in., molecular diffusion). Predicted and measured oxidation rates were shown to have the same dependence upon phi, r, temperature (T), and oxygen partial pressure (P-O2). Use of the model was proposed for other oxidation systems and for chemical vapor infiltration (CVI).