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°C (1,058°F) and 975°C (1,787°F). The model separated the porous Al₂O₃ coating into two gas diffusion regions separated by a flame front, where oxygen (O₂) and carbon monoxide (CO) react to form carbon dioxide (CO₂). In the outer region, O₂ 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 (ϕ) and in average pore radius (r) from 0.077 μm (3.0 x 10^–6 in., Knudsen diffusion) to 10.0 μm (3.9 x 10^–4 in., molecular diffusion). Predicted and measured oxidation rates were shown to have the same dependence upon ϕ, r, temperature (T), and oxygen partial pressure $(PO2)$⁠. Use of the model was proposed for other oxidation systems and for chemical vapor infiltration (CVI).