APPLICATION OF A COUNTERCURRENT GASEOUS-DIFFUSION MODEL TO THE OXIDATION OF HAFNIUM CARBIDE AT 1200 TO 1530-DEGREES-C

A counter-current gaseous diffusion model is presented to describe the oxidation of hafnium carbide between 1200 and 1530-degrees-C. The model separates the porous hafnia scale into two gas diffusion regions separated by a flame front, where O2 and CO react to form CO2. In the outer region, O2 and CO2 counter-diffuse; in the inner region, CO2 and CO counter-diffuse. The concentration gradients of each gaseous specie in the pores of the hafnia are determined and the rate of oxidation is calculated. A porosity of 2% and a pore radius of 0.01 mum are representative of the values observed in hafnia during the early stages of HfC oxidation. These values lead to predictions of parabolic rate constants that are close to those measured by thermogravimetric analysis. In addition, the predicted and measured parabolic rate constants are shown to have the same dependence upon temperature and oxygen partial pressure.