| A mathematical model was proposed for the case in which reactants diffuse out through the oxide product layer and react with oxygen on the surface. This model was derived on the basis of intra and intergranular diffusion in series, also combined with structural parameters such as porosity, grain size and the effective diffusion coefficient of the gas. The rate of oxidation of nickel which forms the P-type oxide, was measured at elevated temperatures. The rate increased with increasing porosity, reaction temperature and also oxygen partial pressure. The computer simulation of the model under the given boundary conditions were found to be in fairly good agreement with experimental results, thereby elucidation the oxidation mechanism. The diffusion coefficient for the intragrnular diffusion process were determined to be 5.0×10＾(-8)㎠/sec and 1.0×10＾(-7)㎠/sec at temperature of 920℃ and 970℃, respectively. |
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