Generalized diffuse interface model for determination of kinetic parameters in high-temperature internal corrosion reactions (gas/solid system)

Abstract

Diffuse Interface Model (DIM) is introduced to describe the mechanism of hightemperature corrosion/internal oxidation. The zone has dissolved oxygen and metal atoms diffuse and react resulting in the inward movement of zone. The high-temperature oxidation data for cobalt, iron, and nickel, which are for metal deficit (p-type), have been analyzed using a nonlinear optimization method to obtain the optimal values for different parameters, diffusivity of metal atoms in metal oxide, diffiusivity of gaseous species in the corrosion product, diffusivity of oxidant in the unreacted solid (metal or alloy), the rate constant for the reaction and the fraction of the zone reacted. The expression for diffusivity in the reaction zone has been modified by relating diffusivities of the product layer and the core as function of fraction of zone reacted. The model was applied to the experimental data for the high-temperature oxidation of cobalt, iron, and nickel by pure oxygen at an atmospheric pressure. The model predicts the values of diffusion coefficients, preexponential factors, and activation energies for diffusion process, which are generally compared favorably with those reported in the literature. Linear relationship are derived relating the diffusion coefficient in the core and product layer to the parabolic rate constant which is valid for iron, cobalt, and nickel for hightemperature oxidation (900-1350ЊC) by oxygen at an atmospheric pressure according to the following equations:

Non-catalytic, gas-solid reversible reaction mechanism implementing diffuse interface model

“…For corrosion reactions, the zone formation has been observed experimentally, where diffusion of both, oxidant gas to solid reactant core and metal ion (cation) to metal-oxide scale are important for determination of corrosion rate. [22] For porous solids, the core diffusivity is large and the values of zone thickness for both zones are significant. The concentration at the product-zone I, z 1 interface, [A i ], is also determined by the diffusional resistance of product layer (1/D P ).…”