Oxidation mechanisms of Cr-containing steels and Ni-base alloys at high temperatures - Part II: Computer-based simulation

摘要

Experimental results, which are reported in part I of this paper on the oxidation behavior of ferritic and austenitic steels containing various concentrations of chromium, indicated that the alloy grain boundaries contribute substantially to the overall oxidation process during exposure to laboratory air in the temperature range between 500 and 750℃. The present part II introduces a mechanism-based computer simulation that has been developed in order to predict the corrosion rate of alloys depending on their chemical composition and microstructure, the type of gas atmosphere, and the temperature aiming at a new approach of life assessment. The computer simulation is based on the numerical Crank-Nicholson solution for the diffusion differential equations in combination with the powerful thermodynamic subroutine ChemApp. The kinetic part of the pro- gram distinguishes between grain-boundary and bulk diffusion of the reacting species while the thermodynamic part of the program distributes the individual equilibrium calculations among parallel processing units. The oxidation kinetics, the distribution of the alloying elements, and the penetrating oxygen are calculated in a two-dimensional way considering the effect of grain size of the substrate. It could be shown that the numerical description of oxide formation governed by a grain-boundary diffusion mechanism is in good agreement with experimental observations on low-alloy ferritic steels, where the oxidation rate increases with decreasing grain size, as well as on austenitic steels, where the oxidation rate increases with increasing grain size.