High-Temperature Oxidation of Stainless Steels
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High-Temperature Oxidation of Stainless Steels J.C. Colson and J.P. Larpin Introduction The first stainless steels, mainly low carbon chromium-iron alloys, have been known since the beginning of this century. These steels show good resistance against wet corrosion and high-temperature corrosion.1 This article focuses on hightemperature corrosion, with emphasis on gaseous sulfidizing and oxidizing environments. The discussion is limited to these two gases since corrosion involving halogen-and/or carbon-containing gases involves other specific processes. The behavior of binary and ternary alloys will be successively examined, then the role of minor elements will be considered. Fundamental Mechanisms of High-Temperature Corrosion of Stainless Steel Usually, a dry corrosion process results in the formation of corrosion products, giving a simple or complex oxide or sulfide scale on a metallic substrate, separating it from the aggressive gaseous environment and, consequently, acting as a protective barrier. Scale growth is controlled by the conductivity of the reaction products which are solid electrolytes.213 Generally, the mechanism of scale growth is governed by outward cation or inward anion diffusion processes.4 This is the basis of the model originally put forward by Wagner5* for a single metal and subsequently developed for alloys, and particularly, for stainless steels. This one-way point-defect diffusion process is responsible for the observed parabolic scaling kinetics characterized by a parabolic rate constant kp. This model is well described in the literature.7'8 In the case of stainless steels, formation of a protective scale is required; this is possible if the oxide or sulfide products have a low diffusivity to cations or anions due to a low density of point defects in the crystal lattice. The protective characteristics of the corrosion products may be experimentally determined by measurement MRS BULLETIN/OCTOBER 1994
of their electrical conductivity,9 although the scales should also be effective against short-circuit transport of ions, atoms, or molecules. The best barriers consist of oxides, such as A12O3, SiO2/ and Cr2O3. Good adhesion of the scale to the alloy substrate is also needed to avoid the disruption effects of thermal shocks, and hence, to conserve the barrier effect of the scale during operation. However, in many situations, the protective nature of the scale may be destroyed by spalling or cracking, due to the development of growth or thermal-cycling strains in the scale. Also, if scale growth occurs via an outward cation diffusion mechanism,10 voids may form in the vicinity of the scale/steel interface which can result in poor adhesion of the protective scale. This phenomenon does not occur if scale growth occurs via an inward anion diffusion mechanism.
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Figure 1. Parabolic oxidation rate constant kp against chromium concentration (wt%) for the oxidation of Fe-Cr alloys in air (pressure, P = 1.013 x 105 Pa) at various temperatures.7
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