Kinetics and mechanism of the reaction of iron-chromium and iron-chromium-molybdenum alloys with chlorine gas
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INTRODUCTION
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THEREis only very limited information in the literature regarding the reaction of chlorine gas with chromium alloys and stainless steels. Lz In addition, the published work shows only average corrosion rates, and there are not kinetic data or other mechanistic information available regarding the reaction of chlorine gas with iron-chromium and ironchromium-molybdenum alloys at elevated temperatures. This paper summarizes the results of an investigation which was aimed toward gaining an understanding of the kinetics of the reaction between chlorine gas and these alloys.
II.
EXPERIMENTAL
The reaction kinetics were determined by measuring the weight loss (before and after exposure) of test coupons inserted in a quartz tubular reactor through which the chlorine gas diluted with prepurified nitrogen was passed. The experimental setup is shown schematically in Figure 1. A quartz tubular reactor (about 3 cm diameter and 70 cm length) was placed in a long horizontal three-zone furnace which was capable of maintaining the temperature in the center reaction zone at the desired value with an accuracy of ---1.5 ~ The gases used were of prepurified grade. Mixtures of chlorine and nitrogen were prepared by using conventional flow meters and a mixing coil. The test coupons (with about 10 sq cm surface area) were polished with 6/0 silicon carbide emery paper, and cleaned with aceton before weighing. Before each experiment, the apparatus, with the test coupons in place, was purged with prepurified nitrogen and then heated for at least an hour in a hydrogen atmosphere at 497 ~ in order to reduce surface oxide films which
Z. A. FOROULIS is Senior Associate with Exxon Research and Engineering Company, P. O. Box 101, Florham Park, NJ 07932. Manuscript submitted December 15, 1980. METALLURGICAL TRANSACTIONS A
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Fig. 1--Experimental setup.
may be present on the test coupon surface. Following this, the reactor tube was purged with nitrogen to remove residual hydrogen, and then the reaction gas mixture was introduced. After each experiment for weight loss determination, the surface film was removed by careful honing. The kinetic data were supplemented with scanning electron microscopic (SEM) examination and EDAX analysis of surface films produced during the chlorination reaction. In several cases, X-ray diffraction and chemical analysis of reaction product surface films removed from the exposed coupons were also carried out. III.
RESULTS AND DISCUSSION
Thermochemical considerations regarding the reaction of iron-chromium and iron-chromium-molybdenum alloys with low partial pressure chlorine gas in the temperature range of 270 to 550 ~ indicate that the following metal chlorides are of significance: FeClz, FeC13, CrClg, CrC13, and CrC14. In the temperature range investigated, FeC12, CrC12, and CrC13 are solids with very low vapor pressure. FeC13 is solid with considerable vapor pressure in the low temperature range studied and exists only in the vapor state, as (F
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