The effect of chlorine on the kinetics of oxidation of cobalt in environments containing 0.5 atmosphere of oxygen betwee
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I.
INTRODUCTION
H I G H temperature mixed oxidation in environments containing chlorine is complicated by the volatility of many chloride reaction products. Thermogravimetric experiments in such environments often give confusing and ambiguous results because of the simultaneous formation of condensed reaction products, which increases the mass of the specimen, and of volatile reaction products, which decreases the mass of the specimen. ~-~ In order to obtain useful information on the kinetics of reactions in such environments, the experiments must be performed under carefully controlled conditions where the number of species which can participate in the reaction can be minimized. In this study, thermogravimetric experiments have been performed using high purity cobalt metal (>99.99 pct purity) reacting in flowing gas mixtures of argon, oxygen, and chlorine, so that only compounds in the Co-O-C1 system can be formed. This has been confirmed by analysis of the reaction products after the experiments using X-ray diffraction analysis and energy dispersive X-ray analysis on a scanning electron microscope. Under the conditions considered in this study, cobalt oxide (either CoO or C0304) is the only condensed reaction product which is thermodynamically stable, and cobalt chloride forms only as a vapor species, s'6 Therefore, an increase in the mass of the specimen can be associated with the formation of oxide reaction products, and a decrease in the mass of the specimen can be associated with the formation of chloride reaction products.
II.
APPARATUS AND PROCEDURES
The apparatus used in this study has been described elsewhere. 7 It consists of a Cahn RG electrobalance with associated furnace, controller, and glassware for purification and M.J. MALONEY,formerly Graduate Research Assistant, CEMM Department, University of Illinois at Chicago, is now Graduate Research Assistant, Department of Materials Science and Engineenng, Massachusetts Institute of Technology, Cambridge, MA 02139 M.J. McNALLAN is Associate Professor of Metallurgy, CEMM Department, University of Ilhnois at Chicago, P.O. Box 4348, Chicago, IL 60680. Manuscrtpt submitted January 28, 1985.
METALLURGICALTRANSACTIONS B
mixing of reagent gases. Cobalt specimens in the shape of discs, 1.3 cm in diameter and 0.1 cm thick, were sliced from Marz grade cobalt rod obtained from Materials Research Corp. of Orangeburg, NY, using a low speed diamond saw. The surfaces of the specimens were ground to a 600 grit finish using SiC emery paper, and were degreased with acetone before each experiment. Reactive gas mixtures were prepared by mixing pure argon and oxygen with chlorine-argon mixtures containing 3.5 pct, 0.5 pct, and 600 ppm C12 by volume in appropriate proportions to produce the experimental compositions. All of the experiments on cobalt metal were performed with the gas flowing upward through the furnace at a superficial velocity of 1.5 cm s l through a 2.2 cm inner diameter quartz tube contained in the fumace. The procedure for initiating the experiments wa
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