The thermodynamics of volatilization of chromic oxide: Part I. the species CrO 3 and CrO 2 OH
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H E high temperature oxidation resistance of chromium-bearing alloys is generally attributed to the low transport properties of the tenacious chromic oxide o r chromium spinel I layer which forms on the surface. One mode of failure of this protection was f i r s t indicated by the study of Gulbransen and Andrew 2 who found that the rate of simple oxidation of chromium was exceeded by the rate of volatilization of the oxide above about 1030 ~ C at 0.1 arm. of oxygen. This was later 3 attributed to the formation of the gaseous species CrOs. Caplan and Cohen 4 confirmed that volatilization was important at about 1000 ~ C in oxygen and discovered that this volatilization was enhanced in the presence of water vapor. In view of the importance of a knowledge of the thermodynamics of formation of volatile compounds in interpreting and forecasting the oxidation of chromium and its alloys, s's'6 the original purpose of the present investigation was to establish if this enhanced volatility was caused by the formation of a gaseous hydrated compound and, if so, to establish its standard Gibbs free energy of formation. It became clear after the initial experiments that p r e s s u r e s of the gaseous oxides calculated from the m a s s - s p e c t r o m e t r i c study of the volatilization of Cr2Os by Grimley, Burns, and Inghram 7 were too low. Accordingly, the work was extended to include r e m e a s u r e m e n t of the thermodynamics of formation of the predominant gaseous oxide (CrOs). Both purposes have been achieved by using the s e m i - m i c r o transpiration technique to m e a s u r e the apparent vapor p r e s s u r e of Cr203 in oxygen-argon and oxygen-argon-wate r vapor mixtures in the approximate temperature range 1300 to 1585 ~ C. Since completion of this work, Graham and Davis 8 have measured the kinetics of vaporization of thin rectangular specimens of Cr2Os in flowing oxygen and oxygen-water vapor mixtures. They applied the r e sults of the present study and showed that the rates were governed by diffusion of the gaseous oxide and Y-W KIM is Senior Research Engineer, Inland Steel Company, East Chicago, Ind. 46312. G. R. BELTON is Professor, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia, Pa. 19174. Manuscript submitted February 1, 1974. METALLURGICAL TRANSACTIONS
hydrated species a c r o s s the hydrodynamic boundary layer. The present paper will accordingly be confined to the equilibrium thermodynamics of vaporization. EXPERIMENTAL DETAILS The transpiration apparatus, saturator system for the establishment of accurate p r e s s u r e s of water vapor, and general experimental procedure were essentially the same as have been described previously, 9'1~ except that a molybdenum-wound furnace was used to permit higher temperature operation. Specimens of Cr2Os were held in yttria-stabilized zirconia tubes, approximately 3 mm bore and 6 cm in length, which were carefully ground to fit into an outer reaction tube and which could be readily r e moved for weighing. The particu
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