High-temperature thermodynamic properties of the chromium carbides determined using the torsion-effusion technique

The pressures of carbon monoxide in equilibrium with a Cr₂₃C₆-Cr₂O₃-Cr mixture and with a Cr₇C₃-Cr₂O₃-Cr₂₃C₆ mixture have been measured in the temperature range 1100 to 1300 K using the torsion-effusion technique. From the equilibrium data, the following equation for ΔG^of of Cr₂₃C₆ (in cal per mole) has been calculated: ΔG _f ° (±1200) = −77,000 - 18.3T (1150 to 1300 K) Combining the results of this study at temperatures between 1100 and 1300 K with those of Kelleyet al., ³ at temperatures between 1500 and 1720 K, the following equation for ΔG^of of Cr₇C₃ (in cal per mole) has been determined: ΔG _f ° (±400) = −35,200 - 8.7T (1100 to 1720 K) ) The above equation for ΔG^of of Cr₇C₃ has been used to re-evaluate the equilibrium data of Kelleyet al., ³ and the following equation for ΔG^of of Cr₃C₂ (in cal per mole) has been obtained: ΔG _f ° (±400) = −16,400 - 4.4T (1300 to 1500 K) CHROMIUM reacts with carbon to form three carbides:^1,2 Cr₂₃C₆, Cr₇C₃, and Cr₃C₂. The chromium carbides are of considerable technical importance because of their precipitation behavior in certain high-chromium steels and superalloys. A precise knowledge of their thermodynamic properties is essential for the understanding and the prediction of their chemical behavior in various environments. This paper is based upon a thesis submitted by A. D. KULKARNI in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of Pennsylvania.