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 ΔGof of Cr23C6 (in cal per mole) has been calculated: ΔGf° (±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 ΔGof of Cr7C3 (in cal per mole) has been determined: ΔGf° (±400) = −35,200 - 8.7T (1100 to 1720 K) ) The above equation for ΔGof of Cr7C3 has been used to re-evaluate the equilibrium data of Kelleyet al.,3 and the following equation for ΔGof of Cr3C2 (in cal per mole) has been obtained: ΔGf° (±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.