Thermodynamic studies on chromium carbides by the electromotive force (emf) method

The Gibbs energies of formation Delta (f)G degrees of the three carbides Cr3C2, Cr7C3 and Cr23C6 existing in the Cr-C system were re-determined by electromotive force (emf) measurements between 930 and 1120 K using the galvanic cells Cr, CrF2\ CaF2\ CrF2, CrmCn, CrpCq which give at 1100 K: Delta (f)G degrees (Cr3C2)=-84.3 kJ/mol, Delta (f)G degrees (Cr7C3)=-177 kJ/mol and Delta (f)G degrees (Cr23C6)=-407 kJ/mol. The third-law enthalpies of formation Delta H-f degrees at 298 K were evaluated to be Delta H-f degrees (Cr3C2)=-72.3 kJ/mol, Delta H-f degrees (Cr7C3)= - 144 kJ/mol and Delta H-f degrees (Cr23C6)=- 344 kJ/mol. Previous investigations by other authors diverge from these results up to +/- 50%. Obsolete thermodynamic data of Cr2O3 used in oxygen combustion calorimetry and in CO equilibrium pressure measurements as well as an oxycarbide formation may be the reasons. The experimental difficulties of emf measurements using a CaF2 electrolyte are outlined. Recommended Delta (f)G degrees values of the chromium carbides are given. (C) 2001 Elsevier Science B.V. All rights reserved.