Thermodynamic Aspects of Cr₂O₃ Reduction by Carbon

In order to save chromium resources, technology of flux-cored wire surfacing is of great practical interest. In this case, Cr₂O₃ chromium oxide and carbon as a reducing agent are used as fillers. The thermodynamic probability assessment of 16 reactions between them under standard conditions, as well as for certain reactions under different standard conditions, was carried out using tabulated thermodynamic data of reactants in the temperature range of 1500–3500 K. The following were considered as standard states for reactants: Cr(ref) (reference state, melting point 2130 K, boiling point 2952 K), Cr(liq), Cr(gas), Cr₂O₃(cr, liq), Cr₂O₃(gas), C(ref), and as possible reaction products and standard states for them CO(gas), CO₂(gas), Cr₂₃C₆(сr), Cr₇C₃(cr), Cr₃C₂(cr). The reaction probability was estimated using standard Gibbs energy and was calculated using the Van Goff isotherm equation. The chromium dissolution in metal of surfacing bath or probable partial pressures of CO and CO₂ in gas phase was considered and then calculated from the equilibrium of carbon gasification reaction. The carbon presence in flux-cored wire with chromium oxide Cr₂O₃ as a reducing agent will necessarily lead to the occurrence of reduction reactions with chromium carbide generation, and possibly chromium itself. The generation of Cr₇C₃(сr) carbide is likely. With a longer life span of chromium oxide and carbon at a temperature above 2500 K, chromium generation as a component of the surfacing bath is more thermodynamically probable than carbide generation. Chromium oxide has the highest reactivity in Cr₂O₃(liq) state. Direct reduction is preferential. The generation of CO(gas) as a carbon oxidation product is more probable. The chromium dissolution in metal increases the thermodynamic reaction probability with its generation, as well as further reduces the reaction probability in which chromium is the starting material.