Thermodynamic aspects of Cr₂O₃ reduction by carbon

In order to save resources of chromium, 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. Thermodynamic assessment of probability of 16 reactions between them under standard conditions and for certain reactions under conditions different from standard was carried out using tabulated thermodynamic data of reactants in temperature range of 1500 – 3500 K. The following states 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). Probability of reactions was estimated using standard Gibbs energy and the Gibbs energy calculated using the Van Goff isotherm equation. Dissolution of chromium in metal of surfacing bath or probable partial pressures of CO and CO₂ in gas phase was taken into account and was calculated from equilibrium of carbon gasification reaction. Presence of carbon in flux-cored wire with chromium oxide Cr₂O₃ as a reducing agent will necessarily lead to occurrence of reduction reactions with generation of chromium carbides, and possibly chromium itself. Generation of Cr₇C₃ (сr) carbide is likely. With longer life time of chromium oxide and carbon at a temperature above 2500 K, generation of chromium as a component of the surfacing bath is more thermodynamically probable than generation of its carbides. Chromium oxide has the highest reactivity in Cr₂O₃ (liq) state. Direct reduction is preferential. Generation of CO(gas) as a product of carbon oxidation is more probable. Dissolution of chromium in metal increases thermodynamic probability of reactions with its generation and further reduces probability of reactions in which chromium is the starting material.

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