Abstract: The equilibrium of gaseous CO-CO₂ mixture with liquid iron-Niobium-Carbon alloys and NbO₂(s) contained in Al₂O₃ or ZrO₂ crucible is discribed. It shows that the deposition of carbon is inevitable in the low temperature zone(from 1073 to 1273K). The equilibrium partial pressure of oxygen in gaseous on the surface of the melt has been determined with the solid electrolyte cell.The oxide in equilibrium with the niobium in the melt is considered to be NbO₂ (s). The equilibrium constant for the reaction[Nb] + O₂=NbO₂ (s)at 1823K is:K=6.31×10¹⁰ therefore the standard free energy is
△G°=-377150 (J/mol)
The interaction coefficients of activity evaluated, are
${\rm{e}}\frac{{\rm{C}}}{{{\rm{Nb}}}}$=-0.74;${\rm{e}}\frac{{\rm{C}}}{{{\rm{Nb}}}}$=-0.092.