A Study of the Activity of Niobium in Molten Iron by the Solid Electrolyte Oxygen Cell Technique

The activity of Nb in molten iron was studied at three temperatures, 1823, 1853 anp 1873K with the solid electrolyte oxygen cell technique, the following cell assembly being adopted:
Mo, Ho + ZrO₂ Cermet|Mo.MoOt||ZrO₂ (MgO)||(Nb), NbO₂|Mo To equilibrate the (O) Content of the Nb-containing iron, Pure Solid NbO₂ particles were scattered over the surface of the melt under Purified argon atmosphere, and in some cases, the surface of the melt was left bare under the purified argon atmosphere, the oxide, which was ascertained thermodynamically as NbO₂, being formed by itself as the deoxidation product. By elaboration with the experimental data of a₀ determined, the following results were achieved:
1. The free energy of the reaction of deoxidation:
Nb+2O=NbO_2(S); ΔG°=-89710 + 28.27T
2. The free energy of sulution of Nb in iron:
Nb_(s)=(Nb)%; ΔG°=-32090 + 7.9T;γ_{\mathop 1\limits^\rm^\circ 873}=1.60
Nb_(l)=(Nb)%l ΔG°=-38520 + 10. 24T,γ_{\mathop 1\limits^\rm^\circ 873}=0.92
3. The self activity interaction coefficent of Nb:
\rme_\rmNb^\rmNb=\frac2274\rmT-1.44
When the (Nb) content in the iron was less than about 0.2%, similar to the behavoir of other alloying elements such as Al. Cr, V etc, a complex oxide, probably FeO · NbO₂, was formed. The free energy of this complex oxide from its elements was estimated to be:
Fe₍₁₎+\frac32O₂+Nb_(s)=FeO·NbO_2(s);△G°=-383800+121.95T
The possibility Of the presence of other deoxidation products, as the (Nb) content in the molten Iron was decreased still further, was also discussed.