The Effect of Oxygen on the Compatibility of Niobium With Potassium

Abstract

Abstract Refractory metals are highly resistant to dissolutive corrosion attack by liquid and boiling alkali metals when the alkali metals and refractory metals are free of impurities. Dissolution increases drastically, however, when the alkali metals contain oxygen. This phenomenon has been attributed to the formation of ternary oxides containing atoms of the two metals. As an alternative to the ternary oxide hypothesis, we have examined the possibility that at equilibrium only two condensed phases are present and that the oxygen affects the solubility of the refractory metal by decreasing the activity coefficient of the refractory metal in the liquid. The interplay between solute species in refractory metal-oxygen-alkali metal systems can be characterized in terms of interaction parameters as developed by Wagner. This approach was evaluated using data from the niobium-oxygen-potassium system at 600 C (1112 F). The observed solubility of niobium as a function of the oxygen content in potassium was consistent with that expected on the basis of calculated interaction parameters. Analysis of the observed behavior in terms of the phase rule also indicated that only two condensed phases exist at equilibrium.