Effect of Oxygen on Niobium-Sodium Compatibility

The effect of oxygen on the compatibility of niobium and sodium at 600 C (1112 F) was investigated. An increase in the oxygen concentration of the sodium leads to increased dissolution of the niobium. It was also observed that the experimentally determined oxygen equilibrium distribution coefficient (i.e., the ratio of the oxygen concentration in the refractory metal to that in the alkali metal at equilibrium) is much smaller than the theoretically calculated value. In the past, these observations have generally been explained by postulating the formation of a ternary oxide containing refractory metal, alkali metal, and oxygen atoms. As an alternative to the ternary oxide hypothesis, it was proposed that at equilibrium, a liquid ternary solution exists and oxygen affects the solubility of niobium by decreasing the activity coefficient of the niobium in the sodium. The phase rule was used to check this hypothesis at 600 C. When the concentration of oxygen in niobium exceeded a threshold level, the niobium was penetrated by sodium. At low oxygen concentrations, attack proceeds along grain boundaries, and as the oxygen concentration is increased, the number of grain boundaries attacked and the depth of attack increases.