The effect of oxygen on tantalum-sodium compatibility

Abstract

The effect of oxygen in both phases on the compatibility of tantalum with sodium was investigated at 600°C by exposing tantalum specimens to liquid sodium in static capsules. Increasing the initial oxygen concentration of the sodium from 50 to 12,000 ppm led to increased dissolution of the tantalum in the sodium; oxygen migrated from the sodium to the tantalum. As the initial oxygen concentration of the sodium increased, the accompanying tantalum specimens first gained and then lost weight. We concluded that the addition of oxygen to sodium results in the formation of a loosely adhering ternary oxide scale on the tantalum surface. The increased dissolution of tantalum and the weight change behavior of the tantalum specimens when the initial oxygen concentration of the sodium is increased indicates that the nonadherent ternary oxide flakes off in progressively greater amounts as its thickness increases. When the concentration of oxygen in tantalum before test exceeded 200 ppm, sodium penetrated the tantalum. Penetration, as in other refractory metal-oxygen-alkali metal systems previously investigated, proceeds with the formation of a ternary oxide by a wedging mechanism. At low oxygen concentrations attack occurs along grain boundaries, and as the oxygen concentration of the tantalum is increased, the number of grain boundaries attacked and the depth of attack increase. Still higher oxygen concentrations induce transgranular attack.