Oxygen effects on the corrosion of niobium and tantalum by liquid lithium

Abstract

The effect of oxygen on the corrosion of niobium and tantalum by liquid lithium at 600°C was studied with static capsules. An increase in the oxygen concentration of the lithium from 100 to 2000 ppm had no measurable effect on the dissolution of either refractory metal. Exposure to lithium reduced the oxygen content of niobium and tantalum to less than or equal to 20 ppm regardless of the oxygen concentration of the lithium, results that agree with thermodynamic calculations. When the initial oxygen concentration of niobium and tantalum exceeded a threshold level, lithium (with no oxygen added) penetrated the refractory metal. Penetration resulted from the formation of a ternary oxide on grain boundaries or preferred crystallographic planes and proceeded by a wedging mechanism, which is caused by stresses generated by the corrosion product. Penetration was along grain boundaries at low oxygen concentrations, the attack depth and number of affected boundaries increasing with increasing oxygen concentration. At higher oxygen concentrations, transgranular attack also occurred. The threshold oxygen concentrations for attack were 400 and 100 ppm for niobium and tantalum, respectively.