Oxygen effects on the corrosion of niobium and tantalum by liquid lithium
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LIQUID alkali metals, because of favorable thermal properties, are attractive as heat transfer media for nuclear r e a c t o r s and as working fluids in power conversion systems, x Another potential application for lithium is in the production of power by nuclear fusion, where lithium could be used as a tritium breeder, neutron moderator, and coolantfi 's For both applications, r e f r a c t o r y metals are being considered as containment materials. Corrosion studies of r e f r a c t o r y metal-alkali metal s y s t e m s have shown that these metals are highly r e sistant to mutual dissolution. However, impurities such as oxygen can produce corrosion effects, depending on whether the impurities are present in the alkali metal or the r e f r a c t o r y metal. An increase in the oxygen concentration of the alkali metal increases the dissolution of the r e f r a c t o r y metal in the alkali metal. F u r t h e r m o r e , for niobium and tantalum in potassium 4-6 and s o d i u m / - 9 oxygen migrates from the r e f r a c t o r y metal to the alkali metal, an observation that is cont r a r y to thermodynamic prediction. Two mechanisms have been proposed to explain the effect of oxygen in the alkali metal: 1) oxygen increases the solubility of the r e f r a c t o r y metal in the alkali metal, and 2) a nonadherent t e r n a r y oxide (i.e., an oxide that contains atoms of both r e f r a c t o r y metal and alkali metal) f o r m s on the r e f r a c t o r y metal surface. We have reported on the behavior of oxygen in the niobium-potassium, 5 niobium-sodium, 8 tantalum-potassium, 6 and tantalum-sodium 9 systems at 600~ For the systems containing niobium, oxygen increased the apparent solubility of the r e f r a c t o r y metal, and no third condensed phase (ternary oxide) was present when the oxygen concentration of the sodium and potassium ranged from 50 to 2000 ppm. We used the Wagner x~ thermodynamic interaction p a r a m e t e r s to describe the increased solubility in t e r m s of the effect of oxygen on
R. L. KLUEH is Research Metallurgist, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830.
Manuscript submitted June 29, 1973. METALLURGICAL TRANSACTIONS
the activity coefficient of the niobium in the alkali metal. With the interaction p a r a m e t e r formalism, we were able to explain the migration of oxygen from the niobium to the sodium and potassium. For tantalum in sodium 9 and potassium 6 with 50 to 2000 ppm of oxygen, dark surface scales were formed, and we concluded that a t e r n a r y oxide phase was p r e s ent at the test temperature (NaTaOs was identified in the tantalum-sodium studies). The oxygen that is dissolved in tantalum or niobium also plays a major role in the corrosion of either r e f r a c t o r y metal by sodium or potassium. If the oxygen concentration of the r e f r a c t o r y metal exceeds a certain threshold, the metal i s penetl~ated by the alkali metal, either intergranularly or along specific crystallographic planes. The threshold concentration, which is less for intergran
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