The Oxidation of Vanadium-Base Alloys by Liquid Sodium Containing Dissolved Oxides in Temperatures Ranging From 500 to 600 C^*

Vanadium-base alloys suffer oxidation by liquid sodium (500 to 600 C) containing 5 to 10 ppm oxides. The effects are different with differing components of the alloys and the degree of purity of liquid sodium. In liquid sodium of a pumped sodium loop, V-Ti alloys with 5 to 20% Ti suffer embrittlement of surface areas. The zones of embrittlement contain up to 7 to 9% oxygen. Unalloyed vanadium does not underly oxygen diffusion into the metal phase. The different behavior is intelligible from thermodynamic reasons. The free energy of oxygen exchange between Na₂O and the refractory metals following the equation, Na₂O + V = VO + 2 Na, is small in the reaction with vanadium but remarkable in the reaction with titanium in the above range of oxygen concentrations. Depth of the penetration zone of the metallic phase is dependent on Ti content. After 500 hours testing at 600 C (1112 F), the following depths were measured: 200 microns in V-5% Ti, 100 in V-10% Ti, and 70 in V-20% Ti. In alloys with less than 5% Ti, complete embrittlement occurred through the specimen of 0.5 mm thickness. In some tests with cold trapped sodium with plugging temperatures of less than 130 C (266 F), severe corrosion weight losses and oxide layers were observed. Beneath these oxide layers, there were the well known oxygen diffusion layers. X-ray fluorescence analysis showed that oxide layers contained calcium up to 5% concentration.