Corrosion behavior of the cladding tube (AISI Type-316SS) for FBR in high temperature sodium; Experimental studies on the oxygen concentration dependence of the corrosion in sodium

not registered; Kano, Shigeki; Owada, Tetsuro*; Atsumo, Hideo

Some experiments in order to investigate the oxygen concentration dependence of the corrosion in sodium were carried out on the domestic cladding tube (nominal dimension of 6.3 mm outside diameter at 0.35 mm wall thickness) of Type-316 stainless steel. The daughter loops, M-2 and M-11, were used for mass-transfer test of the tube specimen, in which the test condition was individually selected on the oxygen concentration; the cold trap temperature was controlled to keep its concentration at 200C in M-2 and 150C in M-11. Mass-transfer test was made up to about 2000 hours in flowing sodium and the maximum temperature of test sections was held at 650C. After exposure in sodium, the following analyses were made on the specimens: weighing, surface roughness, SEM, X ray, X ray microscan, mechanical properties and others. The results obtained after exposure at 650C are summarlized as follows: (1)General corrosion rate of the cladding tube speoimen is remarkably dependent on the oxygen concentration. It shows distinguished increase as the cold-trap temperature increases. (2)The leaching layer of iron was observed, in which alloying elements were depleted in nickel and chromium. Elemental leaching and depletion in the surface layer was mainly observed inwards along the grain boundary in the neighbour of the surface exposed in sodium. Thickness of the leaching layer was about 30 m. (3)At the surface layer of the specimens exposed in sodium, the presence of ferrite was confirmed by X ray, and the surface roughness of the specimens after exposure in the higher oxygen concentration increased to about 1.8 times as large as those before exposure.