15The corrosion of an austenitic steel in liquid sodium containing 189 µg.g -1 of oxygen was 16 investigated at 650°C as a function of time (122, 250 and 500 h). The steel samples were 17 characterized by means of complementary techniques, namely scanning electron microscopy, X-18 ray diffraction, glow discharge optical emission spectroscopy and transmission electron 19 microscopy. The characterizations showed that a NaCrO 2 oxide scale forms at the steel surface. 20Under this oxide scale, iron and molybdenum rich M 6 C carbide particles together with NaCrO 2 in 21 the grain boundaries and cavities filled with sodium were observed. The stainless steel substrate 22 and / or the chromite scale were dissolved in parallel with the formation of chromite and carbides. 23Thermodynamic calculations showed that NaCrO 2 and M 6 C are equilibrium phases in such a 24 system. NaCrO 2 is formed by the reaction of chromium diffusing from the steel bulk with sodium 25 and dissolved oxygen (external selective oxidation). Mo segregates to the steel surface where it 26 reacts with Fe from the steel and C dissolved in liquid sodium. The dissolution of stainless steel 27 occurred since the liquid sodium bath is not saturated in the dissolving species (pure metals and 28 oxides such as NaCrO 2 , Na 4 FeO 3 ). As for the cavities, vacancies are created at the steel / NaCrO 2 29 interface by Cr oxidation, carburization and dissolution of the other elements present in the 30 stainless steel. The vacancies become supersaturated and this leads to the nucleation of the 31 cavities observed. Part of the vacancies created by Cr oxidation or steel dissolution is annihilated 32 at sinks like dislocations leading to the translation of the oxide / metal interface towards the metal 33 bulk. 34