Oxidation of 316L(N) stainless steel in liquid sodium at 650 °C

Abstract

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

Further insights into the mechanisms involved in the corrosion of 316L(N) austenitic steel in oxygenated liquid sodium at 550 °C

“…The formation of the NaCrO2 oxide surface layer is not uniform. Pits are observed on the surface of ferritic-martensitic steels [34,35] and intergranular corrosion is reported for ferritic-martensitic steels [27,[34][35][36] and austenitic steels at 650 °C [33] and 700 °C [37]. After 3100 h in flowing sodium at 710 °C containing 30 µg g -1 of oxygen and after 2500 h in flowing sodium at 300 °C containing 8,000 µg g -1 of oxygen, i.e in oxygen saturated sodium, intergranular corrosion was observed for 304L steel over an extent of 40 µm [22].…”

“…Indeed, the formation of sodium chromite (NaCrO2) has been reported by several authors [27][28][29], particularly for oxygen concentrations higher than ~15 µg g -1 [14,30,31]. Scanning electron microscope (SEM) observations of the oxidized surfaces indicate the formation of tetragonal crystals [14,32,33]. NaCrO2 was characterised by X-ray Diffraction (XRD) on ferrous alloys [31] and austenitic steels [14,30,33].…”

“…Scanning electron microscope (SEM) observations of the oxidized surfaces indicate the formation of tetragonal crystals [14,32,33]. NaCrO2 was characterised by X-ray Diffraction (XRD) on ferrous alloys [31] and austenitic steels [14,30,33]. In each observation, a preferential crystallographic orientation is reported along the [0001] axis.…”

“…After 3100 h in flowing sodium at 710 °C containing 30 µg g -1 of oxygen and after 2500 h in flowing sodium at 300 °C containing 8,000 µg g -1 of oxygen, i.e in oxygen saturated sodium, intergranular corrosion was observed for 304L steel over an extent of 40 µm [22]. Cavities are also observed on ferritic-martensitic steels [34][35][36] and austenitic steels [33] immersed in liquid sodium containing oxygen at 650 °C. At 550 °C, local corrosion is not observed for ferritic-martensitic steels immersed in liquid sodium containing 1 µg g -1 of oxygen [36].…”

“…Dissolved oxygen would also decrease the chromium composition of the steel surface layer [29,30,[33][34][35][36]. Our previous results [33] show that the oxidation of chromium into sodium chromite is the driving force behind the diffusion of chromium to the steel surface. Cr depletion increases with time and temperature.…”