TEM investigations of the oxide layers formed on a 316L alloy in simulated PWR environment

Atomic scale observations of the oxide formed on stainless steels, under simulated nuclear reactor conditions, are performed to estimate the oxide layer contribution on stress corrosion cracking (SCC) mechanisms. A duplex oxide composed of a chromium enriched inner layer (Fe1.5Cr1.5O4) and an outer layer composed of magnetite crystallites (Fe3O4) is found. The oxide layer structure evolves from amorphous, for oxidation times of 1 min, to nano-crystalline at 2 min and mono-crystalline after 5 h. IFFT images, calculated from Cs-corrected HRTEM images recorded on grains oriented in the 〈111〉 direction, highlight a double network of dislocations with ½ 〈10-1〉 and ½ 〈−110〉 Burgers vectors. This network leads to the decrease in non-relaxed deformation and favors an epitaxial growth between steel and oxide. Both crystal structure transformations and epitaxial relations between metal and oxide have provided relevant information which contributed to progress on SCC modeling.