TEM characterization of surface and crack oxidation of a 316L stainless steel in PWR water after hydrogenated-oxygenated cycles

Abstract

The oxide films formed under various hydrogenated-oxygenated cycling conditions at the surface and crack flanks of an austenitic 316L stainless steel were characterized and compared to the results obtained in purely hydrogenated or oxygenated high temperature primary water. The effect of one single transition from oxygenated (high potential) to hydrogenated (low potential) conditions on the surface oxide film composition was investigated using transmission electron microscopy (TEM). The results provide evidence that the initial “oxygenated” film is not modified by the environment change and that an additional layer, with a composition like that obtained in pure hydrogenated conditions, is formed beneath.

Besides, to better understand the effect of oxygenated transients on the cracking scenario, the oxide films formed along cracks observed on SCC tensile specimens after different cycling conditions were characterized. An attempt was made to correlate the evolution of the oxide along the crack flanks and the oxide at the crack tip to the cycling sequence and the final environment, respectively.