Influence of dissolved hydrogen on structure of oxide film on Alloy 600 formed in primary water of pressurized water reactors

In order to investigate the relationship between the susceptibility of primary water stress corrosion cracking (PWSCC) in Alloy 600 and the content of dissolved hydrogen (DH) in the primary water of pressurized water reactors (PWR), structural analysis of oxide films formed under four different DH conditions in simulated primary water of PWR was carried out using a grazing incidence X-ray diffractometer (GIXRD), a scanning electron microscope (SEM) and a transmission electron microscope (TEM). In particular, to perform accurate analysis of the thin oxide films, the synchrotron radiation of SPring-8 was used for GIXRD. It has been observed that the oxide film is mainly composed of nickel oxide, under the condition without hydrogen. On the other hand, needle-like oxides are formed at 1.0 ppun of DH. In the environment of 2.75 ppm of DH, the oxide film has thin spinet structures. From these results and phase diagram consideration, the condition around 1.0 ppm of DH corresponds to the boundary between stable NiO and spinet oxides, and also to the peak range of PWSCC susceptibility. This suggests that the boundary between NiO and spinet oxides may affect the SCC susceptibility.