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 ppm of DH. In the environment of 2.75 ppm of DH, the oxide film has thin spinel structures. From these results and phase diagram consideration, the condition around 1.0 ppm of DH corresponds to the boundary between stable NiO and spinel oxides, and also to the peak range of PWSCC susceptibility. This suggests that the boundary between NiO and spinel oxides may affect the SCC susceptibility.

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In order to investigate the influence of dissolved hydrogen on the oxide film and primary water stress corrosion cracking (PWSCC) of Alloy 600 in primary water of PWR under actual operating temperature range, electrochemical polarization measurement, analysis of the oxide film on the alloy and PWSCC test were carried out. In all cases, the content of dissolved hydrogen was changed from 0 to 45 cc/kg H 2 O. The anodic polarization curve reveals that the peak current density increases with increasing dissolved hydrogen and the change of the peak current density becomes maximum between 11 and 30 cc/kg H 2 O of dissolved hydrogen. According to the results of oxide film analysis, it is seen that the oxide films formed below 11 cc/kg H 2 O of dissolved hydrogen are relatively thick and rich in Ni, but those formed at higher dissolved hydrogen content are relatively thin and rich in Cr and Fe. The susceptibility of the alloy to PWSCC has a peak at 11 cc/kg H 2 O of dissolved hydrogen, which reveals that the property of the oxide film may play important role in PWSCC of alloy.