Influence of Dissolved Hydrogen Concentration on the Protective Property of Oxide Film of Alloy 600 Against Primary Water Stress Corrosion Cracking

Primary water stress corrosion cracking (PWSCC) of Alloy 600 has been a key issue in the field of nuclear energy. It is well known that dissolved hydrogen (DH) plays a crucial role in PWSCC initiation, and some studies showed that PWSCC initiation could be mitigated by decreasing the DH concentration. However, there is no consensus on the mechanism of the PWSCC mitigation in a low DH environment. Hence, to reveal the protective property of the passive film formed in various levels of DH, we analyzed the oxide film formed in simulated primary water at 345°C with the DH concentrations of 5 cc/kg and 30 cc/kg-H₂O by electrochemical measurements (electrochemical impedance spectroscopy and Mott-Schottky test) and transmission electron microscopy (energy dispersive x-ray spectroscopy and electron energy loss spectroscopy) where the specimens of interest were tested by reverse U bend (RUB) tests, and it was confirmed that the PWSCC initiation time was longer for 5 cc/kg-H₂O DH than for 30 cc/kg-H₂O DH. Electrochemical measurements showed that the oxide film for the lower DH environment (5 cc/kg-H₂O) had a higher electrical resistance and lower defect density than those for the higher DH (30 cc/kg- H₂O). The microscopic observation indicated that the intergranular oxidation was relatively insignificant in the lower DH environment. The oxide film for both DH conditions consisted of outer oxides and inner Cr-rich barrier layer. The inner layer for 5 cc/kg-H₂O DH had a higher concentration of Fe and a greater ratio of Fe³⁺/Fe²⁺ than that for 30 cc/kg-H₂O DH. The results suggested that Fe³⁺ contributed to the formation of a less defective spinel-type structure in the inner oxide in the lower DH environment and thereby provided the alloy surface with corrosion protection. This protective oxide film prevented intergranular oxidation and led to the mitigation of PWSCC initiation in a low DH environment.