Effect of KOH and dissolved hydrogen on oxide film and stress corrosion cracking susceptibility of Alloy X-750

The replacement of LiOH with KOH for pH control in the primary water of pressurized water reactors (PWRs) is being considered due to the growing cost of enriched 7Li. This study aims to investigate the susceptibility of the primary water stress corrosion cracking (PWSCC) initiation in simulated primary water with KOH. Firstly, the susceptibilities in KOH and LiOH chemistries were compared by conducting uniaxial constant load tests on Alloy X-750 at 360°C. The results showed no significant difference in the time to initiation between the two chemistries. Secondly, the effect of dissolved hydrogen (DH) concentration in the KOH chemistry was examined. The DH concentration of the test water was varied at 5, 30 or 45 ml/kg-H2O. The results showed that the time to initiation of PWSCC was significantly extended under the low DH condition (5 ml/kg-H2O). This observed effect of DH concentration in the KOH environment agrees with the previously reported effect observed in the conventional LiOH environment. To investigate the mechanisms underlying the PWSCC tests, the oxide films on the test specimens were characterized using electron microscopes. The oxide films formed in the KOH and LiOH chemistries did not show significant differences. Additionally, under the low DH condition, the occurrence of selective internal oxidation under the inner oxide film was relatively minor. The results suggest that the use of KOH would not have an adverse effect on PWSCC initiation, and PWSCC initiation can be mitigated with a low concentration of DH in the KOH environment as well as in the LiOH environment.