Abstract
In order to quantitatively evaluate the resistance of a candidate overpack material for geological disposal of high-level nuclear waste to the crevice corrosion, the optimized test method for determining the corrosion-crevice repassivation potential, ER,CREV, of a Ni-Cr-Mo alloy (Alloy 22) was developed based on that of stainless steels (JIS G 0592). It was found that two restrictions shall be satisfied for determining the valid value of ER,CREV for Alloy 22. Restriction (a) was to avoid transpassive dissolution, and (b) was to obtain a penetration depth of 65 μm or more in creviced areas. The recommended procedure in JIS G 0592 at the corrosion-crevice initiation stage, which involved the potentiodynamic anodic polarization at a scan rate of 30 mV min−1, could not satisfy the restriction (a). Consequently, we adopted the potentiostatic holding at the potential below the critical potential for transpassive dissolution. The recommended procedure in JIS G 0592 at the corrosion-crevice propagation stage, which involved the galvanostatic holding at an applied current of 200 μA for 2 hours, could not always satisfy the restriction (b), and the applied current of 1600 μA or more could not satisfy the restriction (a). Therefore, we adopted the galvanostatic holding at a current of 800 μA for 2 hours. The limits of safety usage of Alloy 22 were evaluated by values of ER,CREV which were measured with the optimized procedure in 0.1 to 4 mol dm−3 sodium chloride solutions at 90ºC.
