Abstract
For preventing the stress-corrosion cracking from occurring in the internal structure materials of the BWR plant, the application of injecting hydrogen into the core-water so as to reduce the free corrosion potential of the materials were proposed. Because of the lack of basic data of stress-corrosion cracking susceptibility in BWR environment on Ni-based alloys in comparison with stainless steels, the slow strain- rate tensile (SSRT) tests and the creviced bent-beam (CBB) test were conducted for a sensitized Alloy 182 weld metal in high-purity water environments containing dissolved oxygen (DO) and hydrogen (DH) to varied concentrations at 288 C, and the stress- corrosion crack initiation process were examined. The susceptibility of a material to stress-corrosion cracking was discussed in terms of the electrode potential effect, and the effects of impurities of the testing water were examined by adding slightly Na2SO4. In high purity waters and in the electrode potential region higher than -0.2 V vs. SHE, the interdendritic stress-corrosion cracks were observed both in the slow strain-rate test and the creviced bent-beam test. SEM observations of sub-cracks at the specimen surfaces revealed that stress-corrosion cracks were initiated when the oxide film had cracked to under-hundred μm wide, that no such individual cracks could grow per se, but that those micro-cracks which happened to be formed in each other’s vicinity would coalesce into large cracks, one of which made propagated as stress-corrosion cracking, and that the stress-corrosion cracking sensitivity became more acute on addition of impurity. In the electrode potential region lower than 0 V, on the other hand, the stress- corrosion cracks were observed to be initiated at bottoms of corrosion pits formed on the specimen surfaces in the former, whereas both type of stress-corrosion cracks were observed between 0 to -0.2V. No stress-corrosion crack was observed even though much the same corrosion pits in the CBB test at -0.4V.
