Mechanistic study on PWSCC of Ni based alloys considering a role of hydrogen in metals

摘要

Results from the hump-SSRT tests and the rapid straining electrode tests using Ni based alloys under high temperature water conditions were analyzed and the relationship between these data and PWSCC mechanism was discussed. It is found that (1) PWSCC is accelerated under cathodic potential and arrested under anodic potential using applied potential technique. (2) The cracking morphology shows one of typical hydrogen embrittlement (HE) type fracture. (3) The threshold hydrogen content for the cracking had been observed at the nearest portion of the fracture surface of the specimen after the test. (4) Intergranular (IG) cracking of alloy 600 (UNS N06600) can be observed not only under simulated primary water but also under high purity hydrogen gas. Both IG crackings have almost the same activation energy. (5) According to rapid straining electrode tests under simulated primary water with and without dissolved hydrogen (DH), rapid repassivation of alloy 690 (UNS N06690) and slow repassivation of alloy 600 are observed under the condition without DH, however repassivation could not be observed and only cathodic current is observed under the condition with DH. These cathodic current of alloy 690 is about 1/5 of alloy 600 and 132 (UNSW86132). According to these results, it is apparent that the basic mechanism of PWSCC is a type of HE. The role of hydrogen in the metal may be to accelerate low temperature creep.