Evaluation of primary water stress corrosion cracking initiation behavior of Ni-base alloy 182 weld metal depending on water chemistry in a simulated primary water environment

Abstract

In the pressurized water reactor (PWR) primary environment, one of the major concerns regarding degradation of structural materials is primary water stress corrosion cracking (PWSCC) of Ni-base alloys which are used for steam generator tube, CRDM penetration nozzles, and dissimilar metal welds in piping system. It has been extensively reported that Alloy 600 and related weld metal Alloy 182 are susceptible to PWSCC. Many researches indicated that PWSCC susceptibility is closely connected to the oxide characteris-tics which in turn are dependent on water-chemistry condition, especially dissolved hydrogen (DH) and Zn contents. Despite many efforts to understand the PWSCC mechanisms, it is not enough to establish a sys-tematic relationship between crack initiation behavior and oxide characteristics. In this study, PWSCC initiation behaviors of Ni-base alloy 182 weld metal were investigated in a simu-lated PWR environment with varying amount of dissolved hydrogen (DH) and Zn. To evaluate PWSCC initi-ation behavior, U-bend type specimen was chosen for constant loading. Three DH conditions (5, 30, 50 cc/kg-$H2_O$) and two Zn content (0 and 30 ppb) were chosen, while other chemistry conditions were fixed at typical PWR primary environment. The PWSCC initiation test without Zn indicated that the cracking susceptibility was greatest at DH of $30 cc/kg-H_2O$, which is the typical PWR condition. Meanwhile, in both low DH (5 cc/kg) and high DH (50 cc/kg) conditions, similar cracking susceptibility was observed. Oxide characterization depending on DH indicated that oxide layer could be separated into two parts. From TEM-EDS cross section-al analysis showed that the oxide layer formed on Alloy 182 was composed of Cr-enriched inner oxide on the substrate and Ni, Fe enriched outer oxide. From Zn injection, it was revealed that oxide containing Zn was relatively more dense and protective. According to the Mott-Schottky analysis, defect density in spinel oxide was the highest in PWR condition. In Zn injection cases, however, the resistance of PWSCC initiation in-creased, especially in PWR condition and high DH condition. It stemmed from increased oxide stability by Zn incorporation into spinel oxide. Therefore, PWSCC initiation behavior of Alloy 182 could be related to the stability of oxide formed on the metal surface.