High-resolution microscopy investigation of early SCC initiation of Alloy 182 in simulated PWR primary water

Abstract

Understanding stress corrosion cracking initiation (SCCI) in Alloy 600 and its weld metal, Alloy 182, is still relevant to the nuclear industry because they remain in service in the pressure boundary structures of certain pressurized water reactors (PWR). As part of a larger study to understand SCCI in these alloys, nine tensile samples prepared from an Alloy 182 weld in 15% cold forged condition were tested in 360°C simulated PWR primary water at Ni/NiO stability line. A direct current potential drop (DCPD) technique was used for in-situ detection of crack initiation. The test results revealed a tailing distribution in SCC initiation times in these samples with five initiated within 100 hours, three initiated between 700-3000 hours, and one that did not initiate after 5126 hours of exposure. To better elucidate the discrepancy in initiation responses, microstructural and microchemical examinations were performed on two specimens: IN233 that initiated in less than 30 hours of exposure and IN169 which did not initiate after 5126 hours of exposure. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were employed to characterize near surface precursor/cracks in detail. From these results, a prominent feature observed in multiple sampling locations in IN233 was the presence of Cl in crack wall oxides whereas no Cl was found in IN169 that had not undergone SCCI after 5126 hours.