Microstructural characterization of primary water stress-corrosion cracks in alloy 182 welds from PWR components and laboratory tests

Intergranular stress-corrosion cracking of Alloy 182 welds in PWR primary water service was investigated by analytical electron microscopy of cracks and their leading grain boundaries. Samples from crack-growth tests with simulated PWR and BWR water were also examined for comparison with service samples from several different PWRs. Observations of the crack-tip microstructures indicated that the cracking followed narrow (nm-scale) attack along carbide particle-decorated grain boundaries and carbide particle/matrix interfaces in the weld metal. Characteristic corrosion microstructures observed in the service samples included oxidized and partially dissolved carbide particles near the crack tips, and 5- to 8-nm-wide oxide-filled corrosion 'tunnels' along dislocations that had been intersected by cracks. Similar microstructures were found in the laboratory sample tested with simulated PWR primary water. In the sample tested with oxidizing water containing sulfate, the intergranular attack leading cracks occurred preferentially along one side of the carbide particles, and no tunnel attack was observed. Comparisons with previous crack-tip observations from Ni-base Alloy 600 suggest that the same underlying mechanism of penetrative IG corrosion may be responsible for PWSCC in both materials.