Effect of Grain Boundary Microchemistry on IGSCC of Alloy 132 in a Simulated BWR Environment

Intergranular stress corrosion cracking (IGSCC) of Alloy 132 weld metal in light water reactor (LWR) structural components is a great concern. The microstructure of Alloy 132 is dendritic and the grain boundaries that are parallel to the direction of the dendrites growth are known to be most susceptible to cracking. In order to correlate the grain boundary microstructure to IGSCC, and in general, improve the understanding to IGSCC of Alloy 132 weld metal, the grain boundary microstructure as well as IGSCC behavior of Alloy 132 was studied in this work. Compositional distributions of the dendritic grain boundaries in sensitized Alloy 132 were studied by using Field Emission Gun Auger Electron Spectroscopy (FEG-AES) analyses. The intergranular (IG) facets were generated by fracturing the specimen using slow strain rate test in high vacuum with the tensile stage attached to the FEG-AES. The results show that chromium and phosphorus concentrations on the IG facets vary significantly. IGSCC behavior of Alloy 132 was tested in a simulated boiling water reactor (BWR) environment. The data of compositional distributions of the dendritic grain boundaries and IGSCC crack length were evaluated statistically. By a comparison of the Weibull distributions of IGSCC penetration data and that the compositional profiles of IG facets, a correlation was found between P segregation and the SCC behavior of Alloy 132. The current study results also imply that the depletion of chromium at grain boundaries is also one cause of IGSCC of Alloy 132 in high temperature water.