1 GRAIN BOUNDARY MICROSTRUCTURE EFFECTS ON STRESS CORROSION CRACK INITIATION MECHANISMS IN ALLOY 600 AND ALLOY 690 Technical Milestone Report: M2LW-19OR0402031 September 2019 Z. Zhai, M. B. Toloczko, M. J. Olszta, and S. M. Bruemmer Pacific Northwest National Laboratory Research Project: Stress Corrosion Crack Initiation of Nickel-Base Alloys in LWR Environments Project Manager: S. M. Bruemmer Pacific Northwest National Laboratory Conducted for: Office of Nuclear Energy, U.S. Department of Energy Light Water Reactor Sustainability Program Materials Aging and Degradation Pathway Pathway Manager: T. M. Rosseel Oak Ridge National Laboratory