Effects of Wear on Stress Corrosion Cracking Initiation Behavior of Alloy 690 Steam Generator Tubes

Tube bundles are the most critical components of steam generators in pressurized water plants. They are the heat exchange interface between primary and secondary circuits, acting as an important safety barrier to prevent the radioactive leakage. Stress corrosion cracking (SCC) initiation behavior of the worn alloy 690 tubes was investigated by slow strain rate tests (SSRT) in the high-temperature alkaline solution. The scanning electron microscope, electron backscatter diffraction and transmission electron microscope were used to analyze the fretting wear and SCC initiation behavior of alloy 690 tubes. Results show that SSRT specimens show typical transgranular SCC characteristics. The number of initiated cracks and their average depth are increased with increase in the wear degree, suggesting a wear-induced SCC initiation. This may have a relationship with the grooves, delamination, micro-cracks and residual strain layer with tens of micrometers in thickness left on the worn surface. Furthermore, a preliminary analysis on the possible procedure of wear-induced SCC initiation and further propagation during SSRT tests was carried out based on slip-dissolution/oxidation mechanism.