MICROSTRUCTURAL CHARACTERIZATION OF ALLOY 690TT EXPOSED TO Pb-CONTAINING CAUSTIC SOLUTIONS

摘要

Lead-induced stress corrosion cracking (PbSCC) is one of the secondary side degradation phenomena that has occurred in the steam generator (SG) of pressurized water reactors (PWR). In order to reproduce the conditions where PbSCC develops, C-rings of Alloy 690TT were loaded to the yield stress or to 6% plastic deformation and exposed at 315°C to caustic solutions (pH_(RT) = 13.7) containing 1000 ppm of PbO. Two different surface finishes (as-received, with a local surface deformation, and Oxide Polishing Suspension) were tested to investigate the effect of the surface deformed layer on the SCC initiation propensity. Advanced electron microscopy (SEM, FIB and ATEM) examinations of the oxidized surface revealed that the surface-deformed layer promoted the formation of a protective passive layer, which, in turn, retarded crack formation. The role of stress, surface condition and environmental interactions on PbSCC are also discussed.