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

摘要

Lead-induced stress corrosion cracking (PbSCC) is oneof the secondary side degradation phenomena that hasoccurred in the steam generator (SG) of pressurizedwater reactors (PWR). In order to reproduce theconditions where PbSCC develops, C-rings of Alloy690TT were loaded to the yield stress or to 6% plasticdeformation and exposed at 315°C to caustic solutions(pH_(RT) = 13.7) containing 1000 ppm of PbO. Two differentsurface finishes (as-received, with a local surfacedeformation, and Oxide Polishing Suspension) weretested to investigate the effect of the surface deformedlayer on the SCC initiation propensity. Advanced electronmicroscopy (SEM, FIB and ATEM) examinations of theoxidized surface revealed that the surface-deformed layerpromoted the formation of a protective passive layer,which, in turn, retarded crack formation. The role ofstress, surface condition and environmental interactionson PbSCC are also discussed.