Environment-induced transgranular stress corrosion cracking of 304L stainless steel instrument line tubes

Publisher Summary A leaking instrument line tube was removed for a root cause failure analysis during a recent outage of one CANDU nuclear generating station. The instrument line in question was one of a bundle of 17 tubes constrained by a carbon steel clamp. The failure analysis of the tube section indicated that the mechanism of cracking was transgranular stress corrosion cracking. The crack appeared to have initiated at the outer diameter surface of the tube, and the cracking species present was chloride/fluoride thought to have leached from the heat resistant paint and/or insulation materials, as a result of a nearby tube leak due to a weld defect. Some white and reddish deposits/corrosion products were seen at the leak area. Stereomicroscopic examination confirmed the presence of a circumferentially orientated crack. Secondary cracks were also seen in the immediate vicinity of the main crack. A common phenomenon of all 17 axially split tubes is discoloration of the ID surface through the clamp area. The discoloration appeared to coincide with a temperature gradient induced by the clamp in this length of the tube.