THE ENVIRONMENTALLY ASSISTED CRACKING OF COLD-WORKED ALLOY 800 AND HIGHER-CHROMIUM VARIANTS IN MILDLY ACIDIC SULPHATE ENVIRONMENTS

摘要

Although performance to date of Alloy 800 steam generator (SG) tubing has been exemplary, concerns exist regarding possible vulnerabilities that could shorten the serviceable life. Recent European operating experience includes observations of crevice-induced environmentally assisted cracking (EAC) of Alloy 800 SG tubes. Furthermore, Alloy 800 is known to be susceptible to degradation in acidic sulphate-containing environments. It is feasible that such environments could exist within SG secondary side crevice environments either due to acute ingress during chemistry excursions or due to the long-term concentration of impurities in heated crevices. The effect of pH on the EAC of Alloy 800 in high-temperature aqueous sulphate environments has been investigated by comparative EAC initiation tests. These tests involve the application of cyclic loading to small, blunt-notch tensile specimens coupled with on-line crack detection via the direct current potential drop technique able to detect cracks of ＜ 10 μm. In addition, EAC initiation tests have been supported by high-temperature electrochemical testing and surface analysis. In addition to Alloy 800, the susceptibility of a higher-chromium variant has been tested in identical environments in a comparative manner. This higher-chromium variant was produced via a collaboration between Atomic Energy of Canada Limited and the Institute of Nuclear Safety Systems and has shown promising resistance to stress corrosion cracking in crack growth rate tests by General Electric in boiling and pressurized water reactor primary side environments.