One of the principle factors affecting the stress corrosion cracking (SCC) of a susceptible metal is its corrosion potential. A number of instances of cracking on th outlet feeder pipes of the primary heat transport (PHT) system at the Point Lepreau Generating Station ([PLGS] Lepreau, NB, Canada), a heavy water reactor, have been observed and are likely associated with either SCC or hydrogen-enhanced creep cracking. The potential of the outlet feeder pipes was inferred from representative electrochemical corrosion potential probes, pre-filmed in the laboratory, and exposed to a PHT coolant during a maintenance outage and reactor restart. Laboratory tests of the probes were also conducted over the full range of plant operating temperatures to provide a reasonable estimate of return-to-service conditions. The laboratory results were generally in good agreement with the PLGS data under deoxygenated and oxygenated conditions or temperatures of less than 80°C. Neither the removal of dissolved oxygen at low temperatures at PLGS, after a maintenance outage, nor in the laboratory simulation was efficient to reduce the potential of the carbon steel below the region of susceptibility to SCC. It was only with the increase in temperature toward normal operating conditions that the potential decreased to that expected for the typical reducing conditions of the PHT coolant.

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