Effect of Environmental and Material Conditions on SCC of Carbon Steels in High Temperature Water

A small number of cracking incidents have occurred in carbon steel outlet feeders of a CANDU^® reactor. The cracks were initiated from both the inside and the outside surfaces of tight-radius cold bends. All cracks were intergranular. One plausible explanation for the inside surface initiated cracking is that it is caused by oxidizing water stress corrosion cracking. This mechanism has been studied extensively under boiling water reactor water chemistry (at neutral pH) conditions and the crack path for this kind of SCC is transgranular. It is not known whether the elevated pH in CANDU^® primary water chemistry affects cracking behaviour, particularly electrochemical potential thresholds and crack path. Also, the possible effects of cold work have been investigated in the context of SCC of carbon steel in high temperature water. The pronounced cold work in the feeder bends has been identified as the main variable likely to affect the crack path. Based on the understanding of the feeder cracking incidents, an experimental program was carried out to address the effects of coolant pH, and cold work from feeder bending process during fabrication, on the SCC behaviour of carbon steels in the context of an oxidizing water SCC mechanism.