IGSCC in a BWR steam line after 30 years of operation

Intergranular stress corrosion cracking was observed in a 406 x 8 mm stainless steel BWR steam pipe line after 30 years of operation. The material is Type 304 with a carbon content of 0.04%. The operating temperature is 170°C. The failure analysis showed that the material was sensitized, and that the cracking had occurred at a location where re-welding due to weld defect removal had been performed during the assembly of the pipe. The crack located at a distance of about 8 - 11 mm from the fusion line. The degree of sensitization, measured next to cracked region, showed clear sensitization, with a maximum Ir/Ia - value of 2.32%. Although IGSCC in sensitized stainless steel is a well-known phenomenon, this failure has characteristics worthwhile reporting, i.e., the failure occurred at a temperature, where low temperature sensitization is not effective, and the crack locates at a higher than typical distance from the fusion line, which is at least partly due to the thinner wall thickness compared to typical primary piping. Factors affecting crack initiation include at least the relatively few transients during especially start-up of the plant after annual outages, showing that although steady plant operation with few transients obviously are beneficial as reducing the risk for events, crack initiation can occur with time in components fulfilling the requirements for SCC, i.e. a susceptible material, an oxidizing environment and a high stress.