Characterization of the stress corrosion cracking behavior of thermally sensitized 20Cr-25Ni stainless steel in a simulated cooling pond environment

Niobium stabilized 20Cr-25Ni stainless steel is used for nuclear fuel cladding in the UK's fleet of advanced gas cooled reactors (AGRs). The cladding can have chromium-depleted grain boundaries as a consequence of irradiation in a reactor core, rendering a small proportion of cladding susceptible to intergranular stress corrosion cracking in cooling pond waters after removal from the reactor. In this work, thermal sensitization was used to simulate chromium depletion and the sensitized material was assessed for its susceptibility to pitting corrosion and stress corrosion cracking using slow strain rate testing (SSRT). Elevated chloride concentrations were used to accelerate corrosion initiation and propagation. In 10ppm chloride and 80 degrees C, the pitting potential was at potentials between +375mV and +400mV (SCE). SSRT appeared to lower the pitting potential, with intergranular corrosion and intergranular stress corrosion cracks observed to nucleate at potentials of +200mV (SCE).