Re-thinking stress corrosion cracking

Stress corrosion cracking (SCC) in high temperature water systems is an important phenomenon, as highlighted in the last five decades of nuclear power plant operating experience, exemplified by concerns for reliability and safety associated with innumerable incidents of SCC in structural materials. SCC has undoubtedly been occurring for centuries, and developed into a major problem during the industrial revolution, most notably with the proliferation of steam boilers. To address this, boiler and pressure vessel codes were created with a focus on mechanical issues, with highly quantitative guidance on fatigue and overload. By contrast, limited progress nas been made on SCC, which is often viewed as mysterious and complex, and the codes address it primarily by insisting that the design ensure SCC won't occur. This has fostered the concept of SCC immunity, although most of the supporting evidence was obtained in relatively simple, short term testing. Detailed studies and long term plant operation reveal that immunity to SCC growth does not exist in most and probably all cases. This paper summarizes the history of SCC and boiler codes, key developments that have led to a more sophisticated understanding of SCC, some SCC dependencies and interdependencies, and the key factors that create vulnerabilities in nuclear plants. The paper proposes that unless a different approach is taken, SCC will continue to be a major factor in safety and reliability. (author)