Understanding and predicting stress corrosion cracking (SCC) in hot water

Abstract Stress corrosion cracking (SCC) is a complex phenomenon that involves at least five disciplines and is affected by 20–50 or more variables—far too many to be tackled by a purely empirical approach. Design and evaluation codes address overload and fatigue, and have made some attempt to incorporate environmental effects on fatigue, but they do not address static load or SCC other than to insist that designs preclude its occurrence. This has put the onus on laboratory experiments to define conditions of immunity to SCC, and simple, short-term tests are inadequate to ensure that SCC will not occur over plant operating lifetimes of 40 or more years. This chapter summarizes efforts to improve the sensitivity and relevance of laboratory experiments, with particular emphasis on the crack growth rate response. The data show that immunity to crack growth is rare, and perhaps nonexistent (clearly, at sufficiently low stress, a design can be immune to crack initiation , but such low-stress conditions do not exist everywhere on large, complex, welded components). What has seemed to be a threshold in corrosion potential, water purity, temperature, stress intensity factor, and so on has been shown to be a well-behaved continuum of response, with growth rates decreasing to values that are more difficult to measure. In honoring Coriou and his career, this chapter also shares his view that good laboratory data foretell vulnerabilities in plant components. As longer lives are targeted, a lower “threshold” condition must be used. The well-behaved continuum of response and the similarity in the effects of parameters across various materials (eg, stainless steels and nickel alloys) and water chemistries (eg, boiling and pressurized water reactors) suggest that the underlying processes and mechanisms of crack advance are similar. This chapter summarizes the common and distinctive elements of SCC in stainless steel and nickel alloys in boiling and pressurized water reactor environments, including the effects of corrosion potential, water chemistry, cold work, stress intensity factor, temperature, sensitization, Si, and grain boundary particles. The processes and mechanisms responsible for SCC are briefly described, along with their integration into a prediction methodology.