The Role of Dealloying as a Precursor to Stress Corrosion Cracking of Alloy 800 in a Boiling Caustic Environment

Abstract

Ni-based alloys and stainless steels have historically been used in corrosive environments due to their resistance to general and localized corrosion and good mechanical properties. For instance, the nuclear industry has used these materials extensively. However, despite their good corrosion resistance, Ni-based alloys and stainless steels can suffer from stress corrosion cracking (SCC) in certain environments, such as caustic environments at 140 °C to 300 °C. SCC is a type of localized corrosion that is difficult to detect, and once initiated often propagates rapidly. Consequently, developing strategies to mitigate and understand SCC initiation is a priority for several industries. To study SCC initiation, it is important to understand the role of corrosion precursor sites for crack initiation. For example, dealloying results in the formation of a nanoporous surface film enriched in the more noble element. This film is more brittle compared with the substrate base material and has been proposed to act as a precursor to SCC. This research investigates the susceptibility of Ni-based alloys and stainless steels to dealloying in boiling caustic solutions. While several alloys are studied, Alloy 800 is a primary focus due to its use for tubing material in CANDU nuclear power plant steam generators. The effect of alloy composition and crystallographic orientation on dealloying susceptibility is examined, and the relationship to SCC is discussed. Results indicate that an increase in Ni-content increases resistance to dealloying in boiling caustic solution. A comparison between several Ni-based alloys shows that the type and concentration of other alloying elements are also important; specifically, Cr may be beneficial while Mo is detrimental. Texture analysis reveals that {111} type planes are more resistant to dealloying and by extension dealloying-induced SCC. Detailed, nanoscale analysis of the SCC observed in Alloy 800 shows that the mechanism is in good agreement with the film-induced cleavage mechanism, in terms of both deformation along the crack path and local chemistry changes at the crack tip.