Alloy 800 (Fe-32Ni-21Cr) has been used as the tubing material in Siemens and CANDU steam generators (SG) for decades with good in-service performance. However, laboratory experiments have shown that
this material, and some other Ni-Fe-Cr alloys, are susceptible to dealloying and stress corrosion cracking (SCC) in hot caustic and Pb-alkaline environments at ~300 °C. In this study, we investigate the effect of
alloy composition on the dealloying of Alloy 800 and Ni-based alloys using electrochemical methods in boiling caustic environments (140 °C). The boiling caustic environment is used to simulate dealloying in
Alloy 800 that may be similar to the dealloying observed at higher temperatures, in laboratory experiments that simulate the extreme end of plausible SG heat-transfer crevice chemistries. The importance of the
dealloyed layer as a precursor to SCC of Alloy 800 is studied using self-loaded U-bend samples, followed by nano-scale electron microscopy characterization. Results show that a brittle dealloyed layer is formed
on the surface of Alloy 800, which can inject a micro-crack into the substrate material and initiate SCC by a cleavage process. The mechanism of dealloying and SCC is discussed and may inform corrosion
phenomena observed in alkaline environments relevant to secondary side heat transfer crevices.