Using Modern Microscopy to “Fingerprint” Secondary Side SCC in Ni–Fe Alloys

Abstract

Aggressive aqueous environments (Pb, S, pH extremes) used in laboratory tests have been shown to induce stress corrosion cracking (SCC) in Ni–Fe–Cr alloys. These conditions are used to simulate the extremes of secondary side crevice environments that are unlikely to occur under normal operating conditions but laboratory testing can still be used to establish sensitivities to abnormal chemistry conditions. Advances in modern microscopy have enabled the characterization of these secondary-side SCC systems at near-atomic resolution, helping to reveal mechanistic characteristics unique to each SCC mode. International progress investigating secondary-side SCC phenomena using analytical transmission electron microscopy (TEM) is reviewed in this paper. The unique chemistry and degradation associated with different modes of SCC are identified and compared among Ni–Fe–Cr steam generator tube alloys of interest (Alloy 690 and Alloy 800). It is revealed that each SCC mode exhibits distinctive characteristics, or a “fingerprint”, which can be used to identify the aggressive environment responsible for inducing SCC.