Nanoscale Precursor Sites and their Importance in the Prediction of Stress Corrosion Cracking Failure

Stress corrosion cracking (SCC) usually initiates at locally compromised surface regions, and ultimately at nanoscale precursor sites. The ability to identify such sites would be instrumental in predicting SCC failure and developing proactive mitigation strategies. Modern microscopy capabilities allow for the requisite micro-to-atomic scale analysis to characterize SCC and identify precursor sites at various length scales. In the latter part of his career, Roger Staehle recognized and emphasized the benefit of modern capabilities in microscopy and computational science for modeling and performing physical characterization of atomic and nanoscale processes related to SCC. Consequently, he developed the quantitative micro-nano (QMN) approach with the goal of attaining a global model of SCC on an atomistic basis. This article reviews recent studies that have applied state-of-the-art microscopy techniques to characterize SCC and associated precursors in the context of the QMN approach. Initial examples used to demonstrate characterization of nanoscale precursors include SCC of Alloy 800 in Pb-containing, caustic, and acid sulfate solutions relevant to secondary side crevice environments in nuclear power plants. In line with the QMN approach, the focus is on characterizing and understanding SCC mechanisms, leading to prediction and identification of associated precursors. Precursors to secondary side SCC of Alloy 800 are shown to include monolayer-level S or Pb at oxide-metal interfaces, the onset of dealloying, or metastable pitting corrosion. Following this, intergranular oxidation embrittlement of Alloy 600 in hydrogenated water/steam environments is explored to demonstrate the benefits of a multitechnique approach to identify SCC precursors and highlight recent advancements in in situ microscopy. Although nuclear-relevant SCC systems are used as examples, the QMN approach and benefit of identifying nanoscale precursors that correlate with SCC failure are applicable to a broad spectrum of SCC systems.