PSI - Issue 68

A. Barabi et al. / Procedia Structural Integrity 68 (2025) 285–291 A. Barabi / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Hydropower accounts for around 95% of the electricity generated in the province of Québec (Canada Energy Regulator, 2021), hydraulic turbines are essential components of the hydroelectric infrastructure. These turbines are typically manufactured from low carbon martensitic stainless steel, a material chosen for its suitable mechanical properties and corrosion resistance. During service, the turbine blades are subjected to cyclic loading in an aqueous environment and under these service conditions fatigue cracks may initiate and propagate within the long crack regime. Current crack propagation models, which inform inspection and maintenance schedules, are based on fatigue crack growth data obtained in air. However, the influence of the aqueous environment on crack propagation kinetics under such conditions remains inadequately characterized. Environmental damages, such as anodic dissolution and/or H embrittlement, may significantly influence crack growth behavior. Environmental damage is highly influenced by local crack tip reactions, with pH being a critical chemical parameter. (Cooper and Kelly, 2007), for instance, claimed that during crack growth in AA7050 an acidic pH at the crack tip indicated anodic dissolution which destabilized the protective oxide film. (A Turnbull, 1983) stated that the solution inside crack or crevice-like crack is locally deaerated due to restricted convection and that could lead to potential ( E ) and pH drop. These examples emphasize the importance of monitoring pH to gain deeper insight into the governing electrochemical reactions. Several studies attempted to predict the local electrochemical behaviour at crack tips. (Turnbull, 2001) and (Kovalov et al., 2018) claimed that crack tip electrochemistry differs from bulk solution electrochmistry, similar to crevice effects. They predicted a E difference between the crack interior and exterior of approximately 300 mV SHE . Oxygen depletion at the crack tip due deaeration of solution shifts the cathodic reaction from oxygen reduction to water reduction, leading to hydrogen generation and a subsequent decrease in pH . The hydrolysis of released metallic ions promotes H + generation at the crack tip, leading to reduced pH levels. (Karlberg and Wranglen, 1971) identified anodic dissolution as a key driver of acidification due to the release of metallic ions and the formation of ferrous and chromium chlorides exacerbating acidification inside the crevice-like cracks. (Bogar, 1974) found that Cr content in stainless steel is a key factor influencing pH , with values dropping from 5 to 1.8 as Cr content increases from 0 wt.% to 25 wt.%. In addition to its relevance in electrochemistry, pH is also crucial from a mechanical properties perspective. Many metals, particularly martensitic steels, are susceptible to hydrogen embrittlement, which reduces ductility, toughness, and resistance to crack propagation (Asadipoor et al., 2020; Donahue et al., 2017; He et al., 2020). A decrease in pH at the crack tip indicates a rise in H + concentration, making materials more vulnerable to hydrogen adsorption and embrittlement. The effect of the environment is often obscured by the complex interaction between electrochemical reactions and mechanical properties. Accurate pH evaluation at the crack tip provides valuable insight to isolate the primary environmental degradation mechanism. Direct measurement of pH is challenging given the very limited access to the crack tip. E and pH values inside the artificially created cracks or crevices are reported in the literature (Bai et al., 2023; Kovalov et al., 2019, 2018; Matsumura et al., 2022; A. Turnbull, 1983) but not in the actual propagating crack. Some researchers attempted to semi-directly or directly measuring the crack tip pH . Proposed semi-direct technique by (A. Turnbull, 1983) consist in freezing and extracting segment of frozen electrolyte located near the crack tip to perform E and pH measurement after testing. In this technique, evaporation during defrost process is a potential cause of inaccuracy. (Li et al., 2020) performed direct measurement technique performed by drilling hole and inserting electrodes inside the material and measured the pH during crack growth. They observed a pH drop from 5.6 to 4.2 as the distance from the crack tip decreased. However, the insertion of electrodes can alter the ion movement, and the drilled holes may affect the crack propagation path, and it is not practical. Direct measurement of parameters, clearly the key to a deeper understanding of corrosion fatigue, remains a modern challenge. Our research introduces a novel approach, combining thermodynamic calculations with post-CFCG corrosion product analysis and E measurements to estimate pH at the crack tip. This innovative method aims to provide critical data for understanding the role of environmental damage mechanisms in fatigue crack propagation.