PSI - Issue 68

290 A. Barabi et al. / Procedia Structural Integrity 68 (2025) 285–291 A. Barabi / Structural Integrity Procedia 00 (2025) 000–000 hydrogen at the crack tip, allowing the hydrogen concentration profile to be determined: ( , ) = % . ( '√ & ).+ ) (1) Where C $ is the H concentration at the crack tip, is the distance from the crack tip, is the time required for the crack to propagate into the highly stressed volume (HSV), and D is the diffusion coefficient ( reported as 9.2×10 ,- ( ' ⁄ ) by (He et al., 2020)). Boundary conditions for this H diffusion case are ( , 0) = 0 and (0, ) = % . To plot the H diffusion profile from the crack tip using the equation (1), H concentration at the solution/crack tip interface is required. Hydrogen concentration at the solution/crack tip interface was determined using the estimated pH ( pH=- log[H . ] ). Variation in hydrogen concentration as a function of distance from the crack tip was then plotted using equation ( 1 ), as shown in Figure 4a. As the figure shows, hydrogen concentration is highest at the crack tip and gradually decreases as the distance from the tip increases. The hydrogen profile indicates the highest adsorption occurs at f =0.1 Hz. The adsorbed H accumulates at the crack tip, therefore, the estimated H in the profile is accumulated and presented as a single value for each f in Figure 4b. The accumulated values are compared with the measured H concentration inherently existing in AISI 415 (dashed line band in Figure 4b). It shows that the estimated hydrogen concentration at 0.1 Hz exceeds the inherent hydrogen concentration in AISI 415. This also demonstrates that more hydrogen accumulates at 0.1 Hz than at other f due to the slower crack speed ( da/dt ). Therefore, it can be concluded that hydrogen embrittlement was the primary environmental damage mechanism responsible for the increased CFCGR at 0.1 Hz. 6

Figure 4 (a) The H concentration profile as a function of distance from crack tip, and (b) the accumulated H under different test conditions. The dashed line shows the H concentration band inherently existing in the material.

4. Conclusions In this research, a novel thermodynamic-based approach was used to estimate crack tip pH and indirectly measure potential drop (E) at the crack tip. Estimation of these parameters led to a better understanding of the primary environmental damage mechanisms during the corrosion fatigue crack growth (CFCG) of AISI 415 at different f . At high f , the primary environmental damage mechanism was identified as anodic dissolution, and CFCGR did not increase significantly compared to FCGR. At low f , on the other hand, CFCGR increased significantly despite the inactivity of anodic dissolution. Hydrogen embrittlement is suggested as the main environmental damage mechanism at these lower f (0.1 Hz). This hypothesis is supported by the estimated H profile at the crack tip, derived from pH estimates using corrosion product analysis and thermodynamic predictions from the Pourbaix diagram. These data were used to estimate pH at the crack tip, indicating a drop in pH to a range of 4.4 to 4.6. Accumulated hydrogen at the crack tip was calculated using the solution to Fick's second law, revealing that the highest H concentrations occur at f =0.1 Hz and further corroborating the suggested environmental damage mechanism at low f . Acknowledgements This research received financial support from the Institut de recherche d’Hydro-Québec (IREQ), SACMI, the Natural Sciences and Engineering Research Council of Canada (NSERC), Mitacs, and the Consortium derecherche et d’innovation en transformation métallique (CRITM). The authors would like to express their gratitude for the