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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The Pourbaix diagram for the 13Cr-4Ni alloy in an aqueous environment is shown in Figure 3. Based on the bulk solution pH and E (marked as a red dot in Figure 3), Fe₂O₃ , FeCr₂O₄ , and NiO(Fe₂O₃) are the thermodynamically stable corrosion products. The XPS analysis revealed otherwise, with no NiO(Fe₂O₃) present on the facture surface. The adjacent zone in the Pourbaix diagram, however, displays oxides similar to those actually formed on the fracture surface, confirming the pH and E drop at the crack tip. By propagation of crack in deaerated water a potential drop at the crack tip from 0.075 V SHE to -0.09 V SHE was measured. This observed E drop allowed us to refine our estimate of the crack tip pH range, which is shown with red stars and black arrows in Figure 3 , falling between 4.4 and 4.6. Such an acidic pH at the crack tip has been reported in the work of (Karlberg and Wranglen, 1971). The authors measured a pH of 4 at the tip of a crevice-like crack in a Fe-13Cr binary alloy while the bulk solution pH was 6. As reported before, the pH drop at the crack tip indicates an increase in the H + concentration locally, which increases the chance of H embrittlement (Cooper and Kelly, 2007). Therefore, it is speculated that H embrittlement is the main environmental damage mechanism at f = 0.1 Hz in the current research where anodic dissolution was absent and maximum CFCGR accelaration observed (Figure 1).

Figure 2 The XPS analysis of the corrosion products at f =10 Hz and 0.1 Hz.

Figure 3 The Pourbaix diagram of the 13Cr-4Ni martensitic stainless steel in the aqueous environment. The red dot shows the pH and E in the bulk solution during CFCG. The dashed arrow indicates the potential drop at the crack tip. The red stars shown by black arrows indicate the pH range at the crack tip. To analytically determine the contribution of hydrogen to the increase in CFCGR, the concentration of hydrogen adsorbed at the crack under each test condition is calculated using the estimated local pH . Diffusion of hydrogen inside the material can be modelled using the solution to Fick’s second law for an inexhaustible hydrogen source diffusing in a semi-infinite solid, as proposed by Fullenwider (Fullenwider, 1983). This equation describes the diffusion of