PSI - Issue 66

Johannes L. Otto et al. / Procedia Structural Integrity 66 (2024) 256–264 Johannes L. Otto et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 6. Normalized course of the electrochemical open circuit potential (E OCP ) measured with a three-electrode setup during the stress controlled constant amplitude test shown in Figure 5 for a) the synthetic exhaust condensate and b) the one-molar NaCl solution.

3.3. Fracture mechanisms Typical fracture mechanisms for the fatigue tests in a moderately corrosive environment are given in Figure 7 and for corrosive environments at elevated temperatures in Figure 8. Without elevated temperature cracks initiate at broken Cr-rich borides and propagate along the diffusion zone with a few jumps into the other diffusion zone and back again. In addition to the mechanical notch effect caused by the fractured phases, crevice corrosion can occur here preferentially. Furthermore, the areas near the Cr-rich borides are depleted of Cr, which weakens the passive layer and even promotes the deformation-induced martensite transformation, which is sensitive to hydrogen embrittlement.

Fig. 7. Failed specimen at σ max = 400 MPa after N f = 1.5∙1 0

5 in synthetic exhaust gas condensate with crack initiation and propagation in the

diffusion zone (blue color) of the brazing seam (solidification zone in red color).

In contrast, Figure 8 shows multiple crack initiations over the entire specimen surface and not only in the diffusion zone. These cracks can be seen particularly at intrusions and other surface defects. Pitting corrosion could also be visible, but these possible pits cannot be clearly distinguished from initial pores of the steel without further investigations. Electron backscatter diffraction (EBSD) investigations on the cross-section of the specimen showed a clearly transcrystalline crack path and often martensite was found on the crack flanks. This suggests that Cr-carbides, which can form on the austenite grain boundaries during the preheating stages of the brazing process, do not have a strong influence, like it was shown by Almubarak et al. (2013). In general, the elevated temperature will have accelerated the chemical reactions of corrosion and the diffusion of chloride ions through the passive layer. This will have promoted crevice corrosion mechanism at notches and other defects on the specimen surface, leading to