# Identifying the Initial Corrosion Fatigue Failure Based on Dropping Electrochemical Potential
## 3 Results and Discussion
### Relating the Electrical Potential to Fatigue Failure  
To correlate early failure of corrosion fatigue specimens in a CCS environment with the drop of the curves determined via in situ measurement of the electrochemical potential (measured directly within the corrosion chamber using the Ag/AgCl wire electrode) 10 free corrosion potentials were analyzed with an insulated experimental test setup (Figure 5). The samples were kept in a reference test stand for 72 h, matching the duration of fatigue testing for 10\({}^{7}\) cycles in the corrosion chamber. Figure 5 represents four potential pathways of different X2 CrNiMoN22-5-3 specimens measured with the Ag/AgCl wire electrode in an insulated fatigue testing setup. It shows the drop in potential over time, highlighting the correlation with the onset of crack propagation in the material. Generally, the measured potential was the same as the initial potential in the chamber, confirming that the test setup did not affect electrochemical measurements. The potential remained largely stable, with significant drops observed during crack formation, leading to specimen failure. This indicates that both mechanical load and crack propagation are key factors influencing potential changes during fatigue testing. In general, the sudden decrease in electrochemical potential before failure reflects the combined effects of rapid crack propagation, localized anodic activity, and electrochemical instability in the material as it approaches its critical failure point. Crack initiation is accom  
Figure 4: Test setup and electric potential of the Ag/AgCl wire electrode in relation to the SHE; the value of the Ag/AgCl glass electrode was adopted from [6; 20]. panied by decreasing frequencies in corrosion fatigue tests. These lower test frequencies enhance crack tip exposure to the corrosive environment, amplifying the potential drop before failure. Rapid crack growth near failure increases the reactive (newly) exposed surface area, intensifying electrochemical activity. In addition, high stress and strain at crack tips create localized anodic regions, accelerating corrosion and affecting potential. Shortly before failure, breakdown of passive films and increased crack interactions lead to a sudden shift in electrochemical potential. The Ag/AgCl electrode detects these changes, reflecting the loss of surface protective properties and increased anodic activity. Frequency dependence:  
In corrosion fatigue experiments, the electrochemical potential decreases with changes in frequency because the rate of mechanical loading influences the interaction between mechanical and electrochemical processes. At lower frequencies, the metal surface is exposed to the corrosive environment for longer periods during each cycle, enhancing electrochemical reactions, such as oxidation and hydrogen evolution, and allowing for more extensive localized electrochemical activity, such as spallation, the accumulation of corrosion products or enhanced anodic dissolution at crack tips. This increases localized anodic dissolution and shifts the potential toward more negative (corrosive) values. Decreasing frequency as means of failure onset therefore always results in decreasing electrochemical potential. For all corrosion fatigue specimens of standard duplex stainless-steel X2 CrNiMoN22-5-3, a potential drop at the free corrosion potential (Figure 5) consistently indicates the onset of failure, both in terms of potential value and angle of decline. Therefore, a sudden drop in potential, along with a frequency change, signals crack initiation. Over the course of the tests, the potential fluctuated by approximately 60 mV (Figure 5). For specimens that did not fail, these potential changes were minimal. Since no clear relationship between surface condition, crack path, or microstructure in the crack region was found, residual stress is the most likely explanation for the final failure [14].