# Identifying the Initial Corrosion Fatigue Failure Based on Dropping Electrochemical Potential
## 1 Introduction
### Corrosion Fatigue of Standard Duplex Stainless-Steel X2 CrNiMoN22-5-3  
Duplex stainless-steel (DSS) X2 CrNiMoN22-5-3 (AISI 2205) with a ferritic-austenitic microstructure, often used in industries such as chemical processing, refineries, and desalination plants [16], is known for its high mechanical strength [16], durability in corrosive conditions [17], and resistance to stress corrosion cracking [18].  
The following general findings refer to earlier experiments summarized by [PERSON] et al. [19] and [PERSON] [20] and characterize the corrosion fatigue CF behavior of X2 CrNiMoN22-5-3 used within this study. Later, necessary results will be mentioned as they are needed to follow the experimental procedure and theoretical evaluation.  
In non-corrosive environments, the fatigue limit of DSS X2 CrNiMoN22-5-3 is around 485 MPa (Pf = 50%, push/pull) after 10\({}^{7}\) cycles. However, when exposed to CCS or corrosive geothermal environments such as the geothermal brine from the Northern German Basin at 98 \({}^{\circ}\)C, and tested under stress between 175 and 325 MPa, the material lasted for up to 9.2 million cycles at a stress amplitude of 240 MPa and free corrosion potential. The maximum number of cycles was 4.2 \(\times\) 10\({}^{6}\) at a stress amplitude of 290 MPa without isolation of the specimen against the test machine and 9.2 \(\times\) 10\({}^{6}\) cycles at 160 MPa with insulation. Cracks followed a curved path mainly degrading the austenitic phase. Fatigue strength is significantly reduced due to local corrosion (pitting) from carbonic acid, rather than mechanical loading, as the primary cause of crack initiation and intercrystalline crack propagation.  
Surface roughness plays a key role in mechanical and corrosion properties, with smoother surfaces, deeper plastic deformations, and compressive surface stresses contributing to a lower endurance limit [14]. Polished surfaces improve fatigue life at low stresses, while technical finishes are more effective at high stresses, where microcracks dominate failure. For polished and technical surfaces, the corrosion rates were below 0.005 mm/year, while rates increased to 0.035 mm/year after shot peening. In a geothermal and CCS environment, the main phases precipitating on the surface, regardless of surface roughness, were carbonates or hydroxides such as FeCO\({}_{3}\) and FeOOH. Technical surfaces endured a higher number of cycles before failure (P50% at S\({}_{4}\) 300 MPa = 5 \(\times\) 10\({}^{5}\), k = 19.006) at stress levels above 275 MPa, compared to polished surfaces (P50% at S\({}_{4}\) 300 MPa = 1.5 \(\times\) 10\({}^{5}\), k = 8.78).  
The failure mechanism of X2 CrNiMoN22-5-3 appears to be similar across surface conditions, given the relatively narrow scatter ranges (TN = 1:1.35 for technical surfaces and TN = 1:1.95 for polished surfaces). Early formation of pits and localized depassivation likely play a role in crack initiation, as they are linked to early mechanical degradation. Earlier, the authors showed that a consistent drop in potential (both in absolute value and angle of inclination) at the free corrosion potential [21] signals the start of failure in standard duplex stainless-steel X2 CrNiMoN22-5-3. Just before crack initiation, the drop in potential aligns with the increasing fatigue crack area and matches the predicted growth in crack propagation speed [20; 21].