# Identifying the Initial Corrosion Fatigue Failure Based on Dropping Electrochemical Potential
## 2 Materials and Methods  
**Material:** corrosion fatigue testing was conducted on standard duplex stainless-steel X2 CrNiMoN22-5-3, 1.4462 (yield strength: 672 MPa, tensile strength: 854 MPa, and PREN number referring to corrosion resistance: 35.1 [19; 20]). The steel was continuously casted, tempered appropriately, and quenched in water resulting in a phase equilibrium of ferrite and austenite (Table 1). Analysis via spark emission spectrometry (SPEKTROLAB M, SPECTRO Analytical Instruments GmbH, Kleve, Germany) and the Electron Probe Microanalyzer JXA8900-RLn, JEOL Ltd., Tokyo, Japan, (Table 1) offered the precise chemical composition. The high pitting resistance number PREN of standard duplex stainless-steel X2 CrNiMoN22-5-3 represents sufficient corrosion resistance based on the critical chemical composition of >12% chromium and a moderate percentage of nickel and sufficient nitrogen and manganese content. Especially, the high chromium content guarantees good corrosion resistance against both surface and local corrosive attack in aggressive geothermal environments. **Samples:** round corrosion fatigue samples of X2 CrNiMoN22-5-3, featuring shoulder heads, were produced through precision turning (Rz = 4), in accordance with DIN EN ISO 11782-1 [22] and the FKM Research Issue 217 guidelines (Figure 1). The same dimensions of the test specimen have been used in previous corrosion fatigue tests to characterize the endurance limit and corrosion fatigue behavior of X2 CrNiMoN22-5-3 [6; 19; 20; 21]. The critical region of the specimen, exposed to the corrosive medium, was polished by first grinding with SiC paper ranging from 180 grit to 1200 grit under water, followed by a final polishing using diamond paste in 6 \(\upmu\)m, 3 \(\upmu\)m, and 1 \(\upmu\)m grades. The surface roughness of the polished samples ranged from 0.58 \(\upmu\)m to 2.01 \(\upmu\)m, with an average of 1.59 \(\upmu\)m.  
**Artificial geothermal brine (corrosion media):** in order to replicate the actual geothermal conditions, geothermal aquifer water--comparable to that of the Stuttgart Aquifer [23] and the Northern German Basin (NGB) [24; 25]--was carefully synthesized. The process followed a precise sequence to prevent the formation of salts and carbonates, as outlined in Table 2. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Phases** & **C** & **Si** & **Mn** & **Cr** & **Mo** & **Ni** & **N** \\ \hline \(\alpha\) \& y **\(\star\)** & 0.023 & 0.48 & 1.83 & 22.53 & 2.92 & 5.64 & 0.15 \\ \(\alpha\)\(\,\)* & 0.02 & 0.55 & 1.59 & 24.31 & 3.62 & 3.81 & 0.07 \\ \(\gamma\)\(\,\)* & 0.03 & 0.47 & 1.99 & 20.69 & 2.17 & 6.54 & 0.28 \\ \hline \end{tabular}
* PREN \(\alpha\) = 37.4; \(\gamma\) = 32.4; \(\star\)\(p\) = 0.024; S = 0.008.  
\end{table} 
Table 1: X2 CrNiMoN22 25 3 (EN/DIN 1.4462, UNS S31803); chemical composition in mass percent. **Test setup and corrosion fatigue testing:** the experimental procedure to determine corrosion fatigue has been described earlier in detail [19; 20]: corrosion fatigue testing was carried out on a Schenck-Erlinger Puls PPV machine, operating at a frequency of 33 Hz, with geothermal brine continuously circulating around the specimen. To eliminate any interaction between the specimen and the testing machine, the corrosion chamber was directly attached to the test specimen [19]. Figure 2 represents schematic drawings of the corrosion chamber from the outside (left) and at operation (middle) as well as potential plugs designed for observation, temperature control, lighting, and electrochemical as well as oxygen partial pressure measurements. The temperature of the corrosion medium, set at 369 K, is controlled by thermal sensors located in both the reservoir and the corrosion chamber. The corrosion medium is circulated from the reservoir to the pump, through the corrosion chamber and back, by