# Identifying the Initial Corrosion Fatigue Failure Based on Dropping Electrochemical Potential
## 3 Results and Discussion
### Estimating Early Failure from Electrical Potential  
The aim is to stop the fatigue testing equipment prior to failure of the specimen. After further and final evaluation, a possible algorithm might then be applied to components in geothermal power plants (e.g., pump shafts in geothermal pumps that are stopped before failure of the component and before severe damage is caused to the entire pump). Because data points displaying the drop of the potential are measured during crack propagation, these are not valuable to determine the early onset of the failure. Therefore, there is a need to interpret the potential values immediately before crack initiation or even crack initiation itself should be detected to correlate both with the onset of failure. Datapoints are collected every five seconds, because, firstly, the number of data points was rather easy to handle; secondly, the intervals are sufficiently appropriate to detect a change in potential values, and thirdly, a sufficient high resolution of the onset of failure was reached. Depending on the potential values, each of these data points needs to be sufficient to stop the fatigue testing immediately at any time of the experiment (before crack propagation). It is non-trivial to interpret the discontinuous process of the free corrosion potential over a long period because it could easily be mistaken as the onset of  
Figure 6: Demonstration of statistical distribution of the potential data of X2 CrNiMoN22-5-3 measured via an Ag/AgCl wire electrode (**left**: short term distribution; **right**: long term distribution). crack propagation (Figure 6, right red box). Therefore, it is highly necessary to distinguish between various potential processes and only stop fatigue testing in the case of actual failure of the specimen. Figure 6 demonstrates the statistical distribution of the potential data during the drop in potential over time, highlighting the difficulty to correlate with the onset of crack propagation in the material due to discontinuous experimental data. The long-term distribution (Figure 6, right, long-term distribution), however, then reveals an onset of failure after a period of decreasing and increasing potential values (Figure 6, left, short-term distribution). Variables such as time and frequency are independent and show no influence on endurance limit and potential for the specimen in a stable passive state (such as X2 CrNiMoN22-5-3 in this case). Note that, up to this time, the failure of each specimen occurring at different times cannot be related to residual stresses, because, at high numbers of failure and low-to-moderate stress amplitudes, corrosion mechanisms clearly dominate the failure procedure. Therefore, the onset is more likely to be related to early and local corrosion phenomena rather than intrinsic mechanical properties. At 33 Hz and a maximum number of cycles set for 10\({}^{7}\), the experiments last 84.175 h (3.5 days) (at frequency 33 Hz: 33 numbers of cycles per second equals 303,030.3 s for 10\({}^{7}\) numbers of cycles) (Equation (1)). \[ \begin{array}{c}D_{V}=\frac{N}{f}\\ D_{V}=\frac{10.000,000}{33}=\frac{303030.3}{\text{ }}\end{array}  \tag{1}\]  
\(D_{V}=test\) _durance_  
\(f=frequency\)  
\(N=number\) _of cycles to failure_. Data points are taken every five seconds resulting in a maximum number of data points of 60,606 according to Equation (2). \[ \begin{array}{c}M_{w}=\frac{D_{v}}{5}\\ M_{w}=\frac{303030.3}{5}=\frac{60606}{\text{ }}\end{array}  \tag{2}\]  
\(M_{w}=number\) _of data points_. Because one single data point (out of more than 60,000) is not significant to determine failure, time series offer a possible and sensible solution. Moreover, time series can be displayed as a function over time. A total of 10 time series each containing 10 data points shifted by 1 comprise all data. Calculations are only conducted with the series followed by a reset to generate the following data series.