# Identifying the Initial Corrosion Fatigue Failure Based on Dropping Electrochemical Potential
## 3 Results and Discussion
### Estimating Early Failure from Electrical Potential
The incubation time is 10 data points (50 s with 1 data point = 5 s); after that, each data point will eventually be number 10 and calculated immediately. Because the number of data points is sufficiently small, it shows reliable validity. Condition 1:  
To ensure that fluctuations of the signal during fatigue testing do not lead to early break-off of the experiment, different trends have to be defined (Figure 7). In Figure 7, the red boxes represent upward trends and green boxes downward trends. Note that not all trends are present in every fatigue curve. Trends describe relations of data points over a period of time and not fluctuations of single data points. Differentiation in positive and negative potential is very important, because failure was initiated only in correlation with negative electrochemical potential. Therefore, all potentials and fluctuations above 0 were eliminated (Figure 8). In Figure 8, the blue line refers to the potential of 0. The example given in Figure 7 shows that 250 s are left to end the experiment before the testing machine will stop its program. This is defined as sufficient to take action giving the time advantage. However, crack initiation may only be defined by fluctuations in frequency from the testing machine which then is no longer possible to observe. Also, this rule only applies for one test setup (time, temperature, pressure, atmosphere, and fluid aquifer). Condition 2:  
Single data points are random values and show strong statistical distribution. Therefore, the arithmetic mean is calculated according to Equation (3) once the time course is completed. This reduces the number of data points by a factor of 10, smoothing the  
Figure 8: Example of elimination of potential data > 0 of X2 CrNiMoN22-5-3 (Ag/AgCl wire electrode, insulated fatigue test setup). All potentials and fluctuations above 0 were eliminated. Figure 7: Grouping trends of the statistically distributed potential data of X2 CrNiMoN22-5-3 (Ag/AgCl wire electrode, insulated fatigue test setup). Red boxes: upward trends; green boxes: downward trends. distribution of data points. Arithmetic means are calculated for all time courses completed with negative numbers. \[\overline{y}_{j}=\frac{1}{n}\sum_{i=1}^{n}y_{i} \tag{3}\]  
\(n=number\)\(of\)\(data\)\(points\)  
\(\overline{y}_{j}=arithmetic\)\(mean\)\(of\)\(particular\)\(time\)\(course\)  
\(y_{i}=data\)\(point\)\(of\)\(a\)\(single\)\(time\)\(course\)  
\(j=number\)\(of\)\(time\)\(course\)  
\(i=number\)\(of\)\(data\)\(point\)\(within\)\(time\)\(course\)  
Arithmetic means define the trend as follows: the balance of time courses according to Equations (4)-(6) show upwards or downwards trends. \[\Delta y=\overline{y}_{1}-\overline{y}_{2} \tag{4}\]  
\[\vdots\]  
\[\Delta y=\overline{y}_{9}-\overline{y}_{10} \tag{5}\]  
\[\Delta y=\overline{y}_{10}-\overline{y}_{1} \tag{6}\]  
If the difference values are constantly positive over a certain period of time, they indicate a downwards trend (zero is interpreted as a positive value). Negative difference values would indicate an upwards trend but are without meaning because, as stated above, the data points greater than zero are not related to failure of the fatigue sample. In the case of eliminated marked time courses, the balances were also eliminated. In general, negative balances could be neglected in trends. However, here they serve as another condition: the 10 following balances have to be greater than the preceding balance over a period of 50 s. Due to the statistical distribution, negative balances also occur during the failure process of the sample and serve as intermission (Figure 9). Figure 9 shows a representative set of data where a negative balance divides data sets. These are only slightly visible in the curve. Figure 9: Example of failure phase and interruption of X2 CrNiMoN22-5-3 (Ag/AgCl wire electrode, insulated fatigue test setup) after 2,350,268 cycles to failure.