## 2 SFECT Measurements of Corrosion Cracks: Experimental Set-Up
To evaluate the SFECT application of the investigation of corrosion inhomogeneities, it is necessary to ensure that the corrosion processes take place under quantifiable conditions. Initial conditions were determined for each corrosion crack. The corrosion process itself was spontaneous, i.e., we can consider these material specimens highly similar to the real cracks. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Attribute** & **Symbol** & **Value** \\ \hline Rx width & \(w_{\text{Rx}}\) & 0.7 mm \\ Rx height & \(h_{\text{Rx}}\) & 2 mm \\ Rx radius & \(r_{\text{coil}}\) & 1.4 mm \\ Rx number of turns & \(N_{\text{Rx}}\) & 140 \\ \hline Tx width & \(w_{\text{Tx}}\) & 0.5 mm \\ Tx height & \(h_{\text{Tx}}\) & 2 mm \\ Tx radius & \(r_{\text{coil}}\) & 1.4 mm \\ Tx number of turns & \(N_{\text{Tx}}\) & 80 \\ \hline Tx-Rx distance & \(h_{\text{z}}\) & 0.8 mm \\ Core height & \(h_{\text{c}}\) & 5.7 mm \\ Core radius & \(r_{\text{c}}\) & 0.55 mm \\ Core relative permeability & \(\mu_{\text{core}}\) & 13,000 \\ Shielding width & \(w_{sh}\) & 1 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The SFECT probes dimensions and properties. Figure 3: Microscopic images of the inspected AISI 316L corrosion cracks: cross -section (**a**) and the view from above (**b**). The formation of a corrosion crack takes place in a corrosion solution of magnesium chloride (MgCl\({}_{2}\)). Magnesium chloride is a white hygroscopic substance. It is a typical ionic halide, well soluble in water. The plates were immersed in this solution for \(t_{1}\) = 6 h, \(t_{2}\) = 12 h and \(t_{3}\) = 24 h. A static force acts on the plates immersed in the solution. When preparing the specimens used in the measurements, three static force values are precisely defined. The first defined force is \(F_{1}\) = 1.2 kN, the second \(F_{2}\) = 1.5 kN and the third \(F_{3}\) = 1.8 kN. In this way, we have created a standard of material specimens of corrosion cracks with which we can compare the parameters of their formation. In Table 2 are all formation times and applied static forces of all corrosion cracks on which measurements are made. There are 7 specimens prepared and used for measurement experiments. Because with real cracks, it is not possible to determine the exact position or dimensions of the crack, it is necessary to define the area of interest in which the measurements will be made, Figure 4a. The area that is delineated consists of various measuring points where measurements are carried out step by step because the SFECT method uses a statically located probe. Eleven measurement points are chosen for measurement, which creates a regular measurement network. The distribution of points is shown in Figure 4b. The points are marked with numbers from 1 to 11. Each of the points is distant from the neighboring point a = 5 mm. Point 10 is equidistant from point 2 by a = 5 mm; points 10, 1 and 3 then form an isosceles triangle, similarly points 7, 9, and 11. Points 10 and 1 are then b = 7.07 mm apart. When measuring cracks, the position and distance of the points did not change. Point 5 is always axially above the area with the presumed crack.