## 2 SFECT Measurements of Corrosion Cracks: Experimental Set-Up  
The experimental setup consists of a Lock-in amplifier (Ametek, Signal Recovery DSP 7280, Wokingham, UK) and a PC (personal computer) with the LabVIEW platform (National Instruments, Austin, TX, USA). The lock-in amplifier is also used as a generator of the excitation signal and, at the same time, as a sensor of the detected signal (digital selective voltmeter). The frequency range of the generated signal is \(f=\) \(<\) 1 kHz; 800 kHz \(>\) with step \(\Delta f=\) 5 kHz. The amplitude of the excitation signal is set to the value \(V=\) 0.1 V. The sweep time is the value during which the probe or excitation coil is excited by a signal with a given frequency. The duration of one frequency sweep is set to \(T=\) 2 s. Then the total time of one measurement is \(T=\) 320 s (160 frequencies times 2 s). The output of the Lock-in amplifier is connected to a PC, using a measuring card on which both analogue and digital inputs and outputs are used. The collection of measured data is carried out on a PC using the LabVIEW program. In LabVIEW, 1000 samples with a sampling frequency \(f_{\text{s}}=10\) kHz are read from the detected signal of one frequency. The algorithm is set so that for each frequency, the collection of the given amount of data is ensured using appropriate synchronization. The specimen values for the real and imaginary components of the induced voltage are averaged and then saved in files. Mathematical operations are subsequently implemented in the Matlab environment. Figure 1 shows a block diagram of the connection of the measuring apparatus. A probe with a ferromagnetic core is used to measure corrosion cracks, Figure 2a. In addition to being placed above the crack, the measurements are also performed on the material without a crack and a separate probe in the air, Figure 2b. The two-coil SFECT probe is used. The first coil generates the electromagnetic field by acting as a transmitter (Tx). The second coil, often known as the receiver, is utilized to detect the response signal (Rx). Table 1 displays the probe's dimensions. Material specimens used for the experiments are made of stainless steel, marked AISI 316L (austenitic stainless steel, without titanium stabilization, low Carbon content). The plates are all the same thickness, \(h=10\) mm. The width of the plate is \(w=115\) mm, and the length is \(l=200\) mm. Each plate contains a corrosion crack, Figure 3.  
Figure 1: Data acquisition and signal processing devices during the SFECT measurements. Figure 2: The probe used in experiments (**a**) and probe positioning (**b**). All examined material samples were prepared in the laboratory of microstructural analysis of materials, Department of Materials Engineering, Faculty of Mechanical Engineering, University of Zilina. Material samples from AISI 316L austenitic steel were prepared in the initial state according to the pre-required geometry. Individual corrosion defects were created by the application of two physical factors at the same time: the application of a static force load of defined parameters and duration and the presence of an aggressive environment, which was a magnesium chloride solution. The combination of these influences led to the formation of corrosion defects in the investigated structures. Destructive methods were used to verify the presence of defects: sample cutting and subsequent microstructural analysis. The presence of corrosion defects, spreading from the surface of the material towards its interior, was verifiably detected in all samples. Shape variability or geometry of the defects as a whole varied depending on the applied physical factors. By comparing the defects induced in this way with real corrosion defects, it was possible to claim that their structure and properties are in good correlation. Based on these conclusions, the presented study was carried out, where the thus, produced samples with the presence of corrosion defects were considered \"real\".