## 2 Experimental Conditions  
The investigations were conducted on the three different low-alloyed steels of nominal yield strength: 235, 700, and 1100 MPa. Twenty-four samples of each steel (3 repetitive samples for 8 different corrosion attacks), 200 mm long, 20 mm wide, and 5 mm thick, were cut along the rolling direction (RD) of the delivered sheet thickness of 5 mm. The thickness of samples made of MC1100 was 6 mm. The sheet of yield strength 235 MPa represents a conventional low-strength low-alloyed steel that is widely used and commercially available. The sheets with yield strengths of 700 and 1100 MPa were produced by hot rolling and consecutive accelerated cooling. The microstructure of MC700 is a mixture of ferrite and bainite. Apart from ferrite grain refinement and the presence of the bainite phase as the major aspects linked with high strength, the matrix is also strengthened by Nb and Ti precipitates [1; 2]. The microstructure of MC1100 is fully composed of martensite without additional strengthening by Nb or Ti. The martensite microstructure is due to an accelerated cooling rate after hot rolling, which is higher than that of MC700. The chemical composition, as well as the microstructure images of these steels, can be found in the previous study [16]. The true mechanical properties were verified by the standardized uniaxial tensile test, as illustrated in Figure 1, on dog-bone-shaped samples (gauge length of 40 mm and width of 12 mm). The true elastic strains were checked by the use of an Instron dynamic strain gauge extensometer 2620-602 on the length of 25 mm. The tensile tests, as well as the loading of the samples after corrosion attack, were performed using an Instron 5985 device. The true yield strength of S235 was 312 \(\pm\) 6 MPa, the ultimate strength was 406 \(\pm\) 7 MPa, and the elongation at break was 39 \(\pm\) 1.5% (obtained from three repetitive measurements). The true yield strength of MC700 was 764 \(\pm\) 23 MPa, the ultimate strength was 829 \(\pm\) 12 MPa, and the elongation at break was 24 \(\pm\) 1.2%. The true yield strength of MC1100 was 1232 \(\pm\) 15 MPa, the ultimate strength was 1333 \(\pm\) 12 MPa, and the elongation at break was 12 \(\pm\) 0.6%. The samples were subjected to corrosion attack, which varied in terms of the number of days in the corrosion chamber (see Table 1). The specimens were artificially corroded in a neutral salt spray atmosphere (in accordance with the STN EN ISO 9227 standard) in a VSC KWT 1000 corrosion chamber (temperature of 35 \({}^{\circ}\)C, pH in the range from 6.5 to 7.2,  
Figure 1: Engineering stress–strain curves of investigated alloys. pressure in the chamber of 120 kPa). The specimens were cleaned before the experiments and placed into the chamber with the use of plastic strips. For each steel, one sample without corrosion attack was measured as well (as received state). Loose corrosion residues were manually removed from the surface of each sample by a wire brush, and subsequently, the surface of the specimens was water-cleaned and dried. In order to analyze the wall thickness as a result of corrosion attack, 20 mm long pieces were cut along RD after experiments by the Struers Secotom-50. The specimens were taped with tape in order to avoid delamination of the corroded layer from the surface during cutting. The cut specimens were hot-molded, ground, polished, and finally etched with 3% Nital. The wall thickness was observed and measured in five different positions using the light microscope Zeiss AxioCam MRc5 in Olympus SZx16 and Quick Photo Industrial 3.0 software. The MBN signal was acquired using a RollScan 350 in situ of the tensile test. The tensile stress was increased in steps of 50 MPa up to 300 MPa, followed by increments of 100 MPa afterward. MBN was measured within the elastic region (on the stress-strain curves) along the length of the sample (RD) as well as in the transversal direction (TD) using a serial sensor S1-18-12-01. Measurements of MBN in two perpendicular directions were carried out by magnetizing pole rotation.