## 3 Results of Experiments and Discussion
### MBN Measurements  
It has already been reported that corroded bodies emit lower MBN [14; 15; 16]. The physical background of this behavior has been explained as a result of different effects and their superimposing contributions. A brief list can be summarized as follows:  
* The corroded layer on the surface makes a weaker magnetic field in the un-corroded matrix;
* The corroded layer attenuates electromagnetic pulses propagating toward the free surface;
* Increasing the height of surface irregularities also makes the transfer of electromagnetic pulses toward the sensing coil more difficult [31];
* Finally, the corroded layer usually contains un-corroded fragments whose size drops down along with prolonged corrosion attack [14; 15]. Figures 5-7 prove the aforementioned behavior. MBN in RD and TD drops down along with the longer time in the chamber. MBN in RD is mostly greater as compared with TD. It is only in the region of the lower magnitudes of the tensile stresses that MBN values are more balanced and the magnetic anisotropy expressed in terms of MBN is lost. Figure 4: Reduction in wall thickness as a function of days in chamber. The evolution of MBN with respect to different yield strengths is not systematic. MBN for S235 is greater when compared with MC700, and MBN for MC700 is less than that for MC1100. On the other hand, it can be seen that MBN in TD for MC700 is more than that for MC1100 at the higher tensile stresses, especially for the heavily corroded samples (see Figures 6b and 7b). The main reason for this can be viewed in the near-surface region, which can be more or less affected by the hot rolling process and cooling conditions, as discussed in the previous study [16]. The valuable heterogeneity in the microstructure with respect to the different layers beneath the free surface is due to the hot rolling process, and this heterogeneity is most apparent in the case of MC700 [16]. The grain size in the near-surface with a thickness of about 0.1 mm is larger when contrasted against the deeper regions, and the hardness is less as a result of thermal softening. MBN of the low-alloyed steels is very often driven by grain size and the corresponding density of DWs [32]. For this reason, MBN can be expressed as follows:  
\[\text{MBN (rms)}=C_{g}\ d^{-1/2} \tag{1}\]  
where \(C_{g}\) is the grain size and (\(d\)) is a dependent constant. Having in mind Equation (1), it can be easily understood that a rougher grain size results in a lower MBN. Figure 5: Evolution of MBN versus tensile stress for S235. (**a**) RD, (**b**) TD. Figure 6: Evolution of MBN versus tensile stress for MC700. (**a**) RD, (**b**) TD. Figures 5-7 demonstrate that MBN grows along with increasing tensile stresses at the expense of decreasing MBN in TD. This is a result of magneto-elastic coupling and the positive magnetostriction of Fe alloys [17,19]. The increase in MBN in RD (along the direction of tensile stress) is due to the predominance of the energy of magnetocrystalline anisotropy, \(E_{a}\), over the magnetoelastic energy, \(E_{\sigma}\). \(E_{a}\) depends on the magnetocrystalline anisotropy, and for the bodies of the \(bcc\) lattice, it can be expressed as follows [17, 18, 19]. \[E_{a}=K_{1}({\alpha_{1}}^{2}{\alpha_{2}}^{2}+{\alpha_{2}}^{2}{\alpha_{3}}^{2}+ {\alpha_{1}}^{2}{\alpha_{3}}^{2})+K_{2}({\alpha_{1}}^{2}{\alpha_{2}}^{2}{ \alpha_{3}}^{2}) \tag{2}\]  
where \(\alpha_{1}\), \(\alpha_{2}\), and \(\alpha_{3}\) are the direction cosines of the magnetization vector with respect to three cube edges, while \(K_{1}\) and \(K_{2}\) represent magnetocrystalline anisotropy constants. \(E_{\sigma}\) strongly depends on magnetostriction and can be expressed as follows [17, 18, 19]:  
\[E_{\sigma}=(-3\lambda_{s}cos^{2}\varphi)/2 \tag{3}\]  
where \(\lambda_{s}\) is the isotropic magnetostriction and \(\varphi\) defines the angle between the direction of magnetization and the direction of the exerted stress \(\sigma\).