## 3 Results of Experiments and Discussion
### MBN Measurements
An initial gentle decrease in _PP_ for lower stress is followed by a gentle increase for higher stresses, but the evolutions are quite flat, and their sensitivity, especially against tensile stress, is limited. On the other hand, Figure 8 depicts that the differences among _PP_ become more valuable under the tensile stresses as contrasted against unloaded samples. Finally, it can be reported that the gradual growth of _PP_ in RD and TD for MC1100 depicted in Figure 8 can be found approximately at the same tensile stress (about 400 MPa) as compared with the knee in the evolution of MBN shown in Figure 7. In addition, the saturation of MBN in TD seems to coincide with the saturation of _PP_ in TD at higher stresses. A quite interesting evolution of the _FWHM_ of the MBN envelope can be reported with respect to the different microstructures of the investigated steels. The _FWHM_ for all steels grows along with a more developed corrosion attack (see Figure 9). However, the _FWHM_ for S235 drops down (as reported earlier [29]). The _FWHM_ for MC700 drops only gently, or a flat evolution occurs, and finally, this evolution for MC1100  
Figure 8: Evolution of _PP_ versus tensile stress for MC1100. (**a**) RD, (**b**) TD. After an initial, very gentle decrease, the _FWHM_ for MC1100 exhibits moderate growth (see Figure 9). It should be reported that the sensitivity of _FWHM_ against corrosion attack is quite good. However, a lack of sensitivity against the tensile stress can be obtained for MC700 and MC 1100 as contrasted against a valuable decrease in _FWHM_ with tensile stress for S235. The increase in _FWHM_ along with corrosion attack is attributed to the increasing scale (size) of ferromagnetic domains and the corresponding DWs contributing to MBN. The low-amplitude MBN pulses at higher magnetic fields originating from the small un-corroded fragments in the corroded layer are mixed with the larger MBN pulses occurring at the lower magnetic fields originating from the larger corroded particles and/or the underlying uncorroded matrix [14; 15]. More developed corrosion increases the diversity of MBN sources, which in turn increases _FWHM_. The different evolution of _FWHM_ for S235 compared with MC700 and especially MC1100 is driven by the different microstructures of the investigated steels. The SEM as well as metallographic images of the different microstructures of the investigated steels can be found in the previous study [35]. S235 is composed of equiaxed grains in which demagnetizing fields in different directions are nearly the same. Tensile stress aligns DWs in the direction of this stress, and this alignment becomes more developed at higher tensile stresses, which in turn decreases _FWHM_ (conditions for DW's irreversible motion become more similar along their similar directions). However, MC700 is a mixture of equiaxed ferritin grains and plate bainite, and MC1100 is fully composed of plate martensite [16]. The plate characteristics of bainite and martensite preferentially align DWs along the length of these plates as a result of demagnetizing fields [18; 19]. Preferential alignment of the DWs in the unloaded state consumes the aforementioned realignment of the DWs during tensile stressing. For this reason, the _FWHM_ for MC1100 exhibits a reverse evolution as  
Figure 9: Evolution of _FWHM_ versus tensile stress in RD. (**a**) S235, (**b**) MC700, (**c**) MC1100. compared with S235. The evolution of _FWHM_ for MC700 is, therefore, a mixture of those for S235 and MC1100 since the microstructure is also a mixture of equiaxed ferritic grains and plate bainite. The collection of information with respect to the influence of corrosion as well as the superimposing contribution of the stress state is quite complex. For this reason, the summing contribution of corrosion and the stress state on the extracted MBN features is listed in \[\text{MBN (rms)}=C_{g}\ d^{-1/2} \tag{1}\]0  
Table 2 clearly demonstrates the different evolution of the different MBN parameters extracted from the filtered MBN signals.