## 3 Results of Experiments and Discussion
### MBN Measurements
The aforementioned and explained evolution of MBN versus the tensile stress for S235 is valid for all applied elastic stresses. This evolution is nearly the same for the samples being corroded for shorter times (up to 27 days in the chamber) (see Figure 5a). The evolutions are shifted to lower MBN. When the corrosion attack is prolonged, the sensitivity of MBN is lost, and the relationship between MBN and tensile stress becomes flat (see Figure 5a). In addition, the decreasing evolution of MBN versus stress for S235 in TD is apparent for the initial phases of the corrosion attack. However, the lack of sensitivity appears earlier with respect to the number of days in the chamber (see Figure 5b). The flat evolution as that depicted for S235 cannot be found in MC700 as well as in MC1100 (see Figures 6 and 7). Apart from the last sample (69 days in the chamber), all samples exhibit increasing MBN in RD compensated by the decreasing MBN in TD. The increase in MBN in RD and the decrease in TD is more or less constant for MC700 (excluding some evolutions in TD for a longer duration of corrosion attack). On the other hand, the evolutions of MBN in RD for MC1100 exhibit a decreasing gradient, which is more pronounced for the samples being corroded for longer times in the chamber. Heavily damaged samples exhibit a knee in the evolution when the gradient of the increase in MBN drops down. Moreover, a lack of sensitivity can be reported for the samples corroded for 69 days in the chamber beyond the knee (see Figure 7a). The aforementioned knee can be found at a stress of about 400 MPa and appears for all evolutions in TD as well (see Figure 7b) when the decreasing MBN beyond this threshold is replaced by saturation for higher tensile stresses. The lack of sensitivity in TD for higher stresses and/or the decreasing  
Figure 7: Evolution of MBN versus tensile stress for MC1100. (**a**) RD, (**b**) TD. gradient in RD indicates an altering ratio between \(E_{a}\) and \(E_{\sigma}\) when these energies become more balanced. The lack of sensitivity for the evolution of MBN versus tensile stress for the samples being corroded for a longer time in the case of S235 is due to the remarkably higher thickness of the corroded layer on the surface, as reported earlier [16]. The corroded layer on the surface does not bear the tensile stress and acts as a barrier attenuating MBN pulses originating from the un-corroded body. As soon as the corroded layer exceeds the critical thickness with respect to the MBN sensing depth [33; 34], MBN pulses originating from the un-corroded fragments in the corroded layer shadow the contribution of the electromagnetic pulses originating from the uncorroded body. The valuable lower thickness of the corroded layer for MC700 and MC1100 is linked to the later saturation of the evolution of MBN versus stress as contrasted against S235. Finally, it should be mentioned that the heavily corroded samples at the level of the highest nominal elastic stress are yielded since the true stress exceeds the yield strength (valid for S235 and MC700) due to the reduction in the effective wall thickness. For this reason, saturation or the decrease in MBN can be found at the end of the tensile stress due to increasing dislocation density. _PP_ values usually correlate with the mechanical hardness of a body [27]. For this reason, _PP_ grows along with yield strength and the corresponding hardness of the low-alloyed steel. Corrosion on the surface increases _PP_ since the sample behaves as magnetically harder, but this evolution saturates all alloys in RD as well TD. _PP_ for TD is higher as compared with TD since TD is the hard axis of magnetization. The evolution of _PP_ versus tensile stress is quite similar for all investigated alloys, and Figure 8 illustrates an example of the evolution of _PP_ with respect to corrosion attack and superimposing tensile stress. RD exhibits a better sensitivity against corrosion attack as compared with TD.