# Usage of Barkhausen Noise for Assessment of Corrosion Damage on Different Low Alloyed Steels
## 3 Results of Experiments and Their Discussion
### MBN Measurements
On the other hand, increasing \(PP\) for S235 versus the number of days in chamber are a result of increasing opposition against magnetisation process due to presence of corroded layer of increasing thickness on the surface. However, \(PP\) values for thermo-mechanically treated MC700 and MC1100 exhibit poor sensitivity and this feature cannot be proposed for non-destructive monitoring of corrosion damage. Figure 6: Optical images of surface of MC700 steels. Figure 9a demonstrates that _FWHM_ in RD can be considered as a promising MBN parameter for such purpose. _FWHM_ grows for all investigated steels, but the most pronounced increase can be found for S235. Increasing _FWHM_ for S235 depicts also Figure 7b. Progressive grow of _FWHM_ is driven by increasing diversity of DWs being integrated into clustered DWs motion when the differences among the regions contributing to the entire MBN emission increase with corrosion damage due its non-homogenous nature. Figure 8: Evolution of _PP_ along the days in the chamber. (**a**) RD, (**b**) TD. Figure 7: MBN envelopes. (**a**) 1 day in the chamber-RD, (**b**) S235-RD. Figure 10 demonstrates that MBN (its rms values) exhibits good sensitivity against the thickness of corroded layer for all steels in RD as well as TD. As soon as the thickness of corroded layer exceeds a certain threshold (for example 80 \(\upmu\)m in the case S235), a lack of sensitivity can be found. For this reason, the evolution of the extracted MBN parameters indicates that a reliable concept for monitoring of corrosion damage should be based on a combination of _FWHM_ and the effective value of MBN signal when the high MBN and the low _FWHM_ indicate the lower degree of corrosion attack and vice versa (see Figures 10 and 11). Much better sensitivity can be reported for RD in comparison to TD. Corroded layer on the underlying ferromagnetic matrix is a barrier for magnetisation of the sample and transfer of MBN pulses from the untouched surface towards the sensing coil. The corroded layer can contain non-corroded ferromagnetic particles. However, their size as well as the corresponding amplitude of produced MBN pulses drops down along with the increasing degree of corrosion damage due to the increasing fraction of 90\({}^{\circ}\) DWs at the expense of 180\({}^{\circ}\) DWs and the reduced free path of their motion [9]. For this reason, these non-corroded particles cannot fully compensate zero MBN emission originated form the non-ferromagnetic or hard ferromagnetic oxides [10] and MBN emission drops down along with more developed corrosion damage. MBN emission originating from feritic steels is usually higher as compared with martensitic steel [27] as a result of higher dislocation density and high pinning strength of carbides [16; 17] after quenching. However, nearly no precipitates in the form of carbides (as very strong pinning sites) can be found in the low alloyed steels due to the missing elements creating the simple or/and complex carbides (see Table 1). Therefore, the fine-grained structure and the corresponding higher density of DWs in motion can be reported  
Figure 11: Thickness of corroded layers versus FWHM. (**a**) RD, (**b**) TD. Figure 10: Thickness of corroded layers versus MBN (rms). (**a**) RD, (**b**) TD. as the main reason for stronger MBN [28] originating from the martensitic MC1100 as contrasted against the feritic S235. The higher hardness and the corresponding higher dislocation density of MC1100 (as well as MC700) [22] only increases the magnetic field in which DWs are unpinned (_PP_ for MC1100 and MC700 are more as compared with S235, see also Figure 8) but takes only the minor role in the amplitude and density of produced MBN pulses. The MBN technique is very often employed for assessment of stress state of constructions or components [29,30]. It should be noted that the presence of corroded layer on the surface would make difficult assessment of true stress since the contribution of stress is mixed with the contributing role of the corrode layer varying in thickness.