# Usage of Barkhausen Noise for Assessment of Corrosion Damage on Different Low Alloyed Steels
## 3 Results of Experiments and Their Discussion
### MBN Measurements  
Figure 5 shows evolution of MBN along with the number of days in the chamber. This figure clearly demonstrates that strong sensitivity of MBN against corrosion damage can be found especially in the early phases (low number of days in the chamber) followed by saturation when there is only a moderate decrease of MBN along with number of days. MBN for S235 drops down steeply up to 22 days followed nearly a flat region in RD and well as TD. MBN for MC700 exhibits remarkable decrease in RD and TD up to 11 days followed by moderate decrease in RD (or flat region in TD) and finally more remarkable decrease especially in TD. The best sensitivity of MBN for monitoring corrosion damage can be found for MC1100 in RD as demonstrates Figure 5a. The steep descending region, up to 18 days, is followed by less remarkable, but still valuable decreases in MBN. Decreasing MBN along with especially increasing thickness of the corroded layer (see also Figure 3) is driven by multiple superimposing effects. MBN acquired on the surface of a corroded sample is composed of MBN pulses originating from small non-corroded particles embedded in the corroded layer [9] and MBN pulses produced by the underlying non-corroded matrix. Therefore, MBN decreases along with more developed corrosion damage (the thickness of corroded layer) is a result of:  
1. Corroded layer mostly composed of Fe oxides, which are non-ferromagnetic or hard magnetic phases [9; 10] whose DWs motion cannot be initiated by much weaker magnetic field produced by the employed sensor;  
Figure 5: Evolution of MBN (rms) along the days in the chamber. (**a**) RD, (**b**) TD. Decreases in size of non-corroded particles in the corroded layer producing MBN pulses of lower amplitude [9];
* Increasing gap between the sensor and the non-corroded matrix as a result of attenuation of magnetic field in the sample during surface magnetisation (decreases rate of change of magnetic field in time-d\(H\)/d\(\tau\)[24]);
* Increasing height of surface irregularities and therefore worsen condition for pulses acquisition [20];
* Corroded layer, which can be considered as a barrier which attenuates the MBN pulses originating from non-corroded matrix during their propagation towards the sensor. Figure 5 also shows that MBN emission does not develop in a systematic manner with respect of yield strength. MBN emission can be also affected by grain size and phase transformation (strain induced martensite transformation). For this reason, in this particular case, MBN originating from ferrite of finer grain size is richer as a result of higher density of DWs and the corresponding number of MBN pulses [25; 26]. \[\text{RMS}=\text{C}_{g}d^{-1/2} \tag{1}\]  
where \(d\) is the average grain size and \(C_{g}\) is a material constant. For this reason, MC1100 emits stronger MBN as compared with S235 (finer grain size of MC1100 [22]). One might expect that the MBN for MC700 would be richer as compared with S235 due to the finer grain size. However, the microstructure of the near surface region (lying within the MBN reading depth) is rougher as contrasted against the deeper layer which in turn contributes to the lower MBN, see Figures 5 and 6 (see also Figure 1b). MC700 steel can be considered as a transient state with respect of microstructure when grain refinement is gently beyond the critical threshold and the matrix is composed of a mixture of the fine ferrite grains and the isolated martensite islands. Figures 7 and 8 depict that S235 is softer from magnetic point of view. MBN emission is initiated at lower magnetic field as a result of lower density of pinning sites expressed in term of grain boundary density as well as dislocation density. For this reason, MBN envelopes for S235 can be detected at lower magnetic field (see Figure 7), and therefore, the corresponding \(PP\) are lower as compared with MC700 and MC1100 (see Figure 8). Figures 8-10 also demonstrate that sensitivity of \(PP\), \(FWHM\) as well as the number of detected pulses in TD is very poor.