# Usage of Barkhausen Noise for Assessment of Corrosion Damage on Different Low Alloyed Steels
## 3 Results of Experiments and Their Discussion  
### Observations of Corrosion Damage  
Visual observation of corroded samples showed that corrosion attack is not developed in the same rate with respect of the different steels. It is worth mentioning that evolution of corrosion is strongly heterogeneous with respect of the different microanodic and microachodic regions within one sample [1; 9]. For this reason, corroded region neighbouring with non-corroded one or/and extent of corrosion towards the deeper layers strongly varies. Figure 2 indicates the heterogeneity and coverage of corrosion products within the sample's surface in a detail after various exposure times in neutral salt spray atmosphere. All the surfaces of tested steels were completely covered by various types of iron oxides, hydroxides and oxohydroides (proved by different colours and structures of corrosion product) even after 11 h of exposure in salt spray atmosphere (Figure 2d-f) as a result of high humidity, temperature, presence of oxygen and chloride ions accelerating the corrosion process. Depending on local thermodynamic conditions changing over time within the corrosion process, different types of corrosion products are formed or transformed locally on the sample's surface explaining the mentioned heterogeneity. However, as the heterogeneity of corrosion products is very similar within all the tested samples and all the surfaces were completely covered by corrosion products even after short exposure times, the testing method of surface evaluation by visual surface inspection cannot be used as a relevant non-destructive tool for assessing the extent of corrosion attack. Real differences in corrosion extent are visible from the cross section of individual samples (Figure 3) and interpretation of corrosion products thickness growth is presented in Figure 4. This different corrosion rate is related to various microstructures, chemical \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline
1 & 2 & 4 & 6 & 8 & 11 & 14 & 18 & 22 & 27 & 34 & 41 & 48 & 55 & 61 & 69 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Number of days in the chamber. compositions and performed hot rolling as all the other conditions were the same for all the samples. However, it is not possible to use cross-section analysis in real conditions, hence, another non-destructive technique, such as Barkhausen noise emission should be employed to assess the real corrosion extent. Figure 2: Images of corroded surface. (**a**) S235—1 day, (**b**) MC700—1 day, (**c**) MC1100—1 day, (**d**) S235—11 days, (**e**) MC700—11 days, (**f**) MC1100—11 days, (**g**) S 235—27 days, (**h**) MC700—27 days, (**i**) MC1100—27 days, (**j**) S235—69 days, (**k**) MC700—69 days, (**l**) MC1100—69 days. Cross sectional images in Figure 3 demonstrate that the thickness of corroded layer on the samples increases steeply especially in the early phases of corrosion attack and saturates later, see also Figure 5. These figures also depict that corroded layer, which appears dark (grey) on the images, contains in many cases the uncorroded fragments, which appear  
Figure 4: Evolution of the thickness of the corroded layer. Figure 3: Cross sectional metallographic images of corroded, RD. (**a**) S235—1 day, (**b**) MC700—1 day, (**c**) MC1100—1 day, (**d**) S235—11 days, (**e**) MC700—11 days, (**f**) MC1100—11 days, (**g**) S 235—27 days, (**h**) MC700—27 days, (**i**) MC1100—27 days, (**j**) S235—69 days, (**k**) MC700—69 days, (**l**) MC1100—69 days. white (the same colour as the uncorroded matrix) [9]. Figures 3 and 4 also demonstrate that the thickness of corroded layer strongly varies and the regions, in which the corroded layer is quite thick neighbouring with those of limited thickness or/and free of the corroded layer as a result of complex nature of corrosion process in which small regions exposed to the corrosion attack become relatively anodic or cathodic. For this reason, the thickness of corroded layer in some regions is increasing at the expense of the neighbouring ones.