# Usage of Barkhausen Noise for Assessment of Corrosion Damage on Different Low Alloyed Steels
## 2 Materials and Methods  
Three different commercial low alloyed steels of nominal yield strength 235, 700 and 1100 MPa were employed in the study. A total of 32 samples of size 25 \(\times\) 50 mm of each steel were cut from sheets of thickness 5 mm (6 mm in the case of MC1100). The sheets of yield strength 700 and 1100 MPa were produced by hot rolling (further details are unknown). Chemical composition of all steels is indicated in Table 1.  
The matrix of the S235 steel is entirely composed of ferrite grains and limited volume of pearlite. Hot rolling initiated the fine-grained structure with certain volume of strain induced martensite phase in the case MC700 whereas MC1100 steel is entirely composed of martensite phase. Microstructural images of investigated steels can be found in Figure 1. The average grain size obtained from the EBSD (electron back scattering diffraction) observations is 22 \(\upmu\)m for the S235, 15 \(\upmu\)m for the MC700 and 11 \(\upmu\)m for MC1100 [22]. The samples were subjected to variable degree of corrosion attack. The samples were corroded in the neutral salt spray atmosphere with respect of STN EN ISO 9227 standard [23] in the VSC KWT 1000 chamber (temperature of 35 \({}^{\circ}\)C, pH in the range from 6.5 to 7.2, pressure in the chamber 120 kPa). The samples were cleaned before the experiment and placed into the chamber by the use of plastic stripes. Certain fractions of the corroded layer were removed from the surface during the cleaning. However, the corrosion products firmly connected with the underlying matrix \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Yield Strength (MPa)** & **Trade Mark** & **Fe** & **C** & **Mn** & **Si** & **P** & **S** & **Al** & **Nb + Ti** \\ \hline
235 & S235 & bal. & 0.22 & 1.6 & 0.05 & 0.05 & 0.05 & - & - \\
700 & MC700 & bal. & 0.05 & 1.8 & 0.02 & 0.01 & 0.002 & 0.034 & 0.21 \\
1100 & MC1100 & bal. & 0.15 & 1.8 & 0.5 & 0.02 & 0.005 & 0.15 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of investigated steels in wt%. Figure 1: Microstructural images of investigated steels. (**a**) S235, (**b**) MC700, (**c**) MC1100. can be found on metallographic images. The cleaning of the surface was carried out in the manner as that potentially employed before MBN measurements on the real components. The samples were exposed to the different corrosion attack linked with the days in the chamber as it has been indicated in Table 2.  
For each degree of corrosion, Attack 2 samples were analysed. Loose corrosion residues were manually removed from the surface of each sample and subsequently the surface of samples was water cleaned and dried. MBN was measured by RollScan 350 in MicroScan 600 software under magnetising voltage \(16\) V and frequency 125 Hz of sine profile (the corresponding magnetising field altered \(\pm\) 9.87 kA.m\({}^{-1}\)). Sampling frequency of acquired MBN emission was 6.4 MHz. Due to magnetic anisotropy originated from sheets rolling MBN emission was measured in the rolling direction (RD) as well as transversal direction (TD). RD direction represents the easy axis of magnetisation whereas TD represents the perpendicular hard axis (due to the lower MBN in TD as compared with RD). MBN signal was acquired in the frequency range from 10 to 1000 kHz by the use of serial sensor S1-18-12-01. MBN refers to the rms value of the signal. The \(PP\) (peak position) feature is linked with magnetic field in which MBN envelope attains the maximum. _FWHM_ of MBN envelope is the full width of MBN envelope at half of its maximum. In order to analyse the thickness of corroded layer, about 20 mm long pieces were cut along RD (in Struers Sectom-50). The samples were tapped by red tape in order to avoid massive delamination of corroded layer from the surface during cutting. Small pieces were subsequently hot moulded, grounded, polished, and finally etched by 3% Nital. Optical images of corroded samples were observed in Neophot 2 microscope by the use Niss Element software.