## 4 Discussion  
As expected, the modification of surface roughness has a significant influence on the measured signal, regardless of the applied magnetic method. The first and important finding is that the two methods yield very similar correlation between magnetic parameters and roughness parameter. This is a good result, because it offers a fast and reliable electromagnetic method, for quantitative determination of the surface condition. On the contrary, this effect is harmful from point of view of magnetic NDE, because the influence of the surface suppresses the influence of material degradation. The utility of magnetic measurements in non-destructive testing is supported by the fact that very similar result was found by applying two, very different methods. Figure 6: The normalized MAT descriptor with two different slopes of magnetizing current (**a**), BN signal response in transversal (Tx) and in longitudinal (Lx) orientation (**b**) and comparison of transversal BN signal and MAT descriptor wit 0.5 A/s slew rate of magnetizing current (**c**) as functions of the correlation with \(R_{a}\) surface roughness parameter. In case of Barkhausen noise measurement a well detectable difference is observed when the magnetizing field is perpendicular or parallel to sample orientation. This result perhaps can be interpreted if we take into account the orientation of manufactured surface grooves. If the magnetizing field is perpendicular to the grooves (as in case of BNLx measurement), larger influence of surface roughness can be detected, than in parallel orientation. The reason is that MBN method is sensitive only to the surface region. By applying the MAT method, we cannot observe this difference, because in this case a larger volume of sample is magnetized and the source of different magnetic behavior is the effective air gap between magnetizing yoke and sample surface. MAT seems to be more sensitive to the modification of surface condition than magnetic Barkhausen noise measurement if the slew rate of magnetizing current is lower. MBN method seems to be more suitable to reducing the harmful influence of surface quality condition. It is an important observation that by increasing the speed of magnetizing current, the dependence of MAT descriptors on the surface roughness parameter can be reduced. This result is new, compared with previous measurements. In short, if we focus on the determination of the surface roughness itself, slow measurement should be applied, while if we are interested in detection of material changes due to any type of degradation, faster measurement is preferred approach. It is worth mentioning that by using fast MAT measurement up to a certain value of surface roughness (about \(R_{a}\) = 0.35 \(\upmu\)m), the influence of surface roughness on the magnetic descriptor is almost negligible, in spite of the fact that in permeability loops, especially considering their peak values, the difference is clearly seen (see Figure 6). This fact can also be observed in the case of MBN measurements, with reference to the transversal measurements (see Figure 7). The good correlation between MAT and MBN measurements is encouraging. For MAT measurement, increasing the speed of magnetization by ten times results in quasi static type excitation too, since reaching above 1.75 A peak current by 0.5/s speed means 3.5 s of half-period, i.e., 7 s of reverse magnetizing cycle period. Therefore, the skin effect, i.e., the reduction of the penetration depth, can be still considered as negligible; otherwise, this skin effect could emphasize the relevance of the surface layer and could act in contrary to our presented observation. Considering the reduced magnetic coupling (or the equivalent air gap) induced by the surface roughness is independent of the speed of reverse magnetization, the reason of our novel observation can be found in reduction of surface layer relevance for the recorded magnetic response.