## 3 Results of Experiments and Discussion  
### Wall Thickness  
Figure 3 illustrates two different aspects of corrosion attack. The first one is associated with the increasing height of surface irregularities. The surface becomes rougher as a result of the heterogeneity of corrosion attack. The electrochemical process in the neighboring regions makes certain regions relatively anodic or cathodic to the neighboring ones. Moreover, this anodic-to-cathodic relationship can be exchanged as a result of the different elemental compositions. For this reason, the uncorroded regions previously unaffected by corrosion can be attacked. For these reasons, corrosion attack is strongly heterogeneous, which in turn contributes to the variable surface height, as depicted in Figure 3. It was previously reported that progressively developed corrosion attack remarkably increases \(Sa\), \(Sq\), and \(Sz\)[15].  
The second aspect of corrosion attack is associated with a reduction in the effective wall thickness (see Figures 3 and 4). Figure 4 clearly demonstrates that the effective wall thickness is progressively reduced, and the gradient of this reduction is kept nearly constant within the investigated duration of the corrosion attack. Moreover, Figure 4 also illustrates that the reduction in the effective wall thickness is less for MC1100 when contrasted against MC700 and S235 steels. This finding proves that the resistance against corrosion in the case of high-strength low-alloyed steels is better than that of lower-strength steels [1; 2]. This information also proves the previous study [16], which clearly demonstrates that the thickness of a corroded layer on an un-corroded matrix is highest for S235, lower for MC700, and lowest for MC1100.  
Figure 2: The brief diagrammatic sketch of equipment and extracted information.  
Comparing the evolution of the wall thickness reduction (see Figure 4) and the evolution of the thickness of the corroded layer, some differences can be found. The differences among the steels with respect to the thickness of the corroded layer are more pronounced. The evolution of the effective wall thickness, along with the number of days in the chamber, is different. This evolution exhibits continuous growth with a nearly constant gradient, whereas the thickness of the corroded layer increases remarkably within the initial phases of the corrosion attack, followed by its saturation [16]. An explanation of this controversy can be found in the removal of poorly adhered corrosion residues from the surface af  
Figure 3: Cross-sectional views of the steels before and after corrosion attack. (**a**) S235—bulk, (**b**) S235—after 69 days, (**c**) MC700—bulk, (**d**) MC700—after 69 days, (**d**) MC1100—bulk, (**f**) MC1100—after 69 days.  
ter a corrosion attack. Corrosion damage penetrates deeper and deeper along with the prolonged time period within the matrix exposed to the aggressive salt bath. On one hand, the corroded surface layer, which remains on the surface after cleaning, saturates in thickness, whereas the thickness of the corroded layer being removed during the cleaning continuously grows.  
Standard deviations of the measured effective wall thickness increase along with the prolonged corrosion attack as a result of the increasing height of surface irregularities. Reduced wall thickness should be taken into account with respect to the true stresses exerted during uniaxial tensile stressing of the corroded samples. For this reason, the information in Figure 4 is employed for the calculation of true stresses, as analyzed in the next chapter. Metallographic details in which the corroded layer can be distinguished from unaffected matrix can be found in the previous study [16]. SEM observations as well as the chemical analyses of the corroded layer have also been reported earlier [14].