## 3 Results
### Pitting Corrosion  
Pitting corrosion is a local, accelerated dissolution of metal, which occurs as a result of the degradation of a protective passive layer on the metal surface [17]. This type of corrosion occurs only in specific areas in the form of spots or pits, often penetrating deep into the material. Pitting corrosion involves the formation of local pits due to the initiation of an anodic reaction by activating ions and a cathodic reaction in the presence of oxidizing agents. The bottom of the pit acts as the anode, and the metal dissolution takes place there. The walls surrounding the pit function as the cathode, and the oxygen reduction occurs [26]. Pits are a result of the reaction of metal with a corrosive agent on the metal surface. Figure 10 shows cylindrical losses of the material, which are examples of pits. In acid corrosion, iron in a dilute solution of hydrochloric or sulfuric acid undergoes a series of reactions [26]:  
\[\left(\text{Fe}\right)_{metal}+\left(2\text{HCl}\right)_{liquid} \longrightarrow\left(\text{FeCl}_{2}\right)+\left(\text{H}_{2}\right)_{gas}\]  
or  
\[\left(\text{Fe}\right)_{metal}+\left(\text{SO}_{4}\right)_{liquid} \longrightarrow\left(\text{FeSO}_{4}\right)+\left(\text{H}_{2}\right)_{gas}\]  
where the subscripts _metal, liquid_, and _gas_ indicate the following phases: solid metal, liquid aqueous solution, and gaseous, respectively. The amount of material loss in pits is a function of the time of exposure to the corrosive agent. In the presence of oxygen and moisture, a sequence of interdependent reactions of iron oxidation to Fe\({}^{3+}\) and rust formation occur. The corrosion of materials depends on the type of electric conductivity at the material-environment interface, and on the type of environment [27]. Pitting corrosion is a local form of corrosion, which occurs on passivated surfaces. The starting point of destruction may be the inclusions or grain boundaries on the surface. The aggressive anions penetrate through such places in the metal and create small cavities. The surface adjacent to the pitting corrosion places remains intact. The rate of pitting corrosion can be high due to the change in local chemical composition inside the pits [28]. The selected pit was mechanically cut in a direction perpendicular to the plane of the pit, and the elemental composition of three distinguished areas was examined in Figure 11.  
Figure 9: EDS spectrum of cracked discharge electrode material (inset: SEM image of analyzed sample). Figure 11: SEM micrographs of cross-section of ESP discharge electrode (**left**) and EDS spectrum of selected places (**right**): (**a**) electrode fragment, cross-section of ESP discharge electrode (measurement area marked with circle 1); (**b**) close-up view of inner surface of pitting (scanning area marked with rectangle), inner surface of pitting in ESP discharge electrode. Figure 10: (**a**) Image of various forms of pitting on the surface of a degraded metal electrode; (**b**) 3D profile (photos taken under a 3D profilometer Keyence VR-6200; courtesy of Keyence (Japan)). Figure 11 shows a SEM micrograph of a cross-section of the ESP discharge electrode with a pitting. The elemental composition of the electrode was examined using the EDS method in the following areas: the original material (Figure 11a) and the inner surface of the pitting (Figure 11b). In comparing the elemental compositions of these different areas, it can be noticed that in the pit area, the content of basic metal components (iron, chromium, nickel) decreases, while the content of sulfur and oxygen increases. The results are confirmed by XRD measurements of the collected scale (cf. Section 3.7). As a result of corrosion, metal oxides (Fe\({}_{2}\)O\({}_{3}\), CrO\({}_{2}\)) or metal sulfides (FeS, Ni\({}_{3}\)S\({}_{2}\), Cr\({}_{5}\)S\({}_{8}\)) are formed.