## 4 Discussion
The scale formed on the surface of the metal phase is dense, i.e., free from discontinuities and demonstrating sufficient adhesion to the substrate, thereby preventing the periodic detachment of the scale from the metal surface. Corrosion is one of the inevitable problems of structural steel exposed to long-term exposure to waste gasses from the incineration of industrial waste. Corrosion is characterized by a uniform loss of the material of the surface layer. Corrosion cracking can only occur when a corrosive environment combines with stresses and tensile forces (stress corrosion) [17]. In Figure 8, an area of a crack with a white circle is indicated. This crack is probably a result of stresses inside the material. Corrosion cracks can occur both along grain boundaries (in the case of lower stresses) and within the material's components (intracrystalline). Several typical stress corrosion cracks in the investigated electrode material are shown in Figure 12. The adsorption of sulfur compounds may occur in these cracks, resulting in their systematic widening (Figure 9). Crack initiation occurs in brittle surface layers and is followed by a gradual expansion of these cracks into the inner zone of the metal. The cracking process is influenced by microstresses, which may result from temperature gradient or structural heterogeneity of the material. Often, in non-austenitic chromium-nickel steel material, the stress corrosion cracking results in the formation of groups of adjacent cracks. At the beginning of crack development, some dislocations and microdeformations can emerge at the surface (cf. Figure 8). The primary factor for crack development is the accelerated penetration of the corrosion environment into the crack crevices. Stress corrosion cracking can occur under the simultaneous action of a corrosive environment and tensile stresses. The microstresses and structural heterogeneity of the material distort the course of cracking. The development of pitting corrosion occurs by increasing the pitting formed in the nucleation process and creating new corrosion zones. In the process of pitting corrosion, three stages can be distinguished: the nucleation of pits on the metal surface, the development of pits, and finally, the perforation element (open pits). The shape and size of the pits as well as volume of removed material change depending on the corrosive conditions. The quantity of material lost at the pitting site is a function of the acidic components and the moisture content [37]. The author of [37] reviewed the critical factors, which influence the pitting corrosion. The factors discussed include temperature, pressure, and surface roughness. The result of the corrosive environment is the formation of open pits, as illustrated in Figure 13. This mechanism suggests that the areas completely devoid of metal in this electrode are caused by acid gasses produced by waste incineration. The removed material accumulates at the edges of the pit in the form of scale or is removed with the flowing gas in the electrostatic precipitator. Two- or multi-layer scales are formed on the surfaces of metals and alloys, where at least one of the layers, usually the innermost, is compact. The scales formed may be either single-phase or multi-phase, and they are compact throughout their cross-section, closely adhering to the substrate [36]. The scale is filled with fly ash particles, which absorbed Hg, As, and Cu compounds, as suggested by EDS measurement (Figure 14). A XRD diffractogram of the phase composition of scales on the electrode is shown in Figure 15.  
Figure 16 shows that either sulfides or oxides are formed as an effect of corrosion, and these compounds are the most abundant in the scales. Figure 16: _Cont._  
Scale formation is also a common phenomenon observed in exhaust gasses. As a result of fly ash deposition on the discharge electrode, the sulfur reacts with the electrode material via \"migrating\" to the structure of the material.