## 4 Discussion  
The investigation of discharge electrode degradation in an electrostatic precipitator was carried out. Given the lack of knowledge regarding the chemical composition and PM content of the exhaust gasses from an industrial waste incinerator, which can vary significantly, the physical and chemical causes of electrode degradation can be reconstructed ex-post and deduced from the traces of elements remaining at the electrode material only, or via a comparison of the effects of similar phenomena observed in laboratory experiments. In the literature, only the problems of corrosion affecting boilers and heat exchangers, or exhaust pipes, are usually discussed. In these cases, the discussed processes involve high-temperature corrosion (from the boiler side) [31; 32] or low-temperature corrosion (from the gas outlet side) [33; 34]. In coal-fired boilers, the gas composition may vary slightly, but in the case of waste incineration plants, the flue gas composition may change depending on the type of waste being burned. In the case of industrial waste incineration plants, the fuel components may include oil sludge, electronic parts, rubber, soil contaminated with pesticides, etc. Given the diverse nature of these components, it is challenging to ascertain the precise mechanisms, through which the electrostatic precipitator corrosion processes may occur. There is an ongoing discussion in the literature regarding the possible mechanisms of corrosion and the influence of individual components in the flue gas on this process [31]. A review of the literature reveals the absence of information about the course of corrosion in exhaust gas treatment systems, where the temperature should be above the acid dew point. In a solid waste incinerator, in which municipal wastes are used as a fuel, the corrosion of steel by chlorine and chlorides plays the main role. The corrosion process due to  
Figure 15: XRD diffractogram of scale and deposit of blade electrode. the reaction of chlorides with oxides from the protective layer of the electrodes occurs as follows:  
\[2\text{NaCl}+\text{Fe}_{2}\text{O}_{3}+\frac{1}{2}\text{O}_{2} \longrightarrow\text{NaFe}_{2}\text{O}_{4}+\text{Cl}_{2}\]  
\[4\text{NaCl}+\text{Cr}_{2}\text{O}_{3}+\frac{5}{2}\text{O}_{2} \longrightarrow 2\text{Na}_{2}\text{Cr}\text{O}_{4}+2\text{Cl}_{2}\]  
And the effect of chlorine (formed through the oxidation of HCl) [35] occurs as follows:  
\[\text{Fe}+\text{Cl}_{2}\longrightarrow\text{FeCl}_{2}\]  
Due to relatively high vapor pressure at high gas temperatures, a continuous evaporation of \(\text{FeCl}_{2}\) takes place, and metal chlorides diffuse outward from the scales. At the temperature of the flue gas in the electrostatic precipitator, this reaction could be negligible. The \(\text{FeCl}_{2}\) reacts with \(\text{O}_{2}\), resulting in \(\text{Fe}_{2}\text{O}_{3}\) formation with \(\text{Cl}_{2}\) gas emission:  
\[4\text{FeCl}_{2}+3\text{O}_{2}\longrightarrow 4\text{Cl}_{2}+2\text{Fe}_{2} \text{O}_{3}\]  
For this reason, chlorine has not been detected in the corrosion layer with the EDS method. In another way, the corrosion caused by \(\text{SO}_{2}\) occurs at low temperatures of exhaust gasses. EDS and RDX analyses indicated that the sulfur phases occur on the discharge electrodes. General sulfur corrosion reactions could occur, forming metal salts [26]. Most corrosion processes involving water and aqueous solutions, called electrochemical corrosion, which might occur after water or acid condensation onto the electrodes, can be explained by the formation of corrosion microcells onto the surface. Local damage can lead to the perforation of the metal parts [36]. If a metal material is exposed to an environment with oxidizing properties at elevated temperatures, the reaction products are formed and form solid products called scales (cf. Figure 12), mainly on the surface of the material.