# Pitting Corrosion of Steel Pipes in Water Supply Systems: An Influence of Shell-like Layer
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{**pH** */**} & \multicolumn{6}{c}{**Concentration [meq/L]**} \\ \cline{3-4}  & & & **H\({}^{*}\)** & **Cl\({}^{-}\)** & **SO\({}_{4}\)2\({}^{-}\)** & **NO\({}_{3}\)\({}^{-}\)** & **Fe2\({}^{*}\)** & **Mg2\({}^{*}\)** & **Ca2\({}^{*}\)** \\ \hline
1 & 3.32/1.63 & 23.93 & 756 & 1.0 & 0.24 & 5.05 & 150 & 550 \\ \hline
2 & 3.73/2.10 & 7.97 & 647 & 0 & 0.83 & 3.59 & 171 & 471 \\ \hline
3 & 3.28/1.65 & 22.46 & 749 & 0 & 0.97 & 5.91 & 170 & 514 \\ \hline \hline \end{tabThe data obtained using the electron microscope show that the main components of the solution are chlorides, nitrates, and calcium. The contribution of each of the other elements is estimated at 0.3-1.5%. The acidic medium of the solution is determined by the presence of hydrogen cations. The analysis of the obtained results, based on the structural features of the tubercles and theoretical aspects of corrosion, allows for the description of the development of pitting corrosion inside water pipes. The formation of tubercles goes with the first phase, which involves fixing the base of the tubercle on the inner surface of the pipe and forming a dense layer firmly adjacent to the metal. In [43; 44], it was observed that the contour of the tubercle on the metal surface is formed by the water flow in the pipe and fixed after 60-70 days. Anodic and cathodic areas appear on the metal surface; afterward, a loose layer of iron hydroxides with immobilized water grows on the anode sites. The corrosion process is controlled by oxygen depolarization. As the oxygen concentration on the metal surface decreases, the redox environment changes inside the scale layer, which is followed by the formation of a dense layer and the tubercle [15]. The dense layer at this stage is relatively permeable to ions and water. The conversion of iron hydroxides into magnetite, which forms a dense layer, proceeds with the dehydration of hydroxides and leads to the appearance of free water inside the tubercle. In the absence of oxygen, hydrogen ions act as the depolarizer. Hydrogen ions are reduced to molecular hydrogen at the cathode sites of the metal inside the tubercle, forming \"chimneys\" in the structure of the tubercle [44]. The medium inside the tubercle is acidified due to the reaction of Fe\({}^{2+}\) hydrolysis. The second phase includes an increase in salt concentration inside the tubercle. According to the previous research results [27; 28], the pit is stable at a very high solution concentration of approximately 3 mol/L. Thus, it is necessary to explain how a dense layer provides not only the transfer of ions and water but also ensures the appearance of a solution with a high concentration. A well-studied method for obtaining highly concentrated solutions under the action of electric current is electrodialysis. As a rule, devices with \"closed\" brine chambers with no external water supply can be applied, while the  
Figure 5: Spectrogram of the elemental composition (in the area marked by a red cross) of desorbed water. concentration occurs due to osmotic and electroosmotic transfer of hydrated ions through selectively permeable anion-exchange and cation-exchange membranes [44]. Since metal serves as an anode for corrosion, anions are required to neutralize the charge of iron cations. Therefore, the dense layer should act as an anion-exchange membrane. Based on the studies of the qualitative composition of desorbed salts, it can be stated that the dense layer has selective properties, preferably passing monovalent chloride ions (Table 3). Thus, an increase in the concentration of iron cations inside the tubercle (because of corrosion) is provided by the transfer of an equivalent amount of hydrated chloride ions (as well as sulfates and nitrates), leading to an increase in the total concentration of the electrolyte and an increase in its aggressive properties.