# Pitting Corrosion of Steel Pipes in Water Supply Systems: An Influence of Shell-like Layer
## 3 Results and Discussion
From the theory of electrodialysis, it is known that the concentration of fixed ions in the membrane structure limits the maximum concentration of brine on electrodialysis devices [45, 46]. It was experimentally established that the concentration of the salt solution, which can be achieved during membrane concentration, grows within the increase of current density and membrane selectivity. This corresponds to the studies of [PERSON] and [PERSON] on the dependence between the electrolyte concentration inside the pit and the current density [27]. It can be assumed that the high concentration of the solution in the tubercle is also associated with the concentration of fixed charges in the dense layer. Fixed ions are embedded in the voids of the crystal lattice of a dense layer that occurs during its formation. At the same time, the high concentration of fixed cations in the dense layer prevents the access of iron ions into the water flow outside the tubercle. Thus, the total concentration of electrolyte in the structure of the tubercle can reach high values, as follows from the data in Table 3.  
Obviously, the permeability of a dense layer is developed within its formation during the transfer of hydrated anions to a soluble anode. At the same time, the pressure inside the tubercle increases due to the electroosmotic transfer of water, contributing to the dense layer growth and, in some cases, its destruction. The anion selectivity of the dense layer can be confirmed based on the comparison of the quality of the water sample with desorbed salts and the water sample in the water supply system during pipe operation. For example, in Moscow, chloride content in tap water is no more than 10% of the total anionic composition, while chloride content in desorbed salts exceeds 99%. According to the above, the selective properties in a dense layer can be confirmed by the concentration of chloride ions in the desorbed solution and the inhibitory effect of individual anions on the potential of pitting formation. Overall, corrosion rate can be influenced by introducing various inhibitors [29, 47, 48]. The third phase includes the processes preceding the formation of a fistula. This stage can be represented as a complex effect on the metal surface of a multitude of pits that form on a limited metal surface directly under a dense layer of tubercle. At this stage, the tubercle stops growing and goes into the mode of autocatalytic action. The corrosion inside the tubercle intensifies as the metal is now in contact with a concentrated solution of FeCl\({}_{2}\) instead of tap water. Corrosion is supported mainly by water, which is located directly in the volume of the tubercle. At the last stage, the role of hydrolysis of iron chloride with the formation of H\({}^{+}\) and OH\({}^{-}\) ions should be noted. They inhibit the growth of iron concentration, creating crystalline forms of Fe(OH)\({}_{n}\). On the other hand, the reduction of H+ ions to hydrogen gas at the cathode contributes to sealing the pores of a dense layer due to the large size of gas bubbles, which exceed the pore size. Thus, the condition for the fistula emerging is an amount of water within the tubercle core that is sufficient for the oxidation of iron ions, considering the remaining thickness of metal. In case of a shortage of water, separate pits and the overall tubercle are repassivated. Therefore, pipes with a thicker wall (cast iron pipes, for instance) are more resistant to corrosion. When analyzing damage to the pipe walls caused by fistulas, attention should be paid to the formation of a conical crater on the inner wall of the pipe, with a pit in the crater center. If the high concentration of chlorides causes the formation of a fistula, the presence of the hollows on the walls of this crater may be expected, eventually reaching the outer boundary of the pipe wall and forming a fistula. However, the analysis of the crater surface did not confirm such expectations.