# Pitting Corrosion of Steel Pipes in Water Supply Systems: An Influence of Shell-like Layer
## 1 Introduction
The sum of the cations is 42.977 meq L\({}^{-1}\), and the sum of the anions is 56.718 meq L\({}^{-1}\). The inequality of the sums of anions and cations indicates that the steady water may contain other unaccounted cations. According to [19], the pH of the solution inside the pit should have a more acidic environment; therefore, a pH of 5.84 means that selected samples of steady water were partially diluted with the tap water. The undefined content of sulfates in the source water may be an indirect confirmation of the predominant chloride transfer. In [19], the authors identified and studied two types of water: steady (near the tubercles) and occluded (inside the tubercles). The data on the chemical composition of both water types are generally consistent with the studies [20]. The authors also found a lower concentration of nitrates in steady water under stagnant conditions, which is explained by the reaction of nitrates with dissolved iron and the formation of nitrites and ammonium ions. The same effect was described in [21]. The above results of the steady water study reveal that the solution located inside the tubercles has a higher concentration than the water outside the tubercles. However, even within a short period between the extraction of the pipe and sampling, the concentration of salts in the steady water will decrease due to the dilution of the remaining water in the pipe. Therefore, the presence of high-concentration salts in the tubercles requires deep investigation, although the effect of water salinity on corrosion has been known for a long time [20; 22; 23]. The water composition is also important; the content of chlorides, sulfates, hardness, and alkalinity in the transported water affects the composition of corrosive deposits and the rate of iron release from the corrosive scale [24]. In the research of [PERSON] et al. [25] and [PERSON] et al. [26], the focus was on the sequence of pitting occurrence on stainless steel: the destruction of passivity, the emergence of pitting, and the pitting propagation. Two groups of pits are considered: metastable and stable. The difference between how these types of pits grow is that metastable growth ends with repassivation, whereas stable pitting continues growing. The development of a stable pit is determined by its internal chemical composition with a high concentration of salts. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \begin{tabular}{c} Cations \\ [mg L\({}^{-1}\)/meq L\({}^{-1}\)] \\ \end{tabular}  & Fe\({}^{2*}\) & Mn\({}^{2*}\) & K\({}^{*}\) & Na\({}^{+}\) & Mg\({}^{2*}\) & Ca\({}^{2*}\) \\ \cline{2-7} \cline{6-7} \  
Within the study of pitting corrosion on stainless steel samples [27], [PERSON] and [PERSON] established the range of concentration values of the solution inside the pit, which precedes the growth of stable pitting. Research with stainless steel (type 304) showed that the concentration of metal cations in solution should be higher than 75-80% saturation of the chloride salt to prevent repassivation [28]. If the concentration of metal cations is less than about 75% saturation (i.e., below 3 M), the analyte is not aggressive enough to support the rapid dissolution of the alloy, and repassivation is inevitable. The aggressive analyte, which is formed in the growing pit, is the result of low pH and high chloride concentrations. Based on the conducted research, [PERSON] and [PERSON] proposed an equation by which the concentration of salts of corroding metal in pitting can be determined [27]:  
\[\Delta C=\left(\frac{2\pi}{3\times z\times F\times D}\right)\times i\times a \tag{1}\]  
where \(\Delta C\) is the concentration difference of metal cations between the inner surface of the pit and the bulk solution;  
\(D\) is the diffusion coefficient of metal cations in solution;  
\(i\) is the current density at which the pit is dissolving;  
\(a\) is the pit radius;  
\(F\) is the Faraday constant;  
\(z\) is the oxidation state of the cations which are produced by dissolution.