# Pitting Corrosion of Steel Pipes in Water Supply Systems: An Influence of Shell-like Layer
## 1 Introduction
Equation (1) shows that pit growth requires minimal values of multiplying current density \(i\) by depth or radius of pit \(a\). If to apply that \(\Delta C=3\) M; \(z=2.19\); \(F=96\),\(485\) C/mol; \(D=10^{-9}\) m\({}^{2}\)/s, a critical value of \(i\times a\) is equal to 0.3 A/m. The diffusion analysis determines a narrow range of \(i\times a\) values in which the pit is stable. The lower limit requires that \(\Delta C\) should exceed 3 M; the upper limit is determined by the solubility of the metal salt. If a saturation concentration of iron chloride is equal to 4.2 M, the condition for stable pit growth is expressed as:  
\[0.3\text{ A}\cdot\text{m}^{-1}\leq i\times a\leq 0.6\text{ A}\cdot\text{m}^{-1} \tag{2}\]  
A high concentration of salts in the pit can be explained by the pit size, which is small enough to neglect the effect of diffusion and convective transfer from the pit to the bulk solution. Moreover, the impact of the oxide film (partially preserved during the destruction) on the perforated coating has not been disclosed. This coating regulates the density of the corrosive current, providing an additional barrier to diffusivity. In [29], [PERSON] and [PERSON] clarified the role of the perforated coating, which \"seves as an additional barrier to diffusion and maintains a sufficiently aggressive concentration of analyte. Small breaks in this coating as the metastable pitting grows increase the diffusion rate without diluting the pitting analyte and support the growth of the pitting. Such ruptures may occur due to the effect of the difference in osmotic pressure developing through the coating\". In [30], [PERSON] and [PERSON] attempted to model the perforated coating and its effect on diffusion processes. The importance of coating was also highlighted in other studies [31; 32]. In [33], the authors consider in detail the initial stage of pit nucleation and propagation. It was concluded that providing metastable pitting propagation requires a set of conditions to control diffusion: a current and surface roughness (the presence of occluded sites). Research [34] is focused on the effect of chloride ions on pitting corrosion. It was shown that a concentration of chlorides of more than 245 mM in the electrolyte volume is required to maintain the spread of pitting on austenitic 316-L stainless steel. When chloride concentrations were below 245 mM, pitting became metastable. Research [35] showed that the chloride ion creates an intermediate complex with a passive metal, followed by a sequential dissolution of the passive film and pit nucleation. Some authors have noted the role of the solid salt film in the formation of pitting corrosion. The salt film can be formed on the metal surface because of the supersaturation conditions when the ionic concentration in the pit solution grows due to an increase in the pit current density [36]. If a film exists, the rate of pit growth is limited by mass transport, and the pit has a regular hemispherical shape; if not, pits may have an irregular shape. Some studies reveal that the salt film helps to stabilize the pit and provide a buffer of ionic species, for example, in case of a sudden loss of a protective pit cover [37; 38; 39]. Additionally, salt film promotes oxide passivation in solutions that contain perchloric and sulfuric acids at potentials above the passivation potential [39]. Numerous publications were aimed at factors that affect the corrosion potential and their importance for the initiation of pitting and overall corrosion [23; 40; 41]. During stagnation, the corrosion potential of iron decreases noticeably [40]. Despite the quite comprehensive knowledge about the mechanism of pitting corrosion, it should be noted that in the above works, the research was carried out with highly concentrated solutions. However, these studies did not disclose how these concentrations of salts were created at the metal-solution boundary within the tap water with a lower salinity.