# Pitting Corrosion of Steel Pipes in Water Supply Systems: An Influence of Shell-like Layer
## 4 Conclusions  
The corrosion in water-supplying steel pipes with the formation of fistulas, as a rule, proceeds in several stages. The results of the study propose a corrosion model that considers the entire process as a whole. In the first stage, ulcerative corrosion develops, accompanied by the growth of tubercles covered with a dense layer of corrosion products (magnetite). In subsequent stages, corrosion develops by a pitting mechanism inside the tubercles. The emergence of fistulas is preceded by an increase in the concentration of electrolyte inside the tubercles due to the peculiarities of the structure of the dense layer. Within the formation, the dense layer acquires the properties of a selective membrane.  
An increase in the concentration of electrolyte on the metal surface under a tubercle intensifies the pitting corrosion, converting unstable pits into stable ones. At the final stage, the growth of tubercles ceases, and the process becomes autocatalytic. In this case, the presence of sufficient water within the core of the tubercle provides conditions for the formation of a fistula. If there is no external water supply, fistulas may form only if there is sufficient water in the tubercular structure to oxidize the entire iron, considering the thickness of the remaining metal. In the pits, water is consumed by the hydrolysis of  
Figure 8: Microphotographs of corrosion scale, which forms a dry layer of the metal base of the pipe.  
iron liberated during corrosion. In cases of water scarcity, individual pits and, generally, tubercles become repassivated. Consequently, pipes with thicker walls, such as cast iron, are more resistant to corrosion. It is proposed that the dense layer in the tubercles in carbon steels and the perforated coating in the structure of the pitting in stainless steels have identical properties, ensuring the creation of critical conditions for intense corrosion.  
The studies carried out to determine the concentration of salts in the structure of the tubercles after extraction from pipes can be used to better understand the mechanism of corrosion. At the same time, it is necessary to study not only the structure but also the properties of the dense layer, which will allow the determination of the directions of influence on the ongoing corrosion processes.  
The results of the research propose a new direction to assess the influence of a dense layer on the corrosion of steel and cast-iron pipes before the appearance of fistulas. A possible change in the electrical resistance of the layer when applying various inhibitors is also presented.  
Conceptualization, V.C., A.A. and N.M.; methodology, V.C., A.A. and N.M.; software, N.M.; validation, V.C. and A.A.; formal analysis, V.C., A.A. and N.M.; investigation, V.C., A.A. and N.M.; resources, V.C., A.A. and N.M.; data curation, V.C. and A.A.; writing--original draft preparation, V.C. and A.A.; writing--review and editing, N.M.; visualization, N.M. and V.C.; supervision, N.M.; project administration, N.M.; funding acquisition, N.M. All authors have read and agreed to the published version of the manuscript.  
The research was funded by the National Research Moscow State University of Civil Engineering (grant for fundamental and applied scientific research, project No. 17-392/130).  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.