# Pitting Corrosion of Steel Pipes in Water Supply Systems: An Influence of Shell-like Layer
## 1 Introduction  
The corrosion of steel pipelines in municipal water supply systems leads to significant economic losses. In particular, in the Russian Federation, over 1000 km of pipelines are completely replaced annually after decommissioning due to intensive corrosion. According to statistics, about 50% of the accidents and incidents in the field of housing and communal services are related to water supply, in particular to the gusts of water supply networks [1]. Accidents occur either due to a significant excess of the standard service life of pipelines or due to abnormally rapid corrosion, which develops in most cases unpredictably and has many causes [2; 3; 4; 5]. Fistula on steel pipelines is the most common type of damage (68%). The corrosion of steel pipes in cold and hot water supply systems has been quite well studied [6; 3; 7; 8]. During operation, tubercles, which have a specific structure and composition, appear on the inner surface of pipelines. The structure of tubercles normally includes four elements: a loose surface layer; a thin but dense shell layer that creates a rigid framework of the tubercle; a loose core that includes corrosion products; and a corroding base [8; 9; 10; 11; 12]. Figures 1 and 2 show fragments of pipes with corrosive tubercles: the pipe with tubercles with a loose core (Figure 1) and the pipe with hollow tubercles (Figure 2). It was noted that the inner cavity of the tubercle is filled with an acidic solution [6]. The formation of the surface and shell layers of tubercles is quite well studied, as are the core structure and composition [8; 9; 10; 11; 12; 13]. However, there are almost no studies that reveal the processes occurring inside the tubercles during their growth or experimentally prove the transition from ulcerative corrosion to pitting corrosion. The study of the corrosion mechanism has three directions. The first direction is related to the effect of an external electric current on samples of various metals. This method includes electrochemical impedance spectroscopy (EIS). The second direction deals with the field studies of the corrosion of steel pipes, which are in operation. Finally, the third direction aims at the corrosion effects on pipelines that are no longer in service after an accident [14; 15; 16]. Figure 1: A fragment of steel galvanized pipe with corrosion tubercles. Figure 2: A fragment of steel pipe (from the fire-fighting water supply system) with corrosion tubercles [6]. Most of the studies within the third direction are based on scale samples that are released from the environment of their formation. As a rule, it means a relatively long time from the sample extraction to their examination in the laboratory. This means that the accident consequences should be urgently localized; however, within this time, the scale structure undergoes significant changes [17]. Results of the structural and crystallographic composition of corrosive tubercles in a were presented in the research of [PERSON] et al. [18]. These studies revealed the predominant presence of green rust, wherein iron compounds and various anions reflect the mineral composition of natural water. A significant amount of the least stable green rust in chloride form was found in almost all samples of the corrosive scale. During the experiment, the steady water that surrounds and partially fills the tubercles in the pipes was investigated. The study showed that drying the samples before analysis significantly changes the crystallographic phases in the corrosion products. Table 1 shows the chemical composition of the steady water within the research of scale samples taken from cast-iron pipes of the water supply system with a diameter of 80-120 mm. The chloride content in the tap water sample was 130.8 mg/L, and the pH was 5.84 [18]. The total salinity was 3257.66 mg/L, and the concentration of chlorides in steady water was 13.8 times higher than the concentration of chlorides in the source tap water.