# Determination of Corrosion Rate in GAlvanized Pipes in Centralized Hot Water Supply Systems
## 3 Results and Discussion
### Assessment of the Impact of Iron-Containing Sediment on Zinc Corrosion  
An experiment was carried out to investigate the mechanism of corrosion under the influence of iron sediment. A suspension containing crushed sediment removed from water pipes with traces of corrosion and containing iron oxides and hydroxide (0.45 g of sediment and 2.5 mL of distilled water) was poured into a small sample of galvanized pipe (DN15) that was tightly plugged on one side (Figures 8 and 9). The experiment was held under static conditions at room temperature. The composition of the initial dry iron precipitate, selected during the experiment, was analyzed using a scanning electronic microscope. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Area**} & \multirow{2}{*}{**C**} & \multicolumn{6}{c}{**Element Deposit Content, Wt\%/At\%**} & \\  & & **C** & **O** & **Al** & **Si** & **P** & **S** & **Cl** & **Ca** & **Fe** & **Cu** & **Zn** \\ \hline \multirow{2}{*}{1} & 9.82/ & 25.95/ & 0.59/ & 0.62/ & 0.61/ & 0.53/ & 0.43/ & 37.52/ & 20.85/ & 0.21/ & 2.88/ \\  & 21.03 & 41.72 & 0.56 & 0.56 & 0.50 & 0.62 & 0.31 & 24.07 & 9.60 & 0.08 & 1.13 \\  & 4.70/ & 17.85/ & 0.82/ & 0.49/ & 0.25/ & 0.25/ & 0.22/ & 0.39/ & 73.60/ & 0.26/ & 1.36/ \\  & 13.38 & 38.13 & 1.04 & 0.60 & 0.28 & 0.27 & 0.21 & 0.33 & 44.91 & 0.14 & 0.71 \\  & 3.66/ & 9.94/ & 0.52/ & 0.93/ & 0.26/ & 0.31/ & 0.26/ & 0.51/ & 77.66/ & 0.58/ & 5.57/ \\  & 12.19 & 24.88 & 0.77 & 1.33 & 0.34 & 0.38 & 0.29 & 0.51 & 55.66 & 0.37 & 3.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sediment content. Figure 8: Sample of the galvanized pipe with the iron hydroxide sediment. During the experiment (within 2 weeks), parts of the sediment were taken from the pipe to determine their composition. Table 3 shows the results of determining the concentration of zinc in the sediment depending on the time of contact with the zinc coating, and Figure 10 reveals the dependence of the zinc content in the sediment on the time of the experiment. The total residence time of the sample with iron-containing sediment was 28 days. The average zinc content in the sediment increased from 0.81% to 4.07% within two weeks. This confirms the effect of sediment on the rate of zinc corrosion, which is relatively uniform. Meanwhile, in the absence of sediment, the corrosion rate decreased over time (Table 1, tests #3 and #4). Thus, the data obtained indicate a different mechanism of corrosion with oxygen depolarization and contact with iron-containing sediment. After conducting the experiments, the sample was cut in the longitudinal direction, and an analysis of the condition of the sample surface was made. Figure 11 shows the inner surface of the sample, on which three zones with different colors can be distinguished. The upper zone was the boundary between water and air. The zinc coating above this boundary \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Time (d)** & **0 (Initial Sediment)** & **1** & **3** & **7** & **14** \\ \hline Zinc content, wt\% & 0.81 & 1.36 & 1.66 & 2.02 & 4.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Zinc content in the sediment depending to time of contact. Figure 10: Zinc content (obtained values and trend) in the sediment related to the time of exposure. Figure 9: Sample of the sediment. was not broken. The middle zone had a brown color and was covered with a layer of iron. Since the oxygen concentration in the water was highest in the upper part, it can be concluded that the pipe surface in this zone was a cathode and was also not corroded. The brown color of the surface indicates that an alkaline medium was formed at the cathode, leading to the adsorption of iron hydroxide. The lower zone had a gray color, which is typical for zinc coatings. The acidic environment at the anode contributes to the dissolution of the precipitate and the transition of iron into ionic form. The transition of zinc into the precipitate occurs according to the reactions presented below.