# Determination of Corrosion Rate in GAlvanized Pipes in Centralized Hot Water Supply Systems
## 3 Results and Discussion  
### Determination of the Protective Effect and Corrosion Rate of Zinc  
Experiments to determine the barrier role of zinc coating and the corrosion rate of galvanized pipes were carried out in hot water. A sample of a galvanized pipe with a diameter (DN) of 15 mm (Figure 1a) was placed in a container under a hot water jet with a temperature of 50 \(\pm\) 2 \({}^{\circ}\)C for 4 h. The sample was sanded on one end and varnished on the other to prevent corrosion. The areas of surfaces coated and uncoated with zinc in the experiment were treated as S\({}_{\text{Zn}}\)/S\({}_{\text{Fe}}\) = 9:1. The aim of the experiment was to evaluate the effect of the protective action of zinc in relation to the steel base of the pipe (the so-called sacrificial corrosion). After being taken out of the water, the sample was dried. A matte gray precipitate was visually observed on the surface of the section, evenly covering the entire surface of the section (Figure 1b). No traces of rust were found. Figure 2 shows a micrograph and the composition of the sediment completely covering the section. The main component of the sediment is calcium carbonate. The high iron content (52.01% by weight) is most likely determined by the small thickness of the sediment layer through which the electron flow freely reaches the surface of the steel during scanning. The relatively low zinc content (0.81% by weight) in the sediment is explained by the higher concentration of calcium in the source water. It is obvious that zinc, having passed into the ionic form, can be sorbed on the cathode surface due to the action of electrostatic forces or carried away by a stream of water. Since the cross section of the pipe was a cathode, there are also aluminum and magnesium cations on the surface, contained both in the source water and in the structure of the new zinc coating. Along the perimeter of the section between the inner and outer surfaces, the precipitate consists of zinc oxide and carbonate. Calcium and zinc compounds, forming a surface layer on a steel substrate, inhibit the oxygen reduction reaction and reduce zinc corrosion. Figure 1: Photos of galvanized pipe samples (DN15) before (a) and after (b) exposure to hot water for 4 h.  
Figure 2: Micrograph and chemical composition of sediment on the wall of the pipe not protected by zinc. During the manufacture and storage of new galvanized steel pipes, sections containing various impurities (sulfur, oxygen, manganese, silicon, aluminum, and calcium) are formed on the inner surface, which are intermetallic inclusions. Figure 3 shows a micrograph of the surface of a new galvanized pipe. In the presence of moisture, carbon dioxide, and chemical impurities present in the atmosphere, zinc oxide is converted into hydroxide, basic carbonate, and other basic zinc salts. A hard-to-dissolve basic zinc carbonate with a composition similar to ZnCO\({}_{3}\)\(\times\)3 Zn(OH)\({}_{2}\) is formed [1]. Figure 3 shows that the chemical composition of the surface layer is more complex, so it can be assumed that when the pipe is first immersed in water, the corrosion rate will depend on the potential difference determined by the composition of the compounds covering the surface. According to the diagram of the phase composition of the zinc coating obtained by hot-dip galvanizing, the zinc content in the surface layer should be at least 98% [9]. Therefore, after 24 h, the chemical composition of zinc compounds was redetermined. An overview of the various areas of the pipes and their composition is shown in Figures 4 and 5, respectively, which reveal the surface to be significantly heterogeneous. To assess the corrosion rate of new galvanized steel pipes in hot water, a sample cut from a 15 mm (DN) pipe was prepared (Figure 6). The sample was placed horizontally in a plastic container into which a flow entered from a height of 0.4 m, ensuring maximum saturation of water with oxygen in the air.