# Determination of Corrosion Rate in GAlvanized Pipes in Centralized Hot Water Supply Systems
###### Abstract  
To counteract possible corrosion, steel pipes are coated with a protective layer of zinc with a thickness of 20 to 85 microns, depending on the requirements of regulatory documentation. It is proven that hot-deposited zinc can effectively protect steel surfaces for 20 to 120 years. However, in centralized hot water supply (CHWS) systems, the period of protective action can decrease to two to three years, with further damage to the zinc layer and the appearance of fistulas. In order to counteract adverse factors, it is necessary to take into account the operating mode and design features of the system, the properties of the coating, and the environment in which the pipes operate. In this article, the main attention is paid to the study of the behavior of zinc coating. The protective properties and corrosion rate of zinc coating in tap water were evaluated. It was established that the protective effect of zinc is effectively manifested in the ratio of the protected area to the unprotected, as \(\mathrm{S_{Zn}/S_{Fe}}\geq 9\):1. The influence of iron-containing sediment, when it accumulates in the pipes of a CHWS system, on the corrosion rate of the zinc coating was studied. It was also noted that the corrosion rates of the new zinc coating and uncoated steel pipe measured in short-term tests differed slightly. the thickness of the zinc layer should be at least 30 microns. In accordance with [4], the minimum thickness of the zinc layer was increased to 45-55 microns, depending on the diameter and thickness of the pipe wall. The standard [4] establishes general requirements for protective coatings applied by hot-dip galvanizing at temperatures of 430-460 \({}^{\circ}\)C, as well as requirements for the base metal and coating quality control methods. The calculated values of zinc coating thickness and the rate of its destruction may be helpful in predicting the service life of galvanized pipes. Hot-deposited zinc effectively protects steel products for 20 to 120 years [1]. However, in some cases, the service life of galvanized steel pipes is two to three years due to pitting corrosion. This is especially true for centralized hot water supply systems. In terms of corrosion resistance, galvanized pipes occupy an intermediate position between uncoated steel and copper pipes. To assess the kinetics of the destruction of the zinc coating under operating conditions, samples made from sections of galvanized pipes were installed on sections of hot water supply systems in Moscow using Volga water. A year later, the change in thickness of the zinc coating on the samples and galvanized pipes was determined. In addition, the proportion of the surface on which complete destruction of the zinc coating occurred was defined. The tests showed that the zinc coating was destroyed most quickly within the first few months of operation. So, in the first, second, and third months, the thickness of the zinc coating decreased, respectively, by 17, 9, and 6%, whereas in the next nine months, it decreased by 1%. This may be due to the precipitation of an insoluble sediment on the surface of the pipes, which increased the electrical resistance in the circuit comprising the anode, pipe material, cathode, and water containing oxygen. However, if there are areas with a coating thickness about three times lower than the average, the coating on these areas is completely destroyed within the initial period of operation. Such areas (with a completely destroyed coating) can be found within three months of operation. Ensuring the uniformity of the zinc coating in thickness, with an average thickness of at least 70 microns, would contribute to the greater durability of pipes [5,6]. The results of surveys of centralized hot water supply (CHWS) systems located in residential buildings in which pipes began to fail within 2-3 years have been published in a number of studies [6,7]. Examples of abnormally high corrosion rates in galvanized pipes can be found in various studies.