# Determination of Corrosion Rate in GAlvanized Pipes in Centralized Hot Water Supply Systems
###### Abstract
Thus, corrosion defects in hot water installations in Gdansk-Wrzeszcz (Poland) are similar to those described above [8]. The zinc coating was completely destroyed within 3-4 years. The reason was the high level of oxygen and aggressive CO\({}_{2}\) in the water, as well as the content of chloride and sulfate ions of a total amount of more than 50 mg/L. The temperature of hot water and especially temperature differences are also of great importance since they are a source of cracks and destruction of the layers of corrosion products with protective properties. Another study showed that maintaining a constant water temperature below 55 \({}^{\circ}\)C at a pH of 7.4-7.8 may be one of the most effective ways to reduce the corrosion rate [8]. The maximum corrosion rate of the zinc coating is observed at a temperature of 70 \({}^{\circ}\)C [9]. The reasons for the high rate of corrosion of galvanized pipes in hot water systems, in addition to the quality of the source water, include the following:  
* Application of various types of pipe materials, including copper, in the CHWS system [10];
* Welding of galvanized pipes without complying with the requirements of the standard;
* Microbiological corrosion [11];
* Current leakage [12]. The main reason for the accelerated rate of corrosion is the high concentration of dissolved oxygen and carbon dioxide in the water. According to the literature data, when the water temperature increases from 10 to 60 \({}^{\circ}\)C, the oxygen content in the water at atmospheric pressure decreases by about three times. It should be noted that in a closed system, such as a CHWS system, water is oversaturatedwith oxygen and carbon dioxide, which are actively involved in the corrosion process. However, all of these factors have the same effect on pipes if we are talking about the hot water supply system in the same building where cases of abnormally high corrosion rates of galvanized pipes are observed. Consequently, there are factors that are not taken into account acting locally on limited areas of the inner surface of pipes and leading to a high rate of corrosion. When studying the protective properties of zinc, it was found that zinc, when destroyed, prevents corrosion of the protected metal. This mechanism, with the direct contact of two metals and the presence of a conductive liquid, can act over long distances and is called sacrificial corrosion. On the other hand, it was also noted that the released zinc can be deposited on the surface of the protected metal in the immediate vicinity. This mechanism is called barrier protection [13; 14; 15; 16]. Research by [PERSON] et al. revealed the modeling of the mechanism of anticorrosive protection of a steel substrate covered with a zinc primer (96% zinc in mass) that was damaged by scratches [17]. The scratch was a 500-micron line located in the middle and along the entire length of the sample. The results showed that although the middle part of the scratch was protected by sacrificial protection, at the ends of the scratch, local cathodic deposition of zinc led to barrier protection due to changes in the concentration of dissolved oxygen. In studying the local pH change in the scratch area in the period from 2 h to 96 h, the authors conclude that the mechanisms of sacrificial and barrier protection exist together along the scratch surface. The high alkalinity above the scratch center demonstrates that this area is under active sacrificial protection. Small areas near the corners of the scratches that are quite acidic, i.e., pH 5.5-6.5, are the areas under barrier protection where a layer of corrosion products is densely formed. Flat samples of electrolytic zinc were used by immersing them for periods of 8, 10, 30, and 90 days in Caribbean Sea water to study the corrosion rate and determine the composition of compounds formed during zinc corrosion [18].