# 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
Figure 6: Photo of a sample of DN15 galvanized pipe: (**a**) sample before exposure of hot water, (**b**) sample after 24 h in hot water; (**c**) sample (**b**) with the groove; (**d**) sample with the groove after 24 h in hot water. * \(S\) is the surface area of the sample on both sides, excluding the surface not protected by zinc (for tests #4 and 5) (cm\({}^{2}\));
* \(T\) is the exposure time of the sample in water (h). The deep corrosion rate was determined:  
\[DCR=\frac{8.76\times(M_{1}-M_{2})\times 10^{4}}{S\times T\times\rho} \tag{2}\]  
where \(DCR\) is the corrosion rate (mm/year). \(\rho\) is the density of the material (zinc or iron) (g/cm\({}^{3}\)). The \(DCR\) values allow the determination of the service life of the zinc coating. If we assume that the average thickness of the zinc layer is 30 microns (0.03 mm), then the amount of time it takes for all of the zinc to dissolve in hot water is  
\[T_{Zn.diss}=\frac{H_{Zn}}{DCR}=\frac{0.03}{0.278}=0.11\;\mathrm{years}=40\; \mathrm{days}\;(\mathrm{test\;\#4})\]  
Table 1 also shows the values of the corrosion rate of only zinc coatings in cold water (test #1). The sample area was 12.8 cm\({}^{2}\), and all exposed steel surfaces were protected from corrosion by a layer of varnish. Then the same sample was immersed in a container with a falling jet of hot water (test #3). At the same time, the corrosion rate increased by about six times. Figure 7 shows a micrograph of a fragment of the groove area (see Figure 6d), and Table 2 shows the chemical composition of the elements that make up the sediment in three different zones marked in the figure with numbers 1, 2, and 3. The increase in the corrosion rate in test #5 compared with test #4 indicates that the appearance of additional areas unprotected by zinc on the surface of the sample led to a sharp increase in the density  
On the scratch surface, the sediment distribution pattern is almost identical to that formed during the corrosion of the unprotected steel pipe (areas #1 and 2). The difference is observed only in area #3, where more zinc is sorbed on the surface, along with almost two orders of magnitude less calcium. Presumably, calcium precipitation is prevented by the sorbed zinc and, accordingly, a change in the sign of the surface charge is observed. Table 2 shows that when the ratio S\({}_{\mathrm{Zn}}\)/S\({}_{\mathrm{Fe}}\) = 3.55, the protective ability of zinc (total: sacrificial + barrier) is distributed unevenly, being practically absent in the center of the groove. The entire surface of the groove is divided into an anode area (#2, in the center, with a high iron content and no zinc) and cathode areas (#1 and #3, where the zinc content has a higher value). The presence of zinc in area #2 can be explained by the presence of water containing zinc ions on the surface of the sample when it was extracted at the end of the experiment. It should be noted that the comparative analysis of the zinc corrosion rate values obtained by us and the authors of [16; 17] shows that the numerical values of the corrosion rate have large discrepancies. These discrepancies are related both to different experimental conditions and to the zinc samples under study. These differences relate especially to the work [16], in which studies were carried out under static conditions and in seawater.