# 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
In addition to determining the corrosion rate, studies of the protective effect of zinc against the unprotected surface of steel were also continued. The sample (Figure 6a,b) had a \(\mathrm{S_{Zn}/S_{Fe}}\) surface ratio of 6.3, and the exposure time in hot water was 24 h. The sample after the experiment had a gray color without obvious traces of corrosion. However, there was rust on the surface that was not thoroughly sanded. The corrosion rate calculated from the loss of zinc mass and attributed only to the zinc-coated surface was 0.226 \(\mathrm{gm^{-2}h^{-1}}\) (Table 1, test #4). The sample area covered with zinc on the inside and outside of the pipe was 20.8 \(\mathrm{cm^{2}}\). Since the sample was cut from a pipe, to determine the area protected by zinc, a layer of foil was applied to the inner and outer surfaces, and then the total area was determined from the impression. Figure 3: Micrograph and chemical composition of surface layer of the new galvanized pipe DN15. Figure 4: Micrographs of the zinc coating throughout various areas (**a,b**) of the sample after 24 h in hot water. Figure 5: Elemental composition of the zinc coating surface layer throughout various areas of the sample after 24 h in hot water (EDS values are shown for area in red square). Then, on the same sample, a layer of zinc was removed from the base metal in the form of a 5 mm wide groove (Figure 6c), and the precipitate that fell in the previous experiment on the cross section of the sample wall and the ends not covered with zinc was removed. The sample area covered with zinc on the inside and outside of the pipe was 18.8 cm\({}^{2}\). The sample was also kept in a stream of hot water for 24 h. The total residence time of the sample in the water was 48 h. The corrosion rate in this case was 0.346 gm\({}^{-2}\)h\({}^{-1}\) (test #5). Figure 6d shows that part of the groove surface was covered with rusty sediment. The unprotected longitudinal walls and transverse sections of the sample, protected by zinc on both sides, had no traces of corrosion. The higher corrosion rate obtained in test #5 can be explained by the fact that the initial concentration of zinc in the surface layer (Figure 6b) was significantly higher than in test #4. The values of the corrosion rates are determined by Equations (1) and (2). \[SCR=\frac{(M_{1}-M_{2})\times 10^{4}}{S\times T} \tag{1}\]  
* where \(SCR\) is the surface corrosion rate (gm\({}^{-2}\)h\({}^{-1}\));
* \(M_{1}\) and \(M_{2}\) are the mass of the sample before immersion in water and after immersion (g); \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Test** & **T (h)** & \(M_{1}\) (g) & \(M_{2}\) (g) & \(\Delta\)M = \(M_{1}-M_{2}\) (g) & \
\begin{tabular}{c} _SCR ** \\ **(gm\({}^{-2}\)h\({}^{-1}\))** \\ \end{tabular} &
\begin{tabular}{c} _DCR_ \\ **(mm/year)** \\ \end{tabular} & \(\text{S}_{\text{Zn}}\)/\(\text{S}_{\text{Fe}}\)** \\ \hline
1 & 23.7 & 13.6248 & 13.6224 & 0.0024 & 0.0827 & 0.102 & - \\
2 * & 24 & 54.0740 & 54.0490 & 0.0250 & 0.174 & 0.197 & - \\  & & & T\({}_{\text{hw}}\) = 45–47 \({}^{\circ}\)C & & & & \\
3 & 3.5 & 13.6213 & 13.6192 & 0.0021 & 0.469 & 0.576 & - \\
4 & 24 & 21.9299 & 21.9186 & 0.0113 & 0.226 & 0.278 & 6.30 \\
5 & 24 & 21.5688 & 21.5532 & 0.0156 & 0.346 & 0.522 & 3.55 \\
6 * & 20 & 53.5420 & 53.4830 & 0.0590 & 0.492 & 0.560 & - \\ \hline \end{tabular}  
* The values obtained during the corrosion of an uncoated steel pipe sample. The sample area was 60 cm\({}^{2}\), the density of iron was 7.874 g/cm\({}^{3}\), and the density of zinc was 7.133 g/cm\({}^{3}\). ** The values of the corrosion rate given in the table should be taken as indicative of the weight of the sorbed chemical compounds on the cathode areas of the surface of the samples and the amount of iron removed (entered into the tap water) from the anode areas of the disturbed zinc coating.  
\end{table} 
Table 1: Determination of the corrosion rate of pipe samples in tap water at different temperatures.