# The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System
## 3 Results
### Corrosion Product and Element Analysis  
Figure 7 shows the XRD analysis results for corrosion products. Two visually distinct white and red-brown oxides were examined. The white colored oxide was mainly formed on the tube seam and was found on a lower end plate partially. On the other hand, the red-brown colored oxide was mainly formed on the inside of the convection tube and lower end plate. It was found that the white-colored salt was sodium compounds such as NaNO\({}_{3}\), NaCl, and Na\({}_{2}\)SO\({}_{4}\) (Figure 7a), whereas the reddish-brown oxide scale exhibits the formation of iron oxides such as Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\), and FeOOH (Figure 7b). Table 2 shows the IC and ICP analysis results of water quality collected eight times during 11 months. The Na\({}^{+}\) and Cl\({}^{-}\) components were the most contained, followed by the NO\({}_{3}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) components. Na\({}^{+}\), Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) have been detected in solution because of the addition of sodium compounds (NaOH, NaCl, Na\({}_{2}\)SO\({}_{4}\)) and oxygen scavenger (CH\({}_{6}\)N\({}_{4}\)O, C\({}_{2}\)H\({}_{7}\)NO). The components (Na, Cl, N) which do not existed in the material component are mainly from boiler water and LNG (Tables 2 and 3). The sodium compound detected from the internal scale analysis does not form under a dry combustion gas environment. Additionally, sodium compound detected from the XRD analysis does not form under the LNG environment that consists of carbon dioxide and small amounts of nitrogen. Therefore, the sodium compound as a corrosion product results from the fact that boiler water was mixed with the gas. It is concluded that the boiler water vaporized by a leakage flows into the tube, forming a corrosion product on the inner surface of the tube. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Unit: ppm** & **CI\({}^{-}\)** & **NO\({}_{3}\)\({}^{-}\)** & **SO\({}_{4}\)\({}^{2-}\)** & **Na\({}^{+}\)** \\ \hline Turbulator oxide & 27.12 & 5.50 & 255.8 & 73.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: IC/ICP analysis of trace elements in turbulator oxide scales dissolved in solution. Figure 6: Ultrasound test results of the convection tube, showing a thinning in the tube thickness [13]. The presence of these oxides indicates a heterogeneous chemical environment, where sodium compounds, originating from boiler water, contribute to a localized corrosion due to their hygroscopic and reactive nature. Meanwhile, the formation of iron oxides such as Fe\({}_{3}\)O\({}_{4}\) provides temporary passivation, whereas Fe\({}_{2}\)O\({}_{3}\) and FeOOH, typically associated with further oxidation states, signify progressive corrosion under moist conditions. This layered composition of corrosion products exacerbates the corrosion process by promoting uneven surface roughness, increased ionic conductivity, and localized electrochemical activity, thereby accelerating material degradation. These findings align with prior studies that highlight the role of mixed-phase oxides in intensifying corrosion susceptibility under environmental and operational conditions [18; 19; 20; 21; 22; 23]. Figure 7 presents the XRD analysis results of oxide scales collected from different tube locations within the convection section of the PLB. Two visually distinct corrosion products were identified: red-brown oxides, predominantly composed of iron oxides, and white oxides, which primarily contained sodium-based compounds. Figure 7: XRD analysis of internal oxide scales collected from tubes at varying conditions within the convection section of the PLB. (**a**) Oxide scale from the inner surface of a failed tube in the leakage direction, (**b**) Oxide scale from an intact tube adjacent to the leakage site, (**c**) Oxide scale from an intact tube distant from the leakage site, (**d**) White-colored salt from an intact tube adjacent to the leakage site, (**e**) Oxide scale from the surface of the turbulator.