# The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System
## 3 Results
### Corrosion Product and Element Analysis
The red-brown oxide scales were mainly observed on the inner surfaces of both failed and intact tubes (Figure 7a-c). XRD analysis revealed the presence of Fe\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\), and FeOOH, which are common iron oxides formed under varying oxidation conditions in humid environments. These oxides form sequentially through oxidation and dehydration reactions: 2 Fe(OH)\({}_{3}\)\(\rightarrow\) 2 FeCOOH + H\({}_{2}\)O \(\rightarrow\) Fe\({}_{2}\)O\({}_{3}\) + Fe\({}^{2+}\) +2H\({}^{+}\)\(\rightarrow\) Fe\({}_{3}\)O\({}_{4}\) + H\({}_{2}\)O. The transition from FeOOH to Fe\({}_{2}\)O\({}_{3}\) indicates further oxidation, while Fe\({}_{3}\)O\({}_{4}\) formation suggests partial reduction processes under fluctuating oxidation states. The formation of Fe\({}_{3}\)O\({}_{4}\) suggests the presence of an initial protective layer, while Fe\({}_{2}\)O\({}_{3}\) and FeOOH indicates progressive oxidation and material degradation, as these phases typically form under prolonged exposure to moisture and an oxidizing environment. Notably, in the intact tube adjacent to the leakage site (Figure 7b), NaCl and Na\({}_{2}\)SO\({}_{4}\) were detected alongside iron oxides, suggesting that these sodium-based compounds were introduced through the infiltration of boiler water and subsequently deposited on the tube surface. The white salt, primarily detected in tubes near leakage sites and on the turbulator surface, consisted of NaNO\({}_{3}\), NaCl, and Na\({}_{2}\)SO\({}_{4}\) (Figure 7d,e). The segregation of white oxide deposits (Figure 7d) confirmed that the major components of these were sodium compounds, further supporting the hypothesis that boiler water ingress and subsequent evaporation led to the formation of corrosive deposits. Additionally, the turbulator surface exhibited a combination of Fe-based oxides and NaCl (Figure 7e), suggesting that it was subjected to both high-temperature flue gas exposure and condensate deposition. Given its structural design, the turbulator provides surfaces where moisture can accumulate, making it more susceptible to localized cooling and corrosion. Figure 8 compares the turbulator conditions in intact and near-leak regions. The intact turbulator (Figure 8a-c) retained its original shape, and its cross-section maintained a uniform, circular structure with minimal oxide formation. In contrast, the turbulator near the leakage site (Figure 8d-f) exhibited significant degradation, with a deformed shape and thick oxide layers covering its surface. The cross-sectional SEM image of the degraded turbulator revealed extensive oxidation, further confirming its exposure to an aggressive environment. XRD analysis of the turbulator's surface oxide (Figure 7e) identified Fe-based oxides and NaCl, but did not reveal other minor elements. To evaluate trace components, over 1 g of turbulator surface oxide was collected and dissolved for solution-phase analysis using IC/ICP-MS (Table 3). The analysis detected Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and Na\({}^{+}\), with SO\({}_{4}\)\({}^{2-}\) reaching 255 ppm, indicating a highly acidic environment that likely accelerated corrosion. The absence of SO\({}_{4}\)\({}^{2-}\) in the XRD results suggests that sulfate concentrations were below the detection limit in solid-state analysis but became detectable in solution. Table 4 summarizes the water quality analysis of the boiler feedwater, revealing the presence of SO\({}_{4}\)\({}^{2-}\), Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and PO\({}_{4}\)\({}^{3-}\), which correlate with the ionic species identified in the system. The detection of these ions in the boiler feedwater analysis strongly suggests that water treatment chemicals played a significant role in the composition of leaked water. This confirms that boiler water leakage led to the reintroduction of these aggressive ions into the system, contributing to continued material degradation.