# The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System
## 3 Results
### Corrosion Failure Mechanisms  
Figure 12 illustrates the progressive failure mechanism of dew point corrosion in peripheral tubes within the convection section of the PLB. This process is driven by a combination of thermal gradients, phase transitions, and ionic interactions, leading to localized corrosion and eventual tube failure. As corrosive species accumulate, the affected region expands, increasing the risk of secondary failures. 1. Primary corrosion and initial leakage occur as carbon steel tubes in the convection section are continuously exposed to high-temperature flue gas and boiler water. Over prolonged operation, localized thinning and material degradation lead to the formation of primary leakage points, near the 90\({}^{\circ}\) position relative to the manhole in this study. The escaping boiler water mixes with the flue gas, initiating further degradation processes. 2. Once in contact with high-temperature flue gas (over vaporization temperature), the leaked boiler water undergoes rapid vaporization, creating a moisture-laden gas mixture that rises along the convection tube bundle following the direction of gas flow. Due to inertial effects, the vapor preferentially accumulates along specific flow paths, particularly near the turbulator region, where localized cooling enhances condensation. Compared to tubes located at the center of the plate, wall-side tubes are more susceptible to condensation and subsequent corrosion due to their lower surface temperatures and proximity to stagnant flow regions. These areas facilitate the accumulation of acidic ions such as Na\({}^{+}\), Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), and NO\({}_{3}\)\({}^{-}\), leading to an increasingly aggressive corrosion environment. 3. As the vaporized water travels through the convection tubes, the gas temperature decreases due to heat exchange with the boiler water inside the tubes. When the local temperature reaches the dew point, condensation occurs. The turbulator region is particularly vulnerable to condensation due to localized temperature drops and flow disturbances induced by its structural features. Since the dew point temperature is influenced by the partial pressure of acidic ions, areas with higher ion concentrations experience condensation at higher temperatures. Under typical flue gas conditions, dew formation generally occurs below 100 \({}^{\circ}\)C (373 K); however, as acidic species accumulate, condensation can occur at temperatures as high as 150-200 \({}^{\circ}\)C (423-473 K), significantly expanding the corrosion-prone region beyond the immediate vicinity of the leak. 4. The condensed water film on the turbulator surface traps corrosive ions such as Cl\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and SO\({}_{4}\)\({}^{2-}\), which were identified in the water quality analysis. The morphological features of the turbulator, including grooves and surface depressions, enhance localized condensate retention, creating highly corrosive microenvironments. These ions originate from boiler water treatment chemicals and enter the system through leakage, leading to localized acidification and intensified electrochemical corrosion reactions. As acidic ion concentrations increase, the local pH drops, and the dew point continues to rise, allowing condensation to occur at even higher temperatures. This results in an expanding corrosion zone, with secondary failures appearing in adjacent tubes. 5. Since internal leakage cannot be immediately addressed during normal operation, corrosive species continue to accumulate over time. The prolonged presence of leaked boiler water, combined with repeated thermal cycles, accelerates acidification and localized corrosion, leading to widespread material degradation. Initially, failure is concentrated near the 90\({}^{\circ}\) position, where the first leakage occurs. Over time, the affected corrosion-prone zone expands, with secondary failures occurring at 0\({}^{\circ}\) and 180\({}^{\circ}\) due to the recirculating nature of the convection flow.