The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System

The peak load boiler (PLB) is a heat production facility that uses SA178 Gr. A and SA516 Gr. 70 low-carbon steels as tube and plate materials, respectively. Recently, failures were frequently observed near plugged tubes due to water leakage, raising concerns about corrosion mechanisms and their impact on tube durability. This work investigates the corrosion failure mechanisms using a combination of endoscopy, ultrasound inspection, oxide scale analysis (X-ray diffraction), chemical analysis (ion chromatography and inductively coupled plasma mass spectrometry), and computational fluid dynamics simulations. The undamaged tube near the leaked tube exhibited oxide scale levels comparable to those directly affected. Surface examinations revealed gas-side pits indicative of localized corrosion, while oxide scales were predominantly composed of iron oxides formed under humid conditions and sodium compounds derived from boiler water. Analysis of the leaked water revealed its mixture with combustion gases, forming an acidic, chloride-rich environment that significantly accelerates corrosion. Computational fluid dynamics simulations demonstrated that leaked water vapor facilitated the condensation of acidic ions near affected tubes, promoting dew point corrosion. These phenomena, driven by localized condensation and chemical concentration at the dew point temperature, exacerbate material degradation, emphasizing the importance of targeted prevention strategies.