# The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System
## 1 Introduction  
District heating systems rely on large-scale infrastructure to efficiently distribute thermal energy across urban areas. Unlike continuously operated power plants, peak load boilers (PLBs) in district heating systems function intermittently to accommodate surging energy demand during peak hours. The PLB investigated in this study is an industrial-scale heat production facility with a capacity of 40 MW and a volume of 25.8 m\({}^{3}\). Since its commissioning in July 1997, the facility has produced 34.4 Gel per hour, supplying heat to approximately 48,000 households. In PLB operation, combustion gases from liquefied natural gas (LNG) burning pass through a convection tube bundle, where heat exchange occurs between the hot gas and boiler feedwater. These tubes, welded to upper and lower end plates, have their outer surfaces exposed to boiler feedwater and inner surfaces subjected to combustion gas flow. Considering cost efficiency and heat transfer performance, low-carbon steel (SA178 Gr. A) is used for the tubes due to its superior thermal conductivity and ease of fabrication. SA516 Gr. 70 is applied to the upper and lower plates, securing the tube bundle while ensuring structural stability under cyclic thermal loads. Despite its operational advantages, the PLB has suffered from persistent and escalating corrosion failures in the convection section, disrupting operations and necessitating frequent repairs. Unlike aqueous environments, where corrosion can be controlled through chemical treatment, high-temperature gaseous environments create unpredictable and aggressive corrosion conditions, often resulting in sudden and severe structural failures [1; 2; 3]. A critical driver of this deterioration is dew point corrosion, where acidic condensates form due to localized cooling of combustion gases. Corrosion in gaseous environments is more challenging to predict and frequently leads to unexpected failures [3; 4; 5]. This phenomenon is exacerbated by fluctuating heat exchange conditions, repeated start-and-stop cycling, and reactive blue gas components [4; 5; 6; 7]. Additionally, intense thermal stresses from rapid temperature variations accelerate material degradation, particularly at weld seams, promoting stress corrosion cracking (SCC) and fatigue-induced fractures that compromise the integrity of the system [8; 9]. Some cases, e.g., stress corrosion cracking and failure in welds by thermal stresses, have also been reported [10]. The PLB considered in this study operates daily start-and-stop (DSS) operations to ensure smooth heat supply. Hence, the facility is exposed to frequent temperature changes, which deteriorates the durability in the material. DSS operation also causes surface defects, which act as a stress concentration point reducing a fatigue lifetime of boiler material [11; 12]. A critical issue in this study is the progressive tube failure occurring after initial leakage events in the convection section. Due to the structural characteristics of the PLB, internal tube leaks cannot be immediately repaired and must wait until scheduled overhauls. As a result, once leakage begins, additional failures tend to develop in adjacent tubes, following a specific directional pattern. These failures have significantly increased maintenance demands. While the facility's total heat supply capacity has remained stable through maintenance, the growing frequency of tube failures has prolonged maintenance downtime and driven up operational costs. Additionally, the accelerating failure cycle has made maintenance planning increasingly challenging, resulting in higher workloads and greater resource demands. As failure intervals continue to shorten, intervention frequency is expected to rise, further compounding costs. Consequently, maintenance and heat supply expenses have already increased by approximately 16%, with further escalation anticipated if the issue remains unresolved.