# The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System
## 2 Materials and Methods
### Non-Destructive Inspection  
Due to the structural constraints of the PLB, direct sampling of damaged tubes was not feasible. Instead, an endoscopic inspection was conducted to assess the internal condition of the convection tubes and characterize gas-side corrosion damage (Figure 2). The inspection focused on regions surrounding pre-existing water leaks, which were predominantly concentrated at 90\({}^{\circ}\) relative to the manhole (defined as 0\({}^{\circ}\)). This selection was based on historical failure records and operational observations, which indicated a tendency for leakage events to occur in this direction. The affected areas were systematically examined as part of a progressive damage chain originating from the initial leak site. Additionally, leak-free tubes were selected from various locations within the convection section to ensure a reliable comparison with leaked tubes. To compare the internal condition of leak-free and leaked tubes, selections were made at symmetric positions relative to the leakage-prone 90\({}^{\circ}\) direction. Specifically, tubes were chosen at 270\({}^{\circ}\) to investigate whether similar corrosion patterns were developing on the opposite side. Each selected tube position was recorded using a row-column notation for precise identification (Figure 2).  
To examine the internal surface condition, a miniature camera was inserted through the lower end plate, capturing images as it ascended toward the upper section. The convection tube was divided into two distinct regions: the lower section (0-4.65 m from the lower  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Element (wt\%)** & **C** & **Si** & **Mn** & **P** & **S** & **Cu** & **Ni** & **Mo** & **Cr** & **Fe** \\ \hline SA178-Gr.A & \(\leq\)0.18 & - & 0.24–0.63 & \(\leq\)0.035 & \(\leq\)0.035 & - & - & - & & Bal. \\ SA516-Gr.70 & \(\leq\)0.30 & 0.13–0.45 & 0.85–1.30 & \(\leq\)0.025 & \(\leq\)0.025 & \(\leq\)0.30 & \(\leq\)0.30 & \(\leq\)0.08 & \(\leq\)0.30 & Bal. \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of boiler materials used in the peak load boiler (PLB) (SA178-A for tubes, SA516-70 for plates).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Composition** & **Methane** & **Ethane** & **Propane** & **i-Butane** & **N-Butane** & **i-Pentane** & **N-Pantane** & **Nitrogen** \\ \hline Average (mol.\%) & 93.38 & 4.665 & 1.221 & 0.2563 & 0.2853 & 0.0175 & 0.0041 & 0.1673 \\ \hline \end{tabular}
\end{table}
Table 2: Chemical composition of LNG used in the peak load boiler (PLB).  
Figure 2: (**a**) Photograph of the top of the convection section, illustrating tube arrangement and failure trend. (**b**) Schematic diagram of tube layout, leakage locations, and measurement points.  
end plate), which consisted solely of the tube structure, and the upper section (4.65-6 m), where a spiral turbulator was installed (Figure 1). Since the presence of the turbulator alters flow characteristics and heat transfer conditions, corrosion behavior was expected to vary between these regions. To further assess material loss due to corrosion, ultrasonic thickness measurements were conducted following the methodology described in [13]. This technique allowed for differentiation between internal (gas-side) and external (boiler water-side) corrosion, providing key insights into the dominant direction of corrosion propagation and the structural integrity of the convection tubes.