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Corrosion-induced tube rupture in boiler flue pipes and preventive measures (case study): A company owned a boiler that, after operating for nearly 19 years, began to leak water from its flue box. Upon inspection, a hole was found in the first row from the top, and the 6th pipe from the right, in the front flue box, at a distance of 110 cm from the front tube sheet； A corrosion perforation was found at a distance of 120 cm from the front tube sheet in the first row counted from top to bottom, and the 5th tube counted from right to left. This boiler has an evaporation capacity of 4 t/h, a pressure of 1.27 MPa, and an operating pressure of 1.0 MPa. The boiler drum is made of 16Mhg material, while the smoke tubes are made of grade 20 steel. The water source is river water; the water treatment equipment is damaged, so chemicals are used for treatment inside the furnace. Inspection: A 500 mm sample was cut from the smoke pipe for inspection, and it was found that there were numerous corrosion products on the upper half of most smoke pipes, with a thickness of 3–8 mm; the thickest sample had corrosion products 12 mm thick. The surface of these corrosion products was brick-red in color, while the interior was brownish-black. They formed a multi-layered structure, were hard and brittle, and had a porous interior. When broken using a hammer, black powder was found inside them. After removing the corrosion products, the area right next to the metal surface is black. By knocking off the black substance on the metal wall, a distinct dent can be seen; there are ulcerative corrosion spots on the water side of the boiler drum and on the downcomers, with moderate depths. Under normal operating conditions, a layer of Fe3O4 film typically covers the surface of the boiler metal. This Fe3O4 film is dense and possesses good corrosion resistance. If the Fe3O4 film is damaged, the metal surface is exposed to the furnace water and becomes highly susceptible to corrosion. An important factor contributing to the degradation of Fe3O4 films is an inappropriate pH value of the boiler water. Since this boiler operates at a high pH level, the reason for the accelerated corrosion is that the Fe3O4 protective layer on the metal surface dissolves in the solution and is thus destroyed. Analysis: The boiler water in this plant has an alkalinity of 131 mmol/L, a Cl- concentration of 1921 mg/L, and a SO2-3 level of 828.2 ppm. When the boiler operates under high alkalinity and high concentrations of Cl- and SO2-3 ions, no protective film forms on the metal surface, which is also one of the factors contributing to corrosion. Since no deoxygenation measures were taken for the water used in this boiler, and due to the frequent starting and stopping of the boiler – especially the water addition before and after shutdown – a large amount of dissolved oxygen entered the boiler drum. When water containing high levels of dissolved oxygen comes into close contact with the metal walls of the smoke tubes, the dissolved oxygen reacts with impurities on those metal walls that have an extremely high electrode potential (cathodes), resulting in the formation of a large number of OH- ions. The secondary products formed as a result of corrosion caused by dissolved oxygen in water are often loose and provide no protection. Once a corrosion site forms on the metal surface, it is impossible to prevent further corrosion from occurring. At this corrosion site, the diffusion of dissolved oxygen from the water to this area is slowed down due to the presence of corrosion products, resulting in a higher concentration of dissolved oxygen around the corrosion site compared to the concentration at that site itself. As a result, the surrounding area becomes the cathode, while the corrosion site itself becomes the anode, and the corrosion process continues. At this point, the Fe2+ generated by corrosion slowly diffuses outward through the loose secondary product layer; when it encounters substances such as OH- or O2 in water, new secondary products are formed and accumulate within the existing secondary product layer. Thus, the secondary products accumulate and thicken, forming bubbles; beneath these bubbles, corrosion becomes more severe, creating pits that eventually lead to perforation. The results of the material identification tests show that the material properties of this furnace meet the corresponding specifications in GBTB-86 \"Carbon and Low-Alloy Steel Plates for Boilers.\" However, the grade 6 banded structure identified through metallographic analysis represents a structural defect, which is also one of the factors that accelerate metal corrosion. Protective measures: 1. Improve water quality management, repair sodium ion exchangers, and enhance training for water treatment personnel. When using boilers, it is necessary to monitor not only the quality of the feed water but also the Cl- content, alkalinity, and pH level of the boiler water. In case these parameters exceed the specified limits, the causes must be identified and appropriate corrective actions taken to ensure compliance with **standards. 2. Strengthen feedwater deoxidation treatment by installing additional deoxidization devices, or by using chemical deoxidation methods such as sulfite deoxidation, sponge iron deoxidation, organic agent deoxidation, catalytic resin deoxidation, and so on. 3. Strengthen maintenance work during boiler shutdowns. For short-term shutdowns, wet protection can be used, while for long-term shutdowns, dry protection or gas-phase corrosion inhibitor methods can be employed to prevent air from entering the unused boiler and to keep its metal surface fully dry. 4. Strengthen regular inspections of the boiler’s smoke pipes. When signs of corrosion or cracking are detected in these pipes during operation, they should be replaced promptly to prevent pipe rupture while the boiler is in use, which could lead to serious accidents. This case is quite typical and holds general guiding significance for the corrosion damage of flue pipes in boilers of the same type.