Talking about corrosion on the flue gas side of boilers
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This post was last edited by Wang Wei2 on 2025-6-9 at 10:30. Corrosion on the flue gas side can be divided into high-temperature corrosion and low-temperature corrosion, depending on the location and conditions in which it occurs. When high-moisture, high-sulfur fuels are burned, the tubes of the high-temperature heating surfaces are corroded, a phenomenon known as high-temperature corrosion. Low-temperature corrosion refers to the corrosion that occurs on the low-temperature heating surfaces at the tail section. 1. High-temperature corrosion: Tube failure caused by high-temperature corrosion on the flue gas side, with the following corrosion mechanisms: (1) Action of corrosive gases such as SO2, SO3, and H2S ； (2) Mechanism of high-temperature corrosion in sulfate form ； (3) Mechanism of sulfide-type high-temperature corrosion. Near the crack, a large area of corrosion can be clearly identified; this corroded area is uneven in texture. The wall thickness decreases near the crack, and the fracture appears to be tear-shaped with long cracks. The difference between corrosion-induced tube failure and wear-induced tube failure is that the wall near a tube that has failed due to wear is smooth, with worn edges, whereas the area around a tube that has failed due to corrosion is uneven, without any distinct edges; there are no significant changes in the microstructure at the fracture site, and the grains at that location are elongated, indicating a ductile fracture; on the smoke-side of tubes that have failed due to corrosion, sulfur-containing deposits are present on the outer surface of the wall, while on the side closer to the substrate, these deposits are usually black and tightly bonded to the wall. 2. Low-temperature corrosion: Low-temperature corrosion-induced tube failure mainly occurs in economizers where the feedwater temperature is low. Near the fracture site of low-temperature corrosion-induced tube failure, there are also uneven corrosion areas; the fracture is of a tearing type, with no significant changes in the microstructure. The grains at the fracture site are elongated, indicating a ductile fracture.
Corrosion on the flue gas side of boilers occurs in two main forms: First, high-temperature corrosion, which takes place in the high-temperature heating surfaces of the boiler, and is particularly evident when burning fuels with high sulfur content and high moisture levels. Common mechanisms: 1. Corrosion resulting from the direct reaction of sulfur-containing corrosive gases such as SO₂, SO₃, and H₂S ； 2. High-temperature corrosion caused by sulfate types (such as the condensation and melting of alkali metal sulfates like sodium and potassium on the tube walls) ； 3. High-temperature corrosion of sulfide types (such as ferrous sulfide FeS). Corrosion characteristics: - Thinned wall, uneven surface, with no distinct smooth edges or corners. - The crack at the tube burst site is long, exhibiting a tensile, ductile fracture pattern. - In the area where the tube burst, there are black or sulfur-containing deposits near the tube wall matrix, which are firmly bonded to the tube wall. II. Low-temperature corrosion: Occurs in the heated surfaces at the rear of the boiler (such as economizers and air preheaters). In this case, SO3 and water vapor in the flue gas condense to form sulfuric acid, which causes acid corrosion of the tube walls. The basic characteristics are: – Corrosion is usually accompanied by condensation; the surface of the corroded tubes becomes moist, showing obvious corrosion spots and marks, and the metal surface becomes rough. Key differences from wear-induced tube failure: - The surface near the wall of a tube damaged by wear is smooth, without any obvious signs of wear or sharp edges ； The surface of a corroded and burst pipe is rough and uneven, with no distinct edges or corners. Control measures: - Control the sulfur content in fuel and optimize combustion air supply ； - Appropriately increase the temperature of the flue gas at the tail end to prevent it from reaching the acid dew point ； - Reasonably design and arrange the heating surfaces and soot-blowing facilities, and strengthen the management of ash accumulation on the flue gas side. .