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Definitions related to dew point corrosion: Dew point corrosion, in a broad sense, refers to the formation of liquid condensation when the process gas reaches its phase transition point as it cools down. Dew point: From a chemical perspective, it is the temperature at which water vapor begins to condense ； From a meteorological perspective, it is the temperature at which cooling occurs without changes in humidity and pressure until saturation is reached. Dew point corrosion is the corrosion of steel caused by saturated steam condensate. Condensed steam type: water vapor, oil and gas. Water vapor containing corrosive agents: Corrosive agents include HCl, CO2, SO2, SO3, NOX, etc. In the areas subject to dew point corrosion, such as heating furnaces and boilers, flue gases containing HCl, CO2, SO2, SO3, NOX, etc., condense at lower temperatures within the furnace, resulting in acidic corrosion; this applies to components like air preheaters, economizers, cold feed in the convection section, furnace walls, and chimneys. 02 Blind areas of equipment and pipes within pipelines or devices, closed bypasses, cold ends equipped with insulation plugs, situations where a cooling medium on one side of a pipe causes localized condensation and corrosion on the other side, as well as the reflux area at the top of a tower. Common hazardous areas include the convection section of the heating furnace, the furnace walls, the boiler air preheater, various monitoring points in the gasifier, the drain lines of the synthesis gas pipeline, pressure tapping points, and temperature measurement points. Characteristics of dew point corrosion: Dew point corrosion is primarily an electrochemical form of corrosion, and its manifestations include uniform corrosion, pitting corrosion, stress corrosion cracking, hydrogen-induced cracking, etc. As the cooling temperature decreases, the acid concentration of the condensate increases from high to low, while corrosion levels change from low to high to low. Dew point corrosion is widespread on the inside and outside of various equipment in coal chemical industries; internally, it is caused by corrosion from the process media, while externally, it results from the combined action of the atmosphere and various corrosive agents. Flue gas dew point corrosion: Impurities in the fuel, including chlorides, sulfides, nitrides, heavy metals (such as vanadium), etc., generate gases during combustion that affect the degree of corrosion caused by the dew point condensate. The V2O5 produced by combustion is a substance with a low melting point that causes condensation and corrosion on the surface of stainless steel; it also acts as a catalyst for the conversion of SO2 to SO3. The dew point temperature of sulfuric acid is proportional to the oxygen content, water vapor content, and sulfur content. The dew point temperature of hydrochloric acid is proportional to the chlorine content, generally ranging from 27 to 60°C. The nitric acid dew point temperature is proportional to the nitride content, generally ranging from 30 to 60°C. The factors that affect the corrosion rate and the laws of low-temperature corrosion in metals are mainly the amount of condensed acid, the concentration of acid exposure, and the temperature of the metal surface. When the wall temperature is high, slightly below the dew point, the amount of acid that condenses on the wall surface is very small, and the corrosion rate is slow. As the wall temperature decreases, the amount of condensed acid increases, leading to a significant increase in the corrosion rate. Typically, the wall temperature at the maximum corrosion point is about 20~45°C lower than the dew point. As the wall temperature decreases further, the amount of acid that condenses becomes sufficient; at this point, the corrosion rate is almost independent of the acid concentration and depends solely on the wall temperature. As the wall temperature decreases, the acid concentration in the acid dew also decreases. Although the decrease in acid concentration in the acid dew increases the corrosion rate, the effect of wall temperature on the corrosion rate is greater than that of acid concentration, resulting in a decrease in the corrosion rate. After dropping to a certain level, since the effect of concentration outweighs that of wall temperature, the corrosion rate increases again as the wall temperature decreases. Measures to prevent flue gas dew point corrosion 1) Use clean fuel ； 2) Low air excess coefficient ； 3) Complete combustion of fuel ； 4) Increase the feed temperature to keep the wall temperature above the dew point temperature (generally, the dew point temperature is set at 150°C when the fuel contains more than 2% sulfur; the temperature of the fluid inside the pipe is set at 135°C, and the flue gas temperature can be 220°C if the area is sufficient) ； 5) Increase the wall temperature of the equipment casing (heating furnace: 90~100°C, FCC regenerator: 180°C) ； 6) Use of corrosion-resistant materials: nickel-based alloys C-276, C-22 ； ND steel, an alloy containing copper and chromium, with an outer coating (fluororubber, enamel, high-temperature paint, sprayed stainless steel + paint sealant, etc.) ； 7) Additives: to neutralize/inhibit the formation of sulfuric acid, such as MgO, Mg(HO)2, which not only neutralizes sulfur but also V5O2 ； 8) Protect the furnace wall with coating ； 9) The furnace insulation lining is made of a dense material to prevent flue gas from leaking through. Other preventive measures: 1) Enhance insulation or raise wall temperatures. After dew point corrosion was detected in equipment and pipelines in many coal chemical plants, the temperature of these equipment and pipelines was increased by providing additional heating to prevent the occurrence of dew point corrosion ； 2) Prevent local supercooling during design, and use corrosion-resistant materials when necessary. For example: the three trays at the top of the atmospheric distillation column are made of MONEL alloy, the top head is lined with MONEL steel plates, and the condenser tubes are made of MONEL or titanium ； 3) Be aware of climate changes, and install shielding facilities in areas where strong winds or heavy rain affect the wall temperature.