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O1 Introduction: Dew point corrosion, in a general sense, refers to the formation of liquid dew when process gases reach their phase transition point as they cool 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. O2 corrosion sites (I) – Heating furnaces and boilers: Flue gases containing HCl, CO₂, SO₂, SO₃, NOX, etc., condense in the cooler areas of the furnace, causing acidic corrosion; this affects components such as air preheaters, economizers, cold feed in the convection section, furnace walls, and chimneys. (II) Inside pipes or equipment: dead ends of equipment and pipes, closed bypasses, cold ends equipped with insulation plugs, areas 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 the 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. O3 corrosion characteristics: 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 temperature gradually decreases, the concentration of condensed acid increases from low to high until an adequate amount is reached. The corrosion rate varies as low-high-low with temperature and acid concentration. Dew point corrosion is widespread on the inside and outside of various equipment in the coal chemical industry; 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. 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 into 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 directly 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. O4 Corrosion rate and low-temperature corrosion patterns: The main factors affecting the metal corrosion rate are the amount of condensed acid, the concentration of acidic dew, 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 drops 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. O5 Measures to prevent flue gas dew point corrosion ● Use clean fuels ； ●Low air excess coefficient ； ●Complete combustion of fuel ； ●Raising the feed temperature ensures that the wall temperature remains 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 135°C, and the flue gas temperature can reach 220°C if the area is sufficient) ； ●Increase the wall temperature of the equipment casing (heating furnace: 90–100°C, FCC regenerator: 180°C) ； ●Use of corrosion-resistant materials: nickel-based alloys C-276, C-22 ； ND steel, a copper- and chromium-containing alloy, with external coatings (fluororubber, enamel, high-temperature paint, sprayed stainless steel + painted sealant, etc.) ； ●Additives: Additives are introduced into the flue gas to neutralize HCl, CO₂, SO₂, SO₃, NOX, etc., and to prevent the formation of acidic vapors ； ● The furnace walls are protected with paint ； ● The furnace insulation lining uses dense materials to prevent the penetration of flue gases. O6 Other preventive measures ● Enhance insulation or increase wall temperature; after dew point corrosion was detected in equipment and pipelines in many coal chemical plants, the temperature of these equipment and pipelines was raised by adding heat tracing to prevent the occurrence of dew point corrosion ； ●Prevent local supercooling during design, and use corrosion-resistant materials when necessary. For example: the three trays at the top of the atmospheric pressure column in distillation units are made of MONEL alloy, the top head is lined with MONEL steel plates, and the tubes of the condenser are made of MONEL or titanium ； ●Be aware of climate changes, and install shielding facilities in areas where strong winds or heavy rain affect the wall temperature.