Dew point corrosion

The dew point temperature of flue gas depends on the vapor pressure of water vapor in the flue gas and the concentration of SO3. The vapor pressure of water vapor alone is around 0.1, with a dew point temperature of 30–60°C. When SO3 in the flue gas combines with water vapor to form sulfuric acid vapor, the dew point temperature can be increased to 90–180°C.

Low-temperature dew point corrosion http://bbs.hcbbs.com/thread-95324-1-1.html

When the flue gas temperature is below the dew point temperature of water vapor, the water vapor in the flue gas condenses, and together with SO2 and SO3 present in the flue gas, it causes chemical and electrochemical corrosion of the pipes; this phenomenon is known as dew point corrosion. ⑴Chemical corrosion: H2SO4 + Fe → FeSO4 + H2↑; H2SO3 + Fe → FeSO3 + H2↑. (2) Electrochemical corrosion: Anode (Fe): Fe – 2e → Fe2+ (oxidation); Cathode (Fe3C): 2H+ + 2e → H2↑ (reduction). When SO2 and SO3, along with H2O, condense on the surface of pipes, many microcells are formed on the metal surface. Among them, iron with a lower potential acts as the negative electrode, where oxidation reactions take place; the metallic iron is continuously corroded, and ferrous ions are constantly released into the solution ； Iron carbide (Fe3C) with a high potential or slag impurities act as the anode; reduction reactions take place at the anode, and hydrogen ions in the solution gain electrons to become hydrogen gas. The rate of this electrochemical corrosion is much faster than that of chemical corrosion. The rougher the surface (such as welds) or the more impurities there are, the more severe the electrochemical corrosion will be. Once these areas are corroded, new surfaces are exposed, making them more susceptible to corrosion. The corrosion pits grow deeper until perforation occurs. Methods to prevent dew point corrosion include: ① Using corrosion-resistant materials, such as fiberglass and glass, for air preheaters. ②Anti-corrosion treatment is applied to the surface of the tubes in the air preheater. ③Return some of the hot air from the air preheater outlet to the inlet in order to increase the air temperature. ④Control the appropriate smoke exhaust temperature. ⑤Enhance insulation measures for flues and convection areas. To prevent dew point corrosion of the flue gases, the process design requires that the temperature of the heat exchange surface be higher than the dew point temperature of the flue gases, in order to avoid such corrosion. The dew point temperature of flue gas is related to the partial pressures of water vapor and SO3 in the flue gas; higher partial pressures result in a higher dew point temperature for the flue gas. The flue gas dew point temperature can be found using specialized charts. In practical operation, it should be determined based on factors such as the sulfur content of the fuel and the air superheat coefficient

The occurrence of dew point corrosion depends on the metal wall temperature, rather than the flue gas temperature. For example, the sulfur-containing flue gas from a coking furnace is at around 900°C, and the calculated dew point temperature of this gas is 150°C. If a water injection coil (with an inlet water temperature of 100°C) is placed in the radiation chamber, dew point corrosion occurs in the 900°C flue gas. Therefore, to avoid dew point corrosion, it is necessary to raise the feed temperature so that the wall temperature is above the flue gas dew point temperature.

I used to only be aware of dew point corrosion caused by smoke; I didn’t know that dew point corrosion also includes corrosion caused by hydrogen sulfide