What is dew point (low-temperature) corrosion?

Thread Content

Dew point corrosion occurs in many coal chemical plants, power plants, and similar facilities. When fuel burns, the hydrogen (H) and oxygen (O) in it combine to form water vapor (H2O), and since most burners use steam for atomization, the flue gases in the furnace contain a large amount of water vapor. Furthermore, sulfur (S) in the fuel produces sulfur dioxide (SO2) upon combustion; a small amount of this SO2 is further oxidized to sulfur trioxide (SO3), which combines with water vapor in the flue gases to form sulfuric acid (H2SO4). The dew point of flue gas containing sulfuric acid vapor rises significantly; when the wall temperature of the heating surface is below this dew point, the sulfuric acid-containing vapor condenses on the heating surface to form a sulfuric acid-containing liquid, causing severe corrosion to the heating surface. Since it occurs on the heated surface at lower temperatures, it is called low-temperature corrosion. Since this type of corrosion occurs only after dew forms on the heated surface, it is also known as dew point corrosion. Depending on the corrosive medium present, dew point corrosion is classified into types such as flue gas dew point corrosion, sulfuric acid dew point temperature, hydrochloric acid dew point temperature, and nitric acid dew point temperature. I. Areas prone to dew point corrosion: 1. In heating furnaces and boilers, flue gases containing HCl, CO2, SO2, SO3, NOX, etc., condense in the cooler parts of the furnace, causing acidic corrosion; these areas include air preheaters, economizers, cold feed in the convection section, furnace walls, and chimneys. 2. 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, and the reflux area at the top of a tower, etc. 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. II. 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. III. Corrosion rate and laws of low-temperature corrosion: The factors that mainly affect the metal’s corrosion rate are the amount of acid condensed, the concentration of the acid solution in contact with the metal, 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. IV. 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 housing (90~100°C for the heating furnace, 180°C for the FCC regenerator) ； 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. V. 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 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 ； 3) Be aware of climate changes, and install shielding facilities in areas where strong winds or heavy rain affect the wall temperature. 4) Pay attention to internal management. As the person in charge of the office, faced with a large number of complicated and routine tasks, I am able to organize my work effectively and speed up the pace of tasks.