【Basic Knowledge 28】What is dew point corrosion? (Questionnaire with Prizes)
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Please explain the concept of dew point corrosion. Dew point corrosion, in a general sense, occurs when the process gas reaches its phase transition point as it cools down, resulting in the formation of liquid dew (i.e., the dew point). Conversely, the temperature at which a liquid rises to the point of vaporization–boiling is called the boiling point. The temperatures of the two are the same, but the temperature level differs by the latent heat of vaporization). At this time, if there are some acidic substances in the medium (such as sulfur, chlorine, nitrogen, etc.), they will accumulate in the condensed water to form acids. For example, the most common form of dew point corrosion occurs in the flue gases emitted by boilers; the main acidic substances are sulfides – sulfuric acid and sulfurous acid (with small amounts of chlorides and nitrides present as well). Their concentration can reach 85%, causing severe corrosion of metals, especially stainless steel (and sometimes even stress corrosion).
Dew point corrosion: This type of corrosion occurs only when dew forms on the heated surface, which is why it is called dew point corrosion.
Flue gas dew point corrosion occurs because sulfur in the fuel is converted into SO2 and SO3 during combustion; when the outer surface temperature of the heat exchange surface is below the flue gas dew point temperature, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface.
Dew point corrosion, in a general sense, occurs when the process gas reaches its phase transition point as it cools down, resulting in the formation of liquid dew, that is, the dew point. Conversely, the temperature at which a liquid rises to the point of vaporization–boiling is called the boiling point. The temperatures of the two are the same, but the level of temperature capability differs by the latent heat of vaporization. At this time, if there are some acidic substances in the medium (such as sulfur, chlorine, nitrogen, etc.), they will accumulate in the condensed water to form acids. For example, the most common form of dew point corrosion occurs in the flue gases emitted by boilers; the main acidic substances are sulfides – sulfuric acid and sulfurous acid (with small amounts of chlorides and nitrides present as well). Their concentration can reach 85%, causing severe corrosion of metals, especially stainless steel (and sometimes even stress corrosion).
Flue gas dew point corrosion occurs because sulfur in the fuel is converted into SO2 and SO3 during combustion; when the outer surface temperature of the heat exchange surface is below the flue gas dew point temperature, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface.
Flue gas dew point corrosion occurs because sulfur in the fuel is converted into SO2 and SO3 during combustion; when the outer surface temperature of the heat exchange surface is below the flue gas dew point temperature, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface.
Dew point corrosion – As sulfur in the fuel is burned to produce SO2 and SO3, when the outer surface temperature of the heat exchange surface is lower than the temperature of the flue gases, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface.
Flue gas dew point corrosion occurs because sulfur in the fuel is converted into SO2 and SO3 during combustion; when the outer surface temperature of the heat exchange surface is below the flue gas dew point temperature, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface.
During the cooling process, the process gas reaches its phase transition point and liquid condensation occurs, mainly in the form of water, which typically adheres to the surface of the metal components of the equipment. If acidic substances are present in the medium (such as sulfur, chlorine, nitrogen, etc.), they will accumulate in the condensed water. This leads to dew point corrosion.
Please explain the concept of dew point corrosion. Dew point corrosion – As sulfur in the fuel is burned to produce SO2 and SO3, when the outer surface temperature of the heat exchange surface is lower than the temperature of the flue gases, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface.
Flue gas dew point corrosion occurs because sulfur in the fuel generates SO2 and SO3 during combustion; when the outer surface temperature of the heat exchange surface is lower than the temperature of the flue gas, sulfuric acid mist droplets form on that surface, leading to corrosion of the heat exchange surface. When fuel burns, the hydrogen (H2) and oxygen (O2) 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. Additionally, sulfur (S) in the fuel produces sulfur dioxide (SO2) upon combustion; a small amount of this SO2 is further oxidized to sulfur trioxide (SO3). Sulfur trioxide 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. The level of dew point temperature is related not only to the sulfur content in the fuel, but also to factors such as the excess air coefficient and the amount of sulfur trioxide generated. The higher the furnace temperature, the less excess air there is; as a result, a smaller proportion of the SO2 generated from sulfur during combustion is oxidized to SO3, and the dew point temperature becomes lower. This is why the relationship between dew point temperature and fuel sulfur content provided in general data is not exactly the same. Based on the sulfur content of fuels in our country, the dew point temperature is generally in the range of 105–130°C. When possible, it is best to use the dew point temperature for actual measurements on-site. During operation, if the wall temperature at the interface between the heated surface and the soot is below the dew point, in addition to causing corrosion, it also leads to the soot adhering to the heated surface; such sticky ash buildup is difficult to remove using conventional soot-blowing methods. Due to the accumulation of ash, it not only affects heat transfer efficiency and increases the flow resistance on the flue gas side, but it also exacerbates corrosion; in severe cases, the metal corrosion products along with the ash can block the passages. Therefore, when burning sulfur-containing fuels, it is very important to take measures to keep the temperature of the metal in contact with the flue gases above the dew point. Other factors affecting the corrosion rate include the concentration of sulfuric acid and the wall temperature. The corrosion rate of concentrated sulfuric acid on steel is very low, while the corrosion rate of sulfuric acid on carbon steel is highest at a concentration of around 50%. Regarding wall temperature, at higher temperatures, the chemical reaction rate increases, which in turn accelerates corrosion. Therefore, due to the differences in sulfuric acid concentration and wall temperature at various low-temperature areas, the corrosion rates vary. The most important way to reduce corrosion caused by low-temperature dew points is to keep the wall temperature of the tubes or heating elements above the dew point, or to use corrosion-resistant materials. Raising the wall temperature can be achieved by increasing the temperature of the medium outside or inside the tube; for example, the temperature of the oil fed into the furnace at low temperatures should be above 100°C, heat air circulation should be used in air preheaters, or other media can be employed to raise the temperature of the incoming air to above 60°C. Additionally, reducing excess air and using detachable structures in areas with low temperatures are also effective measures that are often applied