About dew point corrosion
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In some data, it is seen that dew point corrosion is due to the presence of sulfides. Isn’t it polythionic acid?
Dew point corrosion is due to the presence of sulfides in the flue gas, which generates sulfur dioxide and sulfur trioxide during combustion. When the outer surface temperature of the heat exchange surface is lower than the flue gas temperature, sulfuric acid dewdrops are formed on the heat exchange surface, causing corrosion. Polythionate causes stress corrosion, not dew point corrosion!
As mentioned on the 2nd floor, there are also other corrosive substances, such as chloride and other acid ions. They are only inferior to sulfide. Different gases have different dew points, mainly due to the temperature at which the gas turns into liquid. For example, the dew point of fluoride is many degrees below zero.
The concept of dew point: The lower the temperature, the smaller the saturated water pressure. Therefore, for a gas containing a certain amount of water vapor, the temperature is lowered while the pressure remains unchanged, so that the saturated water vapor pressure drops to the temperature equal to the actual water vapor pressure at that time, which is called the dew point. Gases of different compositions have different dew points. dew point corrosion: This gas contains components such as sulfide. When the temperature reaches or falls below the dew point, "condensation" occurs and liquid is produced, which has a strong corrosive effect on the steel structure. Therefore, the temperature of the gas should be 30 to 50 degrees above its dew point during design.
When air, water and sulfide exist at the same time, polythionic sulfate H2SxO6 can be formed, aggravating corrosion!