Why is low-temperature, humid flue gas from heating furnaces so highly corrosive to metals?

The flue gas contains components that cause stress corrosion in metals, particularly wet hydrogen sulfide. When used with carbon steel, the corrosion allowance for this area is usually set at 3 mm or more.

Fuel oil contains sulfur, and upon combustion it produces sulfur dioxide. Due to the excess oxygen in the combustion chamber, part of this sulfur dioxide is oxidized to form sulfur trioxide; at low temperatures, sulfur trioxide reacts with water vapor to produce sulfuric acid vapor

If the temperature is too low, below the dew point, dew point corrosion will occur; therefore, it is necessary to keep the temperature above the dew point.

We know that fuel gas contains sulfides and other substances that can cause sulfuric acid dew point corrosion. During combustion in the heating furnace, the fuel oil produces high-temperature flue gases containing SO2, SO3, H2S, and other substances. In the cooler sections of the heating furnace, SO2 and SO3 combine with the moisture in the air at the dew point, forming sulfuric acid, which causes dew point corrosion and severely damages the equipment. According to literature reports, for combustion products containing 0.0025% SO3 by volume, the acid dew point is 132°C, while it rises to 171°C when the concentration is 0.0085%. It can be seen that the higher the SO3 content in the flue gas, the higher the acid dew point temperature. 1. Sensitive times and locations for sulfuric acid dew point corrosion. The corrosion sites vary depending on the SO3 content in the high-temperature flue gas and the surface temperature of the heat exchange tubes. Rust spots and dust accumulation on the surface of the furnace tubes often become sensitive areas that trigger sulfuric acid dew point corrosion. 2. Mechanism of sulfuric acid dew point corrosion. Sulfuric acid dew point corrosion is actually corrosion caused by high-temperature dilute sulfuric acid, and its corrosion behavior follows the principles of corrosion in non-oxidizing acids. FeSO4, which is produced as a result of acid dew point corrosion, can be converted back into ferric sulfate under the action of SO2 and O2 in the flue gas: 2FeSO4 + SO2 + O2 = Fe2(SO4)3. Fe2(SO4)3 adheres to and deposits on the furnace tubes, forming a layer of corrosion products. Ferrous sulfate is an acidic substance that is prone to absorbing moisture. When the heating furnace is shut down and cooled down, Fe2(SO4)3 begins to absorb moisture and deliquesce, creating a highly acidic corrosive environment on the surface of the furnace tubes. If the corrosion products are not removed in a timely manner, the furnace tubes will be corroded by dilute acid during the shutdown period. That is, Fe2(SO4)3 itself will also cause metal corrosion to produce FeSO4. This thus creates a corrosion cycle of FeSO4→Fe2(SO4)3→FeSO4, **accelerating the rate of corrosion. 3. Hazards of H2S: H2S can be present in flue gases as a result of the decomposition of organic sulfides contained in the fuel gas in the heater at high temperatures, as well as from that portion of H2S in the fuel gas which is not completely converted into SO2 during combustion. H2S readily reacts with most metals to form sulfides, and its corrosive strength is related to the corrosion resistance of the resulting sulfide film. When carbon steel furnace tubes come into contact with H2S, a loose and brittle sulfide corrosion layer forms on their surface; due to the action of air currents and other corrosive gases, this sulfide layer tends to peel off easily.