Introduction to the analysis of the causes of sulfuric acid dew point corrosion

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Through the observation of corrosion products and testing using modern physical techniques, the cause was sulfuric acid dew point corrosion resulting from the presence of sulfides in the fuel gas. During combustion in the heating furnace, the fuel oil generates high-temperature flue gases containing SO2 and SO3. In the cooler sections of the heating furnace, SO2 and SO3 combine with moisture in the air to condense at the dew point, forming sulfuric acid, which causes dew point corrosion and severely damages the equipment. It is reported that 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. Widespread sulfuric acid dew point corrosion is most likely to occur at the beginning of operation or shutdown of the heating furnace, as well as when the furnace’s load is low ； During the normal operation of the heating furnace, the areas subject to corrosion vary depending on the SO3 content in the high-temperature flue gas and the temperature of the surface of the heat exchange tubes. Rust spots and dust accumulation on the surface of the furnace tubes often become the cause of sulfuric acid dew point corrosion, which is the mechanism behind such 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. Ferric 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 (the high acidity can be seen from the pH value of 2.3 of the corrosion products). If the corrosion products are not removed in a timely manner, the furnace tubes will also be subjected to corrosion by dilute acid throughout 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 corrosion process.