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The core cause of corrosion of heating furnace air heaters (also often called air preheaters) is low-temperature dew point corrosion, which is mainly caused by the sulfur content of the fuel and equipment operating conditions. It can be divided into three categories:: 1. Core chemical corrosion (dew point corrosion) The fuel used in the heating furnace (crude oil, fuel oil, gas, coal, etc.) usually contains sulfide, which will generate sulfur dioxide and sulfur trioxide after combustion. They combine with the water vapor in the flue gas to generate sulfuric acid vapor. The dew point of acid vapor is much higher than that of water vapor (usually above 100°C). When the air heater is in the area with the lowest flue gas temperature, if the temperature of the metal pipe wall is lower than the dew point temperature of sulfuric acid, sulfuric acid vapor will condense on the pipe wall to form liquid sulfuric acid/sulfurous acid, which will directly corrode metal equipment. 2. A vicious cycle in which ash deposits aggravate corrosion. After fuel is burned, solid ash, metal salts and incompletely burned residual carbon particles will remain. These particles are easily deposited on the air heater tube bundles to form ash deposits. The condensed sulfuric acid liquid will adhere to dust, forming sticky ash that is difficult to clean and blocking the flue. The heat insulation effect of the dust will further reduce the temperature of the pipe wall, prompting more sulfuric acid to condense. At the same time, sulfuric acid vapor can diffuse through the ash layer to the metal wall, continuously accelerating corrosion, forming a vicious cycle of "dust accumulation - cooling - easier dust accumulation and more corrosion". 3. Design and environmental factors: The heat transfer elements of the air heater itself are closely arranged, and they are prone to accumulate dust and retain corrosive substances, aggravating corrosion. ； If it is a ducted air heater and the water quality is not up to standard, chlorine ions in the water will accumulate at the pipe turns and welded joints, causing increased corrosion at the joints. ； If the shell is made of steel-aluminum connection material, it is prone to electrolytic corrosion if it is exposed to moisture or acid and alkali environment for a long time.