The sulfur content in fuel after combustion generates sulphur dioxide, which will be oxidized to sulfur trioxide. Sulfur trioxide and steam in the fume gas form sulfate. When the wall temperature of heating surface is less than the dew temperature of sulfuric acid steam, the sulfuric acid steam will condense on the pipe wall and the corrode the heating wall. 

Low temperature corrosion mainly depends on the content of sulphur trioxide in flue gas and tube wall temperature. 

The increasing of sulfur trioxide in flue gas will not only improve the dew point of flue gas but also will increase the amount of sulfuric acid condensation, thus improving the degree of corrosion. 

As long as the wall temperature of the heating surface is less than the dew point of flue gas, the sulfuric acid will begin to condense on the heating surface and corrode the heating surface.