Fuel constituents and metal temperatures are important factors in the promotion of fireside corrosion. Fireside corrosion can be classified as either low-temperature attack or high-temperature oil-ash corrosion. Corrosion may occur on the flue-gas side of economizer and air preheater tubes. The severity of this corrosion depends on the amount of sulfur oxides or acid in the fuel burned and on the temperature of the flue gas and of the media being heated. When sulfur oxides are present in the flue gases, corrosion tends to be severe if the gases cool down to the dew-point temperature. The gas temperature in economizers and preheaters should be kept above about 325°F (163°C) to prevent condensation of corrosive liquid. Actual dew point can be calculated from the flue gas composition and should be performed for fuels with high sulfur levels. This may be best affected by designing the tubing and the water flow in the tubing so that the gas temperatures are controlled as noted in the preceding text.
External corrosion of boiler parts may be expected when boilers are out of service for long periods of time. The sulfurous acid formed from the reaction of condensed moisture with the sulfur in ash deposits can cause rapid corrosion of boiler parts. Also, if a unit remains idle for an appreciable length of time, a warm humid atmosphere tends to corrode boiler parts and supports, unless adequate mothballing procedures are followed.