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Acid Dewpoint Corrosion
Sulfur and chlorine species in fuel form sulfur trioxide and hydrogen chloride within the combustion products. At low enough temperatures, sulfur trioxide and water vapor in the flue gas condense as sulfuric acid, causing rapid corrosion. At lower temperatures, hydrochloric acid may condense and promote corrosion of carbon steels and stress corrosion cracking of stainless steels.
During fuel combustion, sulfur oxidizes to form SO2 and consequently SO3. The free SO3 in the flue gas reacts with moisture to form sulfuric acid (H2SO4). The concentration of H2SO4 depends on the SO3 and the moisture content; hence the acid dewpoint temperature in the flue gas will vary accordingly. The condensation temperature of H2SO4 is approximately 250-300oF and below. The dewpoint of sulfuric acid depends on the exact concentration of sulfur trioxide in the flue gas but is approximately 2800F. Note that a combination of hydrochloric acid (HCl) and hydrobromic acid (HBr) can condense at a lower temperature and it could be as low as 130oF.
Acid dewpoint corrosion produces tube wall thinning that eventually results in ductile rupture of the steel. The external surface exhibits a gouged appearance where the corrosion activity has occurred. A thin edged fracture surface is produced when the load carrying ability of the steel is exceeded. Sulfuric acid corrosion on economizers or other carbon steel components is evidenced by general wastage with the sulfuric acid condensation perhaps forming broad, shallow pits.
Acid dewpoint corrosion can occur at locations in the economizer that have:
1)Boiler tube metal temperatures below the acid dew point, so that condensate forms on the metal
2)Flue gas temperature below the acid dew point, so that condensate forms on the fly ash particles
Acid dewpoint corrosion is caused by the formation and condensation of sulfuric acid from the flue gases. The amount of sulfur trioxide formed in the combustion process is an important factor since an increase in the S03 concentration results in an increase in the acid dew point temperature. Acid dewpoint corrosion is a more significant problem in oil fired boilers than in coal fired boilers due to the vanadium in the oil ash deposits and the smaller quantity of constituents.
Corrective actions involve raising the gas and metal temperatures above the acid dew point temperature, lowering the acid dew point temperature by reducing the S03 below 1%, or injecting magnesium oxide based fireside additives, which has also been successful in preventing acid dewpoint corrosion but may not be feasible for many types of boilers. Chemical injection has been applied primarily in oil fired boilers.
Similar damage occurs in oil fired boilers when the units are water washed to remove ash and the final rinse does not neutralize the acid salts. Sodium carbonate or washing soda should be added to the final rinse as a basic solution to neutralize the acidic ash constituents.
Boilers that burn fuels containing sulfur/chlorine have the potential for sulfuric acid dewpoint corrosion in the economizer sections, air heaters and the stacks. Also, fuel additives such as calcium bromide to help capture mercury can increase the susceptibility to acid dewpoint corrosion (cold-end corrosion). The most common locations for acid dewpoint corrosion are low temperature economizers, air heaters, duct work, and flue gas cleaning equipment.
Preventative Maintenance and Repairs
Visual examination and ultrasonic wall thickness (UT) measurements are used to detect and monitor acid dewpoint corrosion. Upgrading to corrosion-resistant materials such as Inconel or fiberglass-reinforced plastics (if applicable) is recommended if corrosion rates are unmanageable. 300-series austenitic stainless steels are not recommended due to the risk associated with chloride-induced corrosion or stress-corrosion cracking (SCC).