Flue Gas Dew Point Corrosion | Materials And Corrosion Control

Damage Description

·         At low enough temperatures, the formed gases and the water vapor in the flue gas will condense to form sulfurous acid, sulfuric acid and hydrochloric acid which can lead to severe corrosion.

·         Sulfuric acid corrosion on economizers or other carbon steel or low alloy steel components will have general wastage often with broad, shallow pits, depending on the way the sulfuric acid condenses.

·         For the 300 Series SS feed water heaters in HRSG’s, stress corrosion cracking will have surface breaking cracks and the general appearance will be somewhat crazed.

Affected Materials

Control Methodology

·         For HRSG’s, avoid the use of 300 Series SS in the feed water heaters if the environment is likely to contain chlorides.

·         Similar damage occurs in oil-fired boilers when the units are water-washed to remove ash if the final rinse does not neutralize the acid salts. Sodium carbonate should be added to the final rinse as a basic solution to neutralize the acidic ash constituents.

Monitoring Techniques

·         Stress corrosion cracking of 300 Series SS can be found using VT and PT inspection.

Inspection Frequency

KPIs

·         The dew point of sulfuric acid depends on the concentration of sulfur trioxide in the flue gas, but is typically about 280°F (138°C).

·         The dew point of hydrochloric acid depends on the concentration of hydrogen chloride. It is about 130°F (54°C).

Reference Resources (Standards/GIs/BPs)