Glossary
Boilers
Cold-end corrosion and dew-point corrosion
Also known as cold end corrosion, dew point corrosion, sulphuric acid corrosion (boiler).
Cold-end corrosion (also dew-point corrosion) is the attack on boiler air-heater baskets, economiser tubes and downstream ducting where flue-gas temperature falls below the acid dew point of the gas. SO₃ in the flue gas combines with water vapour to form sulphuric acid that condenses on the cooled surfaces and attacks them.
The interplay with fouling
Cold-end corrosion and fouling reinforce each other:
- Condensed acid bonds dust to surfaces — fouling consolidates faster
- Fouled tubes run cooler than design — more acid condenses
- Ammonium bisulphate (ABS) deposits accelerate both processes
The result is a self-feeding cycle: a unit that begins to foul typically also begins to corrode, and both worsen until the cold end is water-washed or rebuilt.
Mitigation
- Maintain cold-end metal temperature above the acid dew point
- Manage fuel sulphur and SCR SO₂/SO₃ conversion
- Use corrosion-resistant materials (Cor-Ten, enamel-coated baskets) at the cold end
- Periodic water-washing of cold-end baskets and tubes
- Sonic horns to keep deposits from consolidating
Related terms
Related terms
- Air heaterAn air heater (also air preheater, APH) recovers low-grade heat from flue gas to preheat combustion air. Cold-end fouling and corrosion are the dominant operational challenges.
- EconomiserAn economiser is the final tube bank in a boiler's convective pass that recovers heat from the flue gas by preheating feedwater. Ash bridging in the economiser is a routine cleaning challenge.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
- Acid dew pointThe acid dew point is the temperature at which sulphuric acid condenses from flue gas containing SO3 and water vapour. Cold-end metal temperatures must be kept above it.
- Boiler tube failureBoiler tube failures are the leading cause of forced outages on industrial boilers. Causes range from creep and erosion to corrosion and overheating; cleaning practices contribute to several.