Definition
Cold-end corrosion and dew-point corrosion
Cold-end corrosion is the attack on air-heater and economiser surfaces below the acid dew point, where SO3 condenses as sulphuric acid. The leading cold-end failure mechanism.
- Subject
- Boilers
- 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
Mechanism
Cold-end corrosion occurs when gas temperature or metal temperature falls below the acid dew point. Sulphur trioxide combines with water vapour to form sulphuric acid, which condenses on air-heater baskets, economiser surfaces, ducts or stacks. Chlorides and ammonium salts can create related low-temperature corrosion and fouling problems.
The acid dew point is not a fixed number. It depends on sulphur content, sulphur trioxide concentration, moisture, excess air and catalytic effects from ash or SCR catalyst. A unit that is safe at one fuel blend or load can become vulnerable after fuel sulphur changes, SCR operation changes or low-load operation lowers cold-end temperatures.
Operating symptoms
Operators see rising air-heater differential pressure, sticky deposits, basket plugging, stack acid mist, corrosion products, fan load increase and sometimes visible plume changes. During outages, affected surfaces can show wet, black or brown deposits and metal loss concentrated at the coldest gas paths.
Low-load operation and frequent cycling increase risk because surfaces cool and stay in the condensation range for longer. Ammonium bisulphate can add a sticky fouling layer that captures fly ash, making both heat transfer and corrosion worse.
Design and cleaning implications
Mitigation includes maintaining exit-gas temperature above the acid dew point, controlling sulphur trioxide, using corrosion-resistant materials, washing air-heater baskets correctly and managing SCR ammonia slip. Cleaning must remove deposits without leaving corrosive wash water trapped in baskets or ducts.
Sonic horns can reduce dry and semi-dry deposit accumulation on cold-end surfaces, especially where ash and ammonium salts are building pressure drop. They cannot prevent acid condensation if the metal temperature is too low. The root-cause decision is therefore thermal and chemical first, cleaning second.
Field checks
Cold-end corrosion is checked by comparing metal temperature with acid dew-point risk, not by gas temperature alone. Air preheaters, economiser exits, stack liners and low-load sections can fall below the condensation threshold even when the main gas stream appears hot enough. Operators watch sulphur in fuel, sulphur trioxide formation, moisture, excess oxygen, ammonia slip and cold-air leakage because all of them can shift the acid condensation window.
Inspection usually finds wastage at cold corners, leakage paths, basket edges, casing seams, drains and low-flow areas where acidic liquid can remain. The failure mode is often uneven: one sector corrodes rapidly while nearby surfaces remain sound. Acoustic cleaning is relevant because deposits insulate metal and create under-deposit acid pockets, especially in air preheaters and cold-side ducts. Keeping surfaces cleaner helps reduce the time that corrosive salts and moisture stay in contact, but it does not replace temperature control, material selection, sealing or proper wash and neutralisation after shutdown.
Related terms
Explore the subject
Related terms
5 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.
References