Definition
Chloride-induced corrosion
Chloride-induced corrosion is the accelerated tube-wall thinning caused by chlorine-rich deposits on WtE and biomass boilers. The dominant tube-failure mechanism in WtE.
- Subject
- Waste-to-energy and biomass
- Also known as
- Cl corrosion, chloride corrosion, high-temperature chloride corrosion
Chloride-induced corrosion is the accelerated tube-wall thinning caused by chlorine-rich deposits on the steam-side surfaces of WtE, biomass and waste-fired boilers. Chloride corrosion is the dominant tube-failure mechanism in WtE and a major maintenance cost driver.
Mechanism
Chlorine in the fuel enters the gas phase as HCl and metal chlorides. Inside a thin deposit on the tube, chloride and metal-chloride species shuttle electrons between the gas atmosphere and the tube surface. The result is rapid metal loss far in excess of what the temperature alone would predict. The "active oxidation" mechanism describes one variant; chloride attack on the protective oxide scale describes another.
Where it dominates
- WtE superheaters - design temperatures kept low (380-420 deg C) specifically to limit chloride corrosion
- Straw-fired boilers - see straw firing
- RDF / SRF boilers - variable but generally high
- Heavy-petroleum-fired boilers with chloride contamination
Mitigation
- Material selection - Inconel-625 weld overlays, nickel-based alloys on the most-exposed tubes
- Lower steam temperature at the superheater outlet to keep tube-metal below the corrosion threshold
- Fuel control - limit chloride loading where the contract permits
- Sonic horns - preventing deposits from consolidating reduces the chloride concentration immediately adjacent to the tube surface, indirectly slowing corrosion
Mechanism
Chloride-induced corrosion starts when chlorine-bearing salts deposit on hot boiler tubes. In waste-to-energy and biomass boilers, alkali chlorides such as sodium chloride and potassium chloride can be present with zinc, lead, sulphates and unburned ash. These deposits can form low-melting mixtures that stay sticky at normal superheater temperatures.
The deposit changes the local chemistry at the tube wall. Chlorine can participate in active oxidation cycles that break down protective oxide scales and expose fresh metal. The result is accelerated tube wastage under a deposit that may look like ordinary ash from the outside.
Where it appears
The highest-risk zones are superheaters, evaporator screens, furnace nose areas and locations where flue-gas temperature and tube-metal temperature place salts in a sticky or molten range. Lower-temperature convective surfaces can also suffer when deposits hold acidic species or moisture during shutdowns.
Operators detect the problem through tube-thickness surveys, corrosion probes, deposit sampling, steam-temperature drift and inspection of wastage patterns. The damage is often localised: one gas lane, one leading edge or one sootblower shadow can fail faster than the average tube bank.
Design and cleaning implications
Mitigation includes lower steam temperatures, corrosion-resistant overlays, fuel blending, additive injection, improved combustion, better gas distribution and regular deposit control. Cleaning is a balance. Removing deposits reduces corrosive residence time, but aggressive sootblowing can erode already weakened tubes.
Acoustic cleaning helps by reducing the time sticky chloride deposits remain attached. It is most valuable as a gentle, frequent cleaning layer between sootblower events. It cannot change fuel chemistry or repair corrosion, but it can reduce deposit thickness, improve heat transfer and make inspection intervals less reactive.
Field checks
Chloride corrosion is investigated by combining tube-metal temperature, deposit chemistry, flue-gas composition and wastage mapping. A deposit sample that looks harmless can become aggressive if it concentrates chloride at the metal surface or traps moisture during shutdown. Inspectors look for localised wastage under deposits, pitting near welds, and corrosion bands that align with temperature windows rather than with simple gas-flow patterns.
The main design variables are material selection, tube temperature, gas temperature, oxygen level, deposit residence time and shutdown humidity. Stainless steels may resist general corrosion but still suffer chloride stress corrosion cracking in the wrong wet or warm conditions. In waste-to-energy, biomass, cement and recovery units, cleaning strategy matters because a thin mobile deposit is usually less damaging than a thick salt-rich insulation layer. Acoustic cleaners help when they keep chloride-bearing ash from staying on hot surfaces long enough to concentrate and react. They do not remove the need for material upgrades, fuel control or careful wash-down and dry-out procedures.
Shutdown practice is a frequent weak point. A unit that survives hot operation can corrode quickly during a cool, damp outage if chloride salts absorb moisture on the metal surface. Draining, ventilation, dry-out heating and inspection timing can be as important as the online cleaning system.
Related terms
Explore the subject
Related terms
5 terms
- Waste-to-energyWaste-to-energy plants burn municipal or prepared waste to generate heat and power, with variable fuel chemistry driving fouling, corrosion and cleaning demand.
- Alkali metals in ashAlkali metals (Na, K) in biomass and waste-fuel ash form low-melting compounds that bond to boiler tubes as sticky deposits and poison SCR catalysts.
- Low-melt sticky ashLow-melt sticky ash forms when alkali-rich ash softens and bonds to cooler tube surfaces. It drives fouling, corrosion and online-cleaning requirements.
- Cold-end corrosion and dew-point corrosionCold-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.
- Tube erosion / tube wastageTube erosion and wastage describe boiler tube metal loss from ash impact, sootblower jets, corrosion under deposits and aggressive cleaning.
References