Corrosion Protection for Air Pollution Control Equipment

Air Pollution Control Corrosion: Key Points

• Air pollution control equipment corrodes where steel runs below the acid dew point, which rises with sulfur trioxide and moisture content, and acid condenses on casings, hoppers, ducts and stacks.
• Emissions upgrades often trigger it: cooler gas narrows the margin above the dew point, and better filtration removes the alkaline dust that used to neutralize condensate on the clean-gas side.
• Electrostatic precipitators (ESPs) are listed in two Polylloy bulletins: FlueGard®-225SQC to 225 °C (437 °F) and FlueGard-425S to 425 °C (797 °F) with peaks to 500 °C.
• Grade follows temperature and wet or dry duty: FlueGard-225SQC and CorrosionGard®-160S are immersion-rated; FlueGard-425S, StackGard®-255SQW, KilnGard®-600SCW and WearGard®-625S are not, and FlueGard-455CHT states no immersion rating.
• Continuously wetted, high-chloride absorber zones often stay with rubber, glass-flake vinyl ester or nickel alloys; mild, dry, ambient steel needs only a conventional coating.
• Polylloy formulates, supplies, specifies, trains and supervises; certified partners apply the coatings.

Air pollution control equipment corrosion protection keeps steel above the acid dew point where possible and coats the surfaces that still condense acid, with the grade matched to gas temperature. FlueGard-225SQC lists baghouses, ESPs, ducts and flue gas desulfurization (FGD) scrubbers to 225 °C; FlueGard-425S covers hotter units to 425 °C, and StackGard-255SQW covers stacks and fans.

Air pollution control equipment corrosion protection is the combination of coatings, linings and temperature control that keeps ESP, baghouse, scrubber, duct, fan and stack steel from acid condensate, chlorides and abrasive dust.

Air Pollution Control Equipment Challenges and How Polylloy Matches Them

The air pollution control (APC) train runs from the hot duct through the collector and scrubber to the fan and stack, and each unit fails in its own way.

Challenge vs Solution Table by Equipment

Each unit is paired with its main attack and grade fit:

Two numbers decide the grade: the hottest excursion and the coldest metal temperature.

Why Air Pollution Control Equipment Corrodes After Emissions Upgrades

Air pollution control equipment often corrodes after an emissions upgrade cools the gas.

Cooler, Cleaner Gas Condenses More Acid

Cooling the gas to cut emissions created acid condensation inside the gas path. Sulfur trioxide (SO3) and water form sulfuric acid, and the acid dew point depends on SO3 concentration and moisture; in cement kiln gas it is typically 120 to 160 °C.

High-efficiency filtration removes alkaline dust, so clean-gas sections lose the buffering that neutralized condensate. Air in-leakage cools steel locally, and the visible symptoms are set out in acid dew point corrosion warning signs.

Where Electrostatic Precipitators Corrode

An electrostatic precipitator charges dust particles and collects them on plates inside a steel casing.

Cold Surfaces in the ESP Casing

The casing, hoppers, roof, inlet and outlet transitions and access doors are the steel exposed to flue gas. Corrosion concentrates where that steel runs below the acid dew point. Typical locations include:

• Hopper walls and valleys.
• Roof and corners.
• Door frames and seals.
• Uninsulated stiffeners.
• The outlet side, where the gas has already been cleaned of alkaline dust.

Air drawn in at worn door seals and hopper discharge valves cools the casing locally. Every shutdown and start-up takes the whole casing through the dew point with damp dust still in place.

Coating an Electrostatic Precipitator: Grades and Scope

Two Polylloy bulletins list electrostatic precipitators, split by temperature.

FlueGard-225SQC for ESPs to 225 °C

The FlueGard-225SQC bulletin lists baghouses, ESPs and ducts in cement plants and waste incinerators. It is a two-component, sprayable polymeric alloy rated to 225 °C (437 °F), resistant to SO3, sulfur dioxide (SO2), hydrogen chloride (HCl), carbon dioxide (CO2), hydrogen fluoride (HF) and nitrogen oxides (NOx).

Published data include Taber abrasion below 30 mg and adhesion above 3,000 psi (ASTM D4541), and it is immersion-rated. In Polylloy's own six-month immersion in 56% sulfuric acid at 60 °C, it showed no observed deterioration; this was not an independent test.

FlueGard-425S for Hotter ESPs

The FlueGard-425S bulletin lists baghouses, ESPs and ducts in metal smelters and waste incinerators. It is a single-component inorganic polymeric coating rated to 425 °C (797 °F) with peaks to 500 °C, with Taber abrasion below 150 mg, and it is not immersion-rated.

Scope and Cause-Side Measures

Neither bulletin names ESP internals, so the coated scope (casing, hoppers, roof, transitions) is set per project with the ESP owner and Polylloy's technical team. A coating works alongside insulation and door and hopper seal maintenance, which limit the air in-leakage and local cooling that drive condensation. Final heat cured activation is completed during start-up using process heat.

Baghouses and Fabric Filters in Pollution Control Service

Baghouses corrode at hoppers, doors, roof and the clean-air plenum.

Summary and Where the Detail Lives

FlueGard-225SQC is the bulletin grade, and FlueGard on three coal mill baghouses at cement plants in Mexico was inspected after about 15 to 20 years. The full comparison sits in baghouse corrosion protection options.

Wet, Dry and Semi-Dry Scrubbers

Wet, dry and semi-dry scrubbers are protected zone by zone.

Match the Zone, Not the Unit

In wet FGD scrubbers, the wet-dry interface and chloride slurry zones are the hardest. FlueGard-225SQC lists FGD scrubbers, ducts and chimneys in power plants and refineries, while FlueGard-425S and StackGard-255SQW list gas scrubbers.

Continuously wetted, high-chloride absorber zones often stay with rubber, glass-flake vinyl ester or nickel alloys, as set out in flue gas desulfurization corrosion protection. Dry and semi-dry scrubber (reactor) shells combine alkaline reagent with acid gas, and the grade is confirmed per zone from temperature and chemistry data.

Only FlueGard-225SQC and CorrosionGard-160S are immersion-rated. FlueGard-425S, StackGard-255SQW, KilnGard and WearGard are not, and the FlueGard-455CHT data sheet states no immersion rating, so none of them is specified for wetted zones.

Flue Gas Ducts and Induced Draft Fans Between Units

Flue gas ducts and induced draft fans link the units and share their chemistry.

Ducts and Fans in Brief

Hot ducts cycle through the dew point at each shutdown while dust erodes the film. FlueGard-425S suits ducts to 425 °C with peaks to 500 °C where its bulletin lists the industry, and FlueGard-455CHT suits ducts to 455 °C (850 °F) where dust is heavy, as covered in hot flue gas duct protection.

ID fan housings see clean gas without alkaline buffering plus erosion where flow accelerates. StackGard-255SQW lists fans in power plants and refineries, while FlueGard-225SQC has a case record of about 24 months on a cement plant fan housing in France exposed to HF, HCl and sulfur oxides.

Fans are not a FlueGard-225SQC bulletin application, so fan duty is confirmed per unit, and other routes are compared in ID fan housing corrosion protection.

Steel Stacks at the End of the Pollution Control Train

Steel stacks run coolest and condense most.

Stacks in Brief

StackGard-255SQW (255 °C / 491 °F) suits dry stacks, with CorrosionGard-160S as the ambient-cure option; saturated stacks after wet FGD need a wet-rated system. Coatings can go on in place, on ground sections or on new sections before assembly, and other options are compared in industrial chimney corrosion protection.

Pollution Control Equipment by Industry: Cement, Incineration, Power, Refining and Smelting

Polylloy bulletins list APC applications by industry.

What the Bulletins List per Industry

In cement, FlueGard-225SQC lists baghouses, ESPs and ducts, and StackGard-255SQW lists chimneys. In cement and lime, FlueGard-425S lists conditioning towers and stacks; the full equipment walk-through is in cement plant corrosion protection.

In waste incineration, FlueGard-225SQC and FlueGard-425S list baghouses, ESPs and ducts, and StackGard-255SQW lists chimneys. The flue path detail is covered under waste-to-energy plant corrosion. In power generation and oil refineries, FlueGard-225SQC lists FGD scrubbers, ducts and chimneys; FlueGard-425S lists gas scrubbers, ducts and chimneys; and StackGard-255SQW lists gas scrubbers, ducts and fans.

For copper, zinc and lead smelters and battery recycling, FlueGard-225SQC lists ducts, filters and stacks, and FlueGard-425S lists baghouses, ESPs and ducts in smelters. These are bulletin applications, and smelter or battery-recycling projects are confirmed per project by technical review.

How Polylloy Supports a Pollution Control Coating Project

Polylloy supports a pollution control coating project by formulating, supplying, specifying, training and supervising, while certified partners apply, with 15 partners in 22 countries.

Delivery, Preparation and Fit

Every project needs an SSPC-SP10 (Society for Protective Coatings) near-white blast with a profile above 3 mils (75 µm), plus start-up process heat for heat-cured grades; CorrosionGard-160S cures at ambient. A site that cannot blast to standard or reach activation is not a fit. The grade family is described on the FlueGard flue gas coatings page.

Mild service is not a fit either: dry, ambient support steel and walkways away from acid gas need only a conventional coating.

Choosing Protection for Your Air Pollution Control Equipment

Choosing protection for air pollution control equipment follows a short set of rules by temperature and wetness.

Decision Rules

Apply these rules per unit:

• Below 225 °C with acid gas, dry or intermittently wet: FlueGard-225SQC.
• Up to 425 °C with peaks to 500 °C, dry: FlueGard-425S.
• Hot and dusty to 455 °C: FlueGard-455CHT.
• Dry stacks to 255 °C: StackGard-255SQW.
• Continuously wetted chloride zones: confirm with Polylloy; established linings or alloys may be the answer.

At design stage, protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions. Pricing depends on the operating environment, selected coating system, equipment condition, project size, and application requirements.

To match grades to your units, request a coating evaluation with equipment type, coated area, continuous and peak temperatures, wet or dry duty, gas chemistry, dust load, location and outage timing.

FAQs

How Do You Protect an Electrostatic Precipitator from Corrosion?

Keep the casing above the acid dew point where you can, with insulation and sound door and hopper seals, and coat the steel that still sees condensate. FlueGard-225SQC lists ESPs among its applications to 225 °C, and FlueGard-425S lists them to 425 °C with peaks to 500 °C. The coated scope is confirmed per unit.

What Coating is Used for Air Pollution Control Equipment?

The coating depends on the unit and its temperature. FlueGard-225SQC lists baghouses, ESPs, ducts and FGD scrubbers to 225 °C, FlueGard-425S covers hotter units to 425 °C with peaks to 500 °C, and StackGard-255SQW covers steel chimneys, ducts and fans to 255 °C. Wet zones need an immersion-rated grade or an established lining.

How Do You Protect Scrubbers and Ducts from Acid Corrosion?

Divide the system into zones by temperature and wetness. Hot, dry ducts take a temperature-rated coating such as FlueGard-425S or FlueGard-455CHT, while gas-side scrubber surfaces below 225 °C suit FlueGard-225SQC, which is immersion-rated. Continuously wetted, high-chloride absorber zones often stay with rubber, glass-flake vinyl ester or nickel alloys after review.

Why Does Pollution Control Equipment Corrode After an Emissions Upgrade?

Upgrades usually cool the gas and remove more dust. Cooler gas narrows the margin above the acid dew point, which depends on sulfur trioxide and moisture, so more steel condenses sulfuric and hydrochloric acid. Better filtration also removes the alkaline dust that used to neutralize condensate, leaving clean-gas sections exposed to direct acid attack.

Which Parts of an Electrostatic Precipitator Corrode First?

Corrosion usually starts where casing steel runs below the acid dew point: hopper walls and valleys, roof corners, door frames and seals, uninsulated stiffeners and the outlet side, where the gas has already lost its alkaline dust. Air drawn in at worn seals cools the steel locally, and each shutdown takes the casing through the dew point.

Who Applies Coatings on Pollution Control Equipment?

Certified Polylloy partners apply them. Polylloy Coatings formulates and supplies the coating, specifies the system, trains applicators and supervises the work, including surface preparation and inspection. The network covers 15 partners in 22 countries, so application stays local while Polylloy's technical team sets and checks the specification.

Does Every Part of an Air Pollution Control System Need a Severe-Service Coating?

No. Support steel, walkways and external surfaces that stay dry and ambient are mild service, and a conventional coating is adequate there. Severe-service coatings belong on internal surfaces that cross the acid dew point, hold damp chloride-bearing dust, see hot acid gas or face dust erosion.