6 Corrosion Protection Options After an Emissions Control Upgrade

Emissions Control Corrosion: Key Points

• Upgrades that cool the gas or remove more dust leave more steel near or below the acid dew point, typically 120 to 160 °C in cement plant gas, and strip the alkaline dust that neutralized condensate.
• Six options are compared: Polylloy coating grades matched by zone (FlueGard®, StackGard® and CorrosionGard®), temperature and air in-leakage control, conventional organic linings, stainless or nickel alloys, wet-zone liners, and corrosion allowance with planned replacement.
• FlueGard-225SQC covers baghouses, ESPs, ducts and FGD scrubbers, ducts and chimneys to 225 °C and is immersion-rated; StackGard-255SQW covers steel stacks to 255 °C; CorrosionGard-160S is the ambient-cure grade to 160 °C.
• Insulation, heat tracing and seal repair come first wherever they work, but they do not protect steel through shutdowns, when surfaces cool through the dew point.
• Continuously wetted, high-chloride zones after an added wet scrubber often stay with rubber, glass block or FRP liners or nickel alloys; dry, ambient support steel needs only a conventional coating.
• Certified partners apply the coatings; Polylloy formulates, supplies, specifies, trains and supervises.

After an emissions control upgrade, protect the steel that now runs near or below the acid dew point with a coating matched to its new temperature, such as FlueGard-225SQC to 225 °C, and fix cold spots and air in-leakage alongside it. Organic linings, wet-zone liners, stainless or nickel alloys and corrosion allowance are the other options.

Emissions control corrosion is acid attack on flue gas path steel that starts or accelerates after an emissions project lowers gas temperature or removes alkaline dust. More steel then runs below the acid dew point, typically 120 to 160 °C in cement plant gas depending on sulfur trioxide (SO3) concentration and moisture (International Cement Review (ICR), May 2026).

What an Emissions Control Upgrade Changes for Flue Gas Path Steel

An emissions control upgrade changes how hot the gas runs and how much dust it carries.

Two Changes That Create Acid Condensation

The first change is cooler outlet temperatures. Gas cooling narrows the margin between operating temperature and the acid dew point, so sulfuric acid (H2SO4) and hydrochloric acid (HCl) condense on more steel for longer (ICR).

The second change comes from added scrubbers or higher-efficiency filters. Better filtration removes the alkaline dust that used to neutralize condensate, leaving clean gas sections exposed to direct acid attack (ICR). Polylloy's co-founders describe it as an industry that solved an emissions problem and created a corrosion one.

Where the Detail Lives

Symptoms are covered in acid dew point corrosion warning signs, and an equipment-by-equipment walk-through in air pollution control equipment corrosion protection. Once the new metal temperatures are mapped, six protection routes are realistic.

How the Six Post-Upgrade Protection Options Were Compared

Each post-upgrade protection option was compared on five criteria:

• Fit to steel newly running near or below the acid dew point.
• Wet or dry duty, since a wet scrubber creates saturated zones.
• Behavior through shutdowns and start-ups.
• Outage and installation impact.
• Honest limits, including where the option is not the answer.

Order is by fit to dry or intermittently condensing steel after an upgrade, on the equipment the Polylloy grade bulletins list. In continuously wetted zones, wet-zone liners and alloys lead instead.

Post-Upgrade Corrosion Protection Options at a Glance

The table lists the six options in order; Polylloy rows give listed service limits, other rows defer to the data sheet or alloy.

Polylloy Coatings Grades Matched by Zone: FlueGard, StackGard and CorrosionGard

Polylloy Coatings, a research and development (R&D) and manufacturing company in California's San Francisco Bay Area, formulates these grades for flue gas service.

Suited To

These grades suit baghouses, electrostatic precipitators (ESPs), ducts, scrubber gas-side surfaces and stacks that now run near or below the acid dew point, dry or intermittently wet, within each grade's listed limit.

What It Is

The grade follows the new temperature. FlueGard-225SQC, a two-component polymeric alloy rated to 225 °C (437 °F), covers cooled baghouses, ESPs and ducts and, in power plants and refineries, flue gas desulfurization (FGD) scrubbers, ducts and chimneys.

StackGard-255SQW, rated to 255 °C (491 °F), covers steel chimneys and scrubbers, ducts and fans in power plants and refineries; it is described on the StackGard stack coating page. CorrosionGard-160S cures at ambient and is rated to 160 °C (320 °F) for cooler structures and vessels, while FlueGard-425S and FlueGard-455CHT cover hotter zones upstream.

Key Features

• FlueGard-225SQC resists SO3, sulfur dioxide (SO2), HCl, carbon dioxide (CO2), hydrogen fluoride (HF) and nitrogen oxides (NOx), is immersion-rated, and lists Taber abrasion below 30 mg and adhesion above 3,000 psi (ASTM D4541).
• Polylloy's own six-month immersion in 56% sulfuric acid at 60 °C showed no observed deterioration of FlueGard-225SQC; it is not an independent test.
• Polylloy laboratory and field testing comparing FlueGard with conventional high-temperature coatings observed undercutting corrosion and delamination in the conventional coatings.
• Coatings can go on inside an existing stack, on replacement sections on the ground, or on new sections before assembly (ICR).

StackGard-255SQW was selected for new FGD ducting at PPL Brunner Island, Pennsylvania, in 2009, an application reference only. A 12-month FlueGard-225 trial on coated coupons in a Taipower FGD exit stack and absorber tower showed no measurable coating thickness loss; both appear in power plant corrosion protection.

FlueGard on coal mill baghouses at three cement plants in Mexico was inspected after about 15 to 20 years.

Cost Factors

Pricing depends on the operating environment, selected coating system, equipment condition, project size, and application requirements. Access, surface preparation and shutdown timing weigh heavily in a retrofit.

Pros and Cons

Pros

• Grade matched to the new temperature profile.
• Protects through shutdown condensation.
• Fits inside the upgrade outage or a following one.

Cons

• SSPC-SP10 near-white blast with a profile above 3 mils (75 µm) is mandatory.
• Heat-cured grades need start-up heat: final heat cured activation is completed during start-up using process heat.
• A coating does not restore wall already lost, and continuously wetted, high-chloride zones are confirmed per zone.

Temperature and Air In-Leakage Control to Keep Steel Above the Dew Point

Temperature and air in-leakage control treats the cause rather than the symptom.

Suited To

It suits every unit after an upgrade: cold hoppers, doors, roofs, stiffeners and leaking seals.

What It Is

Insulation and cladding, heat tracing or hopper heating, and seal and expansion joint repair keep metal above the acid dew point. They also stop false air, which cools steel locally and adds oxygen that can promote SO3 formation (ICR). Design-stage detail is covered in flue gas ductwork design rules.

Key Features

It works alongside any coating or lining, and thermography finds cold patches while the plant runs.

Cost Factors

Cost is project-specific: area insulated, heating energy, access and seal maintenance.

Pros and Cons

Pros

• Reduces condensation in operation.
• Low outage impact for seal work.

Cons

• No protection when the unit cools at shutdown and retained deposits absorb moisture.
• Cannot lift the gas itself above the dew point if the upgrade is designed to cool it.

Conventional Organic Linings: Novolac Epoxy and Glass-Flake Vinyl Ester

Conventional organic linings are the familiar acid barrier for moderate-temperature condensate.

Suited To

They suit moderate-temperature acid condensate zones within the data sheet rating, often outlet sections and hoppers.

What It Is

Novolac epoxy and vinyl ester are organic barrier linings, often glass-flake filled to lengthen the permeation path. How these chemistries differ from hybrids is set out in novolac, vinyl ester and hybrid coatings compared.

Key Features

They are widely available, with broad acid resistance within their data sheet rating.

Cost Factors

Cost is project-specific: number of coats, cure conditions, preparation and outage length.

Pros and Cons

Pros

• Long industry record and many experienced applicators.

Cons

• Application-sensitive; rigid or multi-coat films can crack or embrittle under thermal cycling.
• A primer, build coat and topcoat can mean separate visits to the same surface, as Surjit Gill describes it.

Stainless Steel and Nickel Alloy Construction for Replaced Sections

Alloy construction replaces the steel rather than protecting it.

Suited To

It suits replacement sections or new ducting specified at the upgrade, and severe chloride zones where the owner's standard calls for alloy.

What It Is

The affected sections are built in 316 stainless steel, the usual benchmark, or in nickel alloys. The full case is argued in carbon steel vs stainless steel for flue gas.

Key Features

There is no film to maintain, and nickel alloys are a recognized route in continuously wetted, high-chloride absorber zones.

Cost Factors

Cost is project-specific: alloy grade, fabrication, welding and installation. Protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions.

Pros and Cons

Pros

• Suits zones beyond any coating rating.

Cons

• 316 can pit under chloride-bearing condensate and deposits.
• At Woodland Biomass (Xcel Energy), a stainless-steel baghouse lining had already been unsuccessful before FlueGard-225 was trialled.

Wet-Zone Liners for Saturated Sections After an Added Wet Scrubber

Wet-zone liners are the established route where steel never dries out.

Suited To

They suit continuously wetted, high-chloride absorber zones and saturated stacks and ducts after a wet scrubber.

What It Is

Rubber linings, borosilicate glass block linings and fiberglass-reinforced plastic (FRP) liners are established for wet scrubber and wet stack duty.

Key Features

They are designed for wet, saturated service, with specialist installation per manufacturer.

Cost Factors

Cost is project-specific: specialist installation, liner structure and outage length.

Pros and Cons

Pros

• The established answer in zones that never dry out.

Cons

• Temperature excursion limits for adhesive or resin follow the manufacturer; FRP liners are a separate structure.
• Not a fit for hot, dry upstream ducts.
• Of the Polylloy grades on this page, only FlueGard-225SQC and CorrosionGard-160S are immersion-rated, so any Polylloy grade in a wet zone is confirmed per zone.

Corrosion Allowance and Planned Section Replacement

Corrosion allowance accepts metal loss instead of preventing it.

Suited To

It suits short remaining design life, or steel already below structural minimum.

What It Is

Corrosion allowance is extra steel thickness specified to be consumed by corrosion over the design period. The alternative is planning to cut out and replace corroded sections at set outages.

Key Features

There is no coating to apply or inspect, but replacement steel still corrodes unless protected.

Cost Factors

Cost is project-specific: added steel and weight, cranes, permits and often the longest outages of the six options.

Pros and Cons

Pros

• Simple to specify.
• A valid answer where the asset is near end of life.

Cons

• The post-upgrade corrosion rate is uncertain, so allowance can be consumed early.
• Repeat replacement means repeat outages and more new steel, whereas preserving existing equipment can reduce the need for replacement steel.

How the Six Options Were Chosen and Ordered

Six distinct protection routes for flue gas path steel were compared on the five criteria above. Polylloy data comes from its published bulletins and cleared case studies; the other five options are described by mechanism, with ratings deferred to current data sheets. Polylloy publishes this page and lists its grades first on fit to dry or intermittently condensing duty.

How to Choose Corrosion Protection After Your Emissions Control Upgrade

Choosing protection after an upgrade starts with temperatures, not products.

Decision Path

Work through the train in three steps:

1. Map the new metal temperatures, including the coldest surfaces and shutdown behavior, against the acid dew point.
2. Fix seals, cold spots and deposit build-up.
3. Split the train into zones by temperature and wetness.

Then match each zone to an option:

• Dry or intermittently condensing steel below 225 °C: FlueGard-225SQC.
• Dry stacks to 255 °C: StackGard-255SQW.
• Cooler structures and vessels to 160 °C: CorrosionGard-160S, which cures at ambient.
• Continuously wetted chloride zones: confirm with Polylloy; rubber, glass block, FRP or nickel alloys may be the answer.
• Near end of life: corrosion allowance or replacement.

Build protection into the upgrade scope, not the next failure. Dry, ambient support steel and walkways stay on a conventional coating: mild service is not a fit.

Delivery and Next Step

Certified partners apply the coatings; Polylloy formulates, supplies, specifies, trains and supervises, with 15 partners in 22 countries. Grade detail is on the FlueGard flue gas coatings page. To book a Coating Performance Review, request a coating evaluation and share equipment type, coated area, operating conditions before and after the upgrade (temperature, gas chemistry, abrasion), location and outage timing.

FAQs

What Corrosion Protection is Used After an Emissions Control Upgrade?

Start with the new metal temperatures. Steel that now runs near or below the acid dew point, dry or intermittently wet, is usually coated with a grade matched to that temperature, such as FlueGard-225SQC to 225 °C or StackGard-255SQW to 255 °C for stacks. Seal repair and insulation reduce condensation, while saturated zones may need wet-rated liners.

Why Did Emissions Controls Create an Acid Condensation Problem?

Emissions projects often cool the flue gas and remove more dust. Cooler gas leaves less margin above the acid dew point, typically 120 to 160 °C in cement plant gas, so sulfuric and hydrochloric acid condense on more steel. Better filtration also removes the alkaline dust that used to neutralize that condensate on downstream surfaces.

Should Corrosion Protection Be Part of the Emissions Upgrade Project?

Yes, where the upgrade will cool the gas or add a scrubber or filter. Mapping metal temperatures at design stage shows which ducts, collectors and stack sections will cross the dew point, and protection can be specified for that outage instead of the next failure. New sections can even be coated before assembly.

Do I Need Stainless Steel After Adding a Scrubber?

Not necessarily. Stainless steel suits some replacement sections and severe chloride zones, but 316 can pit under chloride-bearing condensate and deposits. Protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions, so compare both on the same zone, chemistry and scope.

Which Coating Suits a Stack After a Wet Scrubber is Added?

It depends on whether the stack runs dry or saturated. StackGard-255SQW suits dry or intermittently condensing steel stacks to 255 °C. A saturated stack downstream of a wet scrubber needs a wet-rated system, such as an immersion-rated grade confirmed for that zone, or an established glass block or FRP liner chosen after technical review.

Will Insulation and Seal Repair Stop Corrosion After an Emissions Upgrade?

They reduce it but rarely stop it. Insulation, heat tracing and sound seals keep metal warmer and limit the false air that cools steel locally. They give no protection when the unit cools at shutdown, when deposits absorb moisture and condensate forms, so plants often combine them with a coating or lining.

Who Applies Corrosion Protection Coatings During an Upgrade Outage?

Certified Polylloy partners apply the coatings. Polylloy Coatings formulates and supplies the material, specifies the system, trains applicators and supervises surface preparation, application and inspection. With 15 partners in 22 countries, application stays local while Polylloy's technical team sets the specification.