# Hot Flue Gas Duct Protection Through Repeated Thermal Cycling High temperature coatings survive thermal cycling in flue gas ducts when they stay bonded through expansion and contraction and resist the acid condensate that forms at every shutdown. In a Swiss cement plant, FlueGard-455CHT showed no visible widespread delamination after about 17 months and roughly 50 cycles a year; FlueGard-425S covers ducts to 425 °C. **Category:** Use cases **Written by:** Surjit Gill **Last updated:** Oct 12, 2026 **Source:** https://signals.polylloycoatings.com/use-cases/hot-flue-gas-duct-protection --- ## Hot Flue Gas Duct Protection: Key Points - In hot flue gas ducts, each shutdown takes the steel through the acid dew point while dust erodes a film already stressed by expansion and contraction, so cycling, not only peak temperature, decides the coating. - An exterior heat rating does not establish suitability for internal acidic flue gas; ask for acid gas, condensate and abrasion data at the duct's temperature. - FlueGard®-425S is a single-component inorganic polymeric coating for corrosive flue gas to 425 °C (797 °F) with peaks to 500 °C; its bulletin lists conditioning towers and stacks in cement and lime plants. - FlueGard-455CHT is a two-component ceramic coating to 455 °C (850 °F) with 20 mg Taber abrasion, suited where dust load is high. - In a Swiss cement plant, FlueGard-455CHT applied in October 2024 to about 75 m² of new duct showed no visible widespread delamination after about 17 months and roughly 50 shutdown cycles a year; heavy dust prevented full inspection. - Polylloy specifies and supervises; certified partners in Europe and elsewhere apply. High temperature coatings survive thermal cycling in flue gas ducts when they stay bonded through expansion and contraction and resist the acid condensate that forms at every shutdown. In a Swiss cement plant, FlueGard-455CHT showed no visible widespread delamination after about 17 months and roughly 50 cycles a year; FlueGard-425S covers ducts to 425 °C. ## Hot Flue Gas Duct Challenges and How Polylloy Addresses Them Hot flue gas duct protection is a coating or lining system that keeps the internal steel of ducts, conditioning towers and similar gas-path equipment from corrosion and abrasion at temperature and through repeated shutdowns. The table maps each challenge to the Polylloy approach. | Challenge | What happens to the steel | Polylloy approach | | --- | --- | --- | | Acid condensate at each shutdown | Condensate attacks bare or weakened areas | FlueGard-455CHT: good resistance to acidic and alkaline condensates | | Repeated thermal cycling | Film cracks or lifts, then undercut corrosion | FlueGard-455CHT adhesion above 1,000 psi (7 MPa) | | Dust erosion | Loosened film and corrosion products are stripped | FlueGard-455CHT Taber abrasion 20 mg | | Steady temperatures above 300 °C | Lower-rated films exceed their limit | FlueGard-425S to 425 °C, peaks to 500 °C | | Two-part mixing on site | Mixing adds a step and a source of error | Single-component FlueGard-425S; spray, roll or brush | | Exterior heat paints specified internally | Film rated for dry heat meets acid gas and condensate | Match acid gas data, not only heat rating | ## Why Thermal Cycling Breaks Down Coatings in Hot Flue Gas Ducts Thermal cycling breaks down coatings in hot flue gas ducts through three linked mechanisms: acid condensate at every shutdown, movement of the steel, and dust erosion. ### Each Shutdown Crosses the Acid Dew Point The [acid dew point](https://signals.polylloycoatings.com/listicles/acid-dew-point-corrosion-signs) in flue gas is typically 120–160 °C, depending on sulfur trioxide (SO3) and moisture. A duct running at 220–240 °C passes through it on every cool-down, and condensate forms on the steel. False air at joints and seals adds local cooling. ### Expansion, Contraction and Undercut Corrosion Steel and coating expand and contract with each cycle. A film with weak adhesion or low flexibility cracks or lifts, and condensate then spreads beneath it as undercut corrosion. Polylloy laboratory and field testing observed undercutting corrosion and delamination in conventional high-temperature coatings. ### Dust Finishes What Cycling Starts Erosion strips loosened film and corrosion products, exposing fresh steel. Dust loading and gas velocity therefore belong in the specification alongside temperature. ## An Exterior Heat Rating is Not Internal Acid Gas Resistance in Flue Gas Ducts Exterior high-heat and corrosion under insulation (CUI) coatings are rated for dry heat and atmospheric exposure on the outside of hot steel. Inside a duct, the film also meets sulfur dioxide (SO2), SO3, hydrochloric acid (HCl), dust and condensate. Ask for acid gas and condensate data at the duct's temperature, not only a heat rating. The wider field of options is compared in [high temperature coatings above 400 °C](https://signals.polylloycoatings.com/listicles/high-temperature-coatings-400c). ## Matching FlueGard-425S and FlueGard-455CHT to Hot Flue Gas Duct Conditions FlueGard-425S and FlueGard-455CHT are matched to a hot duct by continuous and peak temperature first, then by dust load and application method. ### FlueGard-425S: Single-Component Coating for Very Hot Ducts and Conditioning Towers FlueGard-425S is a single-component inorganic polymeric coating with reactive fillers, rated from ambient to 425 °C (797 °F) with peaks up to 500 °C. It resists SO3, SO2, HCl, carbon dioxide (CO2), nitrogen oxides (NOx) and dust abrasion, and is applied by air spray, roll or brush. Its bulletin lists conditioning towers and stacks in cement and lime plants, plus baghouses, electrostatic precipitators (ESPs) and ducts in smelters and incinerators, and ducts in power plants and refineries. Its limit is Taber abrasion below 150 mg, higher than the ceramic grades, and it is not immersion-rated, so heavy-dust zones may favour FlueGard-455CHT or WearGard®-625S. Incinerator gas paths bring their own chemistry, covered under [waste-to-energy plant corrosion](https://signals.polylloycoatings.com/use-cases/waste-to-energy-plant-corrosion). ### FlueGard-455CHT: Two-Component Ceramic for Hot, Dusty Ducts FlueGard-455CHT is rated to 455 °C (850 °F), with 20 mg Taber abrasion, Shore D above 80 and Elcometer adhesion above 1,000 psi (7 MPa) on sandblasted carbon steel. It is water-based, applied in two coats, and has good resistance to acidic and alkaline condensates. The two coats add an application step, and it is not for continuous service above 455 °C. Grade details are on the [FlueGard hot flue gas coatings](https://www.polylloycoatings.com/fluegard) page. ### Steel at 425 °C in Flue Gas Service At a steady 425 °C, FlueGard-425S sits at its continuous limit and FlueGard-455CHT gives 30 °C of continuous headroom. FlueGard-425S lists peaks to 500 °C, while the FlueGard-455CHT technical data sheet states no peak rating, so check excursions against both. Final heat cured activation is completed during start-up using process heat. ## Swiss Cement Plant Duct: FlueGard-455CHT Through About 50 Cycles a Year The Swiss cement plant duct is a European reference for FlueGard-455CHT under frequent thermal cycling and continuous dust loading. ### Duty FlueGard-455CHT was applied in October 2024 to about 75 m² of newly installed internal flue gas ductwork at a cement plant in Switzerland. The duct runs at 220–240 °C continuous with 255–270 °C peaks, about 50 shutdown and start-up cycles a year, and about 1–2 g/Nm³ of dust at about 13 m/s. Those temperatures sit well inside the 455 °C rating. The customer's stated requirements were corrosion protection, thickness retention and durability through cycling and dust. ### 17-Month Inspection, Stated Precisely The customer inspected the duct in March 2026, about 17 months after application. Visible inspected areas were in good condition, with no visible widespread delamination and no apparent coating-thickness loss in accessible areas. Heavy dust accumulation prevented full inspection of all surfaces, so the result covers the areas that could be seen. Results are project-specific. ### Hot Duct Protection in Europe Polylloy supports European projects through [certified partners](https://signals.polylloycoatings.com/use-cases/polylloy-partner-network-countries), part of a network of 15 partners in 22 countries. A FlueGard-225SQC induced draft (ID) fan housing in a French cement plant is a second European reference, described in [ID fan housing corrosion protection](https://signals.polylloycoatings.com/listicles/id-fan-housing-corrosion-protection). ## How EPC Engineers Specify Hot Flue Gas Duct Protection for New Ductwork Engineering, procurement and construction (EPC) engineers specify hot flue gas duct protection from process data and a site qualification gate, before any grade is chosen. ### Specification Inputs The specification starts from these duct inputs: - Continuous and peak steel temperature. - Shutdowns per year. - Gas chemistry: SO2 and SO3, HCl, hydrogen fluoride (HF) and moisture. - Dust loading and velocity. - New or existing steel, and the outage or fabrication window. The qualification gate is a near-white blast to Society for Protective Coatings (SSPC) standard SSPC-SP10, a profile above 3 mils (75 µm), and process heat at start-up. A site that cannot meet these is not a fit. ### Outcomes for the Project Engineer The project engineer gets a grade matched by service limit, a partner-applied system under Polylloy specification and supervision, and inspection hold points written into the duct package. ## Choosing Protection for a Hot Flue Gas Duct Choosing protection for a hot flue gas duct starts from continuous and peak steel temperature, then dust load and cycling: - Below 225 °C with acid gas: FlueGard-225SQC. - To 425 °C, peaks to 500 °C, moderate dust: FlueGard-425S. - To 455 °C, or heavy dust and frequent cycling: FlueGard-455CHT. - Severe hot abrasion: WearGard-625S, covered in [abrasion resistant coatings](https://signals.polylloycoatings.com/use-cases/abrasion-wear-resistant-coatings). - Beyond coating limits: refractory or alloy. Ducts that run cooler and wetter, such as those around scrubbers, are a different selection, set out in [flue gas desulfurization corrosion](https://signals.polylloycoatings.com/use-cases/flue-gas-desulfurization-corrosion). To book a Coating Performance Review, [request a coating evaluation](https://www.polylloycoatings.com/contact) with duct drawings, coated area, temperature profile and shutdown frequency, gas chemistry, dust load, location and timing. ### FAQs **How Do High Temperature Coatings Handle Thermal Cycling?** A high temperature coating must stay bonded while steel expands and contracts, and resist the acid condensate that forms each time the duct cools through the acid dew point. Adhesion, flexibility and resistance to undercut corrosion decide survival. In a Swiss cement duct, FlueGard-455CHT showed no visible widespread delamination after about 17 months and roughly 50 cycles a year. **What Coating Can Protect Steel at 425 °C in Flue Gas Service?** FlueGard-425S is rated from ambient to 425 °C (797 °F) with peaks up to 500 °C and resists SO3, SO2, HCl and dust abrasion. At a steady 425 °C it runs at its limit; FlueGard-455CHT, rated to 455 °C (850 °F), gives continuous headroom and lower abrasion loss, though its data sheet states no peak rating. **Is There a Single-Component Coating for Very Hot Flue Gas Ducts?** Yes, FlueGard-425S is a single-component inorganic polymeric coating with reactive fillers, applied by spray, roller or brush without mixing two parts. It is rated to 425 °C with peaks up to 500 °C. Its Taber abrasion is below 150 mg, so very dusty ducts may need FlueGard-455CHT instead. **What Coating is Used Inside a Cement Plant Conditioning Tower?** FlueGard-425S lists conditioning towers and stacks in cement and lime plants among its main applications, with a 425 °C limit and peaks to 500 °C. The choice still depends on where condensate forms during water injection and shutdowns, the dust load, and the temperature at the steel. **Can a Heat-Resistant Paint Be Used Inside a Hot Flue Gas Duct?** Usually not on its own. An exterior heat rating shows the film tolerates dry heat; it does not show resistance to SO2, SO3, HCl, dust and acidic condensate inside a duct. Ask any supplier for acid gas, condensate and abrasion data at your duct temperature before specifying it internally. **Who Applies Hot Duct Coatings in Europe?** Certified Polylloy application partners apply them; Polylloy formulates and supplies the coating, specifies the system, trains applicators and supervises. The network covers 15 partners in 22 countries, including Europe. European references include FlueGard-455CHT ductwork in a Swiss cement plant and a FlueGard-225SQC fan housing in France. **How Was the Swiss FlueGard-455CHT Duct Inspected?** By the customer in March 2026, about 17 months after the October 2024 application. Visible inspected areas were in good condition, with no visible widespread delamination or apparent thickness loss in accessible areas. Heavy dust accumulation prevented full inspection of all surfaces, so the result covers the areas that could be seen.