# ID Fan Housing Protection: 5 Options for Acid Gas, HF and Abrasive Dust Five options protect an ID fan housing from acid gas, HF and abrasive dust: FlueGard-225SQC hybrid coating, glass-flake vinyl ester or novolac epoxy linings, wear liners and hardfacing, alloy upgrades, and insulation with cold-spot control. Pair a surface protection with cause-side measures, matched to temperature, gas chemistry and dust load. **Category:** Listicles **Written by:** Edita Rojasova **Last updated:** Oct 12, 2026 **Source:** https://signals.polylloycoatings.com/listicles/id-fan-housing-corrosion-protection --- ## ID Fan Housing Protection: Key Points - Induced draft (ID) fan housings sit downstream of the filter, where clean gas has lost its alkaline dust buffer and every cool-down can push the steel below the acid dew point. - Five options are compared: FlueGard®-225SQC hybrid coating, glass-flake vinyl ester or novolac epoxy linings, wear liners and hardfacing, alloy upgrades, and insulation with cold-spot control. - FlueGard-225SQC lists resistance to hydrogen fluoride (HF), hydrochloric acid (HCl) and sulfur oxides (SOx) in its bulletin, and has about 24 months of service on an HF-rich cement plant fan housing in France whose reported temperatures go beyond its published rating. - Erosion concentrates where flow accelerates; the France project targeted inlet duct sections, transition zones and flow acceleration areas, and erosion-only zones may justify wear liners or a wear coating alongside acid protection. - Alloys and cause-side measures are genuine options: 316 stainless steel is the material benchmark, and insulation reduces condensation whatever surface protection is chosen. - Cost is project-specific; compare options on the same housing, conditions, scope and period. Five options protect an ID fan housing from acid gas, HF and abrasive dust: FlueGard-225SQC hybrid coating, glass-flake vinyl ester or novolac epoxy linings, wear liners and hardfacing, alloy upgrades, and insulation with cold-spot control. Pair a surface protection with cause-side measures, matched to temperature, gas chemistry and dust load. ## Why ID Fan Housings Corrode and Erode in Cement and Power Flue Gas ID fan housings corrode and erode because the gas reaching them has changed. For maintenance and reliability managers whose housing is thinning, pitting or eroding, the drivers are [alternative fuels](https://signals.polylloycoatings.com/articles/alternative-fuels-kiln-corrosion) (AF), waste heat recovery (WHR) and tighter filtration, which together have made fan housings more vulnerable. ### Where the ID Fan Sits in the Gas Path The ID fan pulls gas through the system after the bag filter or electrostatic precipitator (ESP), so it handles cleaned but still acidic gas. In the France case discussed below, the fan housing sits after a heat exchanger, a bag filter and ducting; the case study does not identify it as the induced draft fan. ### Acid Condensate Without the Alkaline Buffer High-efficiency filtration removes the alkaline dust that once neutralised acidic condensate, so clean gas sections now face direct acid attack. The [acid dew point](https://signals.polylloycoatings.com/listicles/acid-dew-point-corrosion-signs) in cement flue gas is typically 120–160 °C, depending on sulfur trioxide (SO3) and moisture, and every shutdown takes the casing through it. WHR and lower gas temperatures narrow the margin above that dew point. AF such as refuse-derived fuel add sulfur, chlorine and fluorine variability, which is one source of HF in cement flue gas. ### Erosion Where the Flow Accelerates Residual dust at high velocity erodes inlets, transition zones and flow acceleration areas. Erosion removes protective films and corrosion products, exposing fresh steel to acid, so erosion and corrosion feed each other inside the housing. ## Selection Criteria for ID Fan Housing Protection Options Each ID fan housing protection option below was assessed against the same five criteria, stated here before the list: - Service temperature, including excursions. - Gas chemistry, especially HF and chlorides. - Dust load and velocity at the housing. - Outage window, and how the system is cured or installed. - Shell condition and access inside the housing. ## ID Fan Housing Protection Options at a Glance The table lists the five ID fan housing options in the order they are described. Cost is project-specific for every option, so it is covered as factors rather than figures. | Option | Suited to | Addresses | HF data to request | Outage fit | Cost | | --- | --- | --- | --- | --- | --- | | FlueGard-225SQC hybrid coating | Housings up to 225 °C | Acid gas and dust erosion | Listed in bulletin | Sprayable quick cure | Project-specific | | Glass-flake vinyl ester and novolac epoxy linings | Acid condensate, low HF | Acid condensate | Resin and flake data | Multi-coat | Project-specific | | Wear liners, hardfacing and ceramic wear coatings | Erosion-dominated zones | Dust erosion | Not addressed | Zone by zone | Project-specific | | Stainless and nickel alloy upgrades | New or replacement casings | Corrosion without coating | Alloy-specific data | Long lead time | Project-specific | | Insulation and cold-spot control | Every ID fan | Condensation cause | Not applicable | Any outage | Project-specific | ## FlueGard-225SQC Hybrid Coating for HF-Rich, Dust-Laden Fan Housings FlueGard-225SQC is listed first because its bulletin names HF resistance and it has a documented fan housing reference. ### Suited To Fan housings, inlet duct sections and transition zones exposed to acid gas including HF, plus moderate to heavy dust, within the 225 °C (437 °F) service limit. ### What It Is FlueGard-225SQC is a two-component polymeric-alloy-inorganic hybrid coating, 100% solids and sprayable with a quick cure, formulated in 2000 and in field use since about 2002. Its bulletin lists resistance to SO3, sulfur dioxide (SO2), HCl, carbon dioxide (CO2), HF and nitrogen oxides (NOx), with one-week immersion weight gain below 1.0% in 3% HF (ASTM D471). Grade details are on the [FlueGard flue gas coatings](https://www.polylloycoatings.com/fluegard) page. ### Key Features - Taber abrasion below 30 mg (CS-17, 1 kg, 1,000 cycles) and adhesion above 3,000 psi (ASTM D4541); immersion-rated. - Applied by Polylloy's [certified partners](https://signals.polylloycoatings.com/use-cases/polylloy-partner-network-countries) under Polylloy specification and supervision. Final heat cured activation is completed during start-up using process heat. The documented reference is a fan housing downstream of the bag filter at a cement plant in France, coated in April 2024 on the housing, inlet duct sections and erosion-prone transition zones. The steel was blasted to SSPC-SP10 / NACE No. 2 near-white, the joint Society for Protective Coatings (SSPC) and NACE International standard, and coated to about 25 mils (635 microns). Exposed to HF, HCl, SOx, high-velocity dust and thermal cycling, the coating showed continued suitability after about 24 months. The reported conditions, below 250 °C continuous with short excursions approaching 280 °C, go beyond the bulletin's 225 °C (437 °F) rating and the brochure's 270 °C spike value. The case does not extend the published rating, and any duty above 225 °C is confirmed project by project with Polylloy's technical team. ### Cost Factors Housing area and geometry, blast access inside the scroll, thickness, scaffolding and confined-space logistics, shutdown timing and freight. ### Pros and Cons **Pros** - One system addresses acid gas including HF and dust erosion. - A documented fan housing reference in HF-rich cement gas. **Cons** - Needs an SSPC-SP10 blast and a start-up heat route. - The 225 °C continuous limit means hotter fans need a different grade. - Not a fix for structurally thinned casing. Two related grades are worth knowing. The [StackGard®-255SQW](https://www.polylloycoatings.com/stackgard) bulletin lists fans in power plant and refinery gas paths to 255 °C but does not list HF, and the WearGard®-625S bulletin lists industrial fans where hot abrasion dominates. ## Glass-Flake Vinyl Ester and Novolac Epoxy Linings for Fan Housings Glass-flake vinyl ester and novolac epoxy linings are the established resin route for acid condensate in flue gas equipment. ### Suited To Fan housings with acid condensate where the resin's published temperature and chemical ratings cover the duty and HF is low or absent. ### What It Is Glass-flake vinyl ester and novolac epoxy linings are thick-film resin linings, often reinforced with glass flake and applied in one or more coats. They are widely specified for flue gas ducts, scrubbers and stacks; the chemistries are set side by side in [novolac, vinyl ester and hybrid coatings compared](https://signals.polylloycoatings.com/comparisons/novolac-vinyl-ester-hybrid-coatings). ### Key Features - Broad acid resistance and a long specification history. - Many applicators know them. - Ask the resin supplier for HF-specific data, including for any glass-flake reinforcement. ### Cost Factors Number of coats and visits, film build, blast standard, cure conditions and outage length. ### Pros and Cons **Pros** - Established chemistries with published data. **Cons** - Temperature limits vary by resin and must be read from the product sheet. - Multi-coat systems can stretch a short outage. - Abrasion resistance at fan inlet velocities should be checked. ## Wear Liners, Hardfacing and Ceramic Wear Coatings for Fan Housing Erosion Zones Wear liners, hardfacing and ceramic wear coatings target erosion rather than acid attack and are applied zone by zone. ### Suited To Fans handling abrasive dust where erosion, not acid attack, is the dominant failure, typically at inlets and transition zones where the flow accelerates. ### What It Is The options are bolted or welded wear liner plates, weld-overlay hardfacing, ceramic tiles, or a ceramic wear coating. WearGard-625S is a two-component nano-ceramic coating to 625 °C with Taber abrasion below 30 mg and Shore D above 95; its bulletin lists industrial fans and ducts in coal power and waste incineration plants. Wider options are covered in [abrasion and wear resistant coatings](https://signals.polylloycoatings.com/use-cases/abrasion-wear-resistant-coatings). ### Key Features - Targets the highest-velocity zones without treating the whole casing. - WearGard-625S is not immersion-rated and its bulletin does not list HF, so pair it with an acid-resistant system where condensate forms. ### Cost Factors Liner material and fixing, welding labour, added weight on the casing, zone area and outage time. ### Pros and Cons **Pros** - Robust in pure erosion. **Cons** - Plates and overlays add weight and create joints where condensate can collect behind liners. - Hardfacing does not stop acid attack on the surrounding steel. ## Stainless and Nickel Alloy Upgrades for Fan Housings Stainless and nickel alloy upgrades change the casing material instead of protecting carbon steel. ### Suited To New fans or casing replacements where the chemistry and the budget favour metallurgy over a coated carbon steel casing. ### What It Is An alloy upgrade replaces the casing, or lines it, with 316 stainless steel or a higher nickel alloy. 316 stainless steel is Polylloy's primary material benchmark, and the trade-offs are set out in [carbon steel vs stainless steel in flue gas](https://signals.polylloycoatings.com/comparisons/carbon-steel-vs-stainless-steel). ### Key Features - No coating to inspect or repair. - Molybdenum improves 316's localized corrosion resistance versus 304, but gives no immunity to chloride pitting, crevice corrosion or stress corrosion cracking. - HF and chloride condensate both need alloy-specific data before selection. ### Cost Factors Alloy grade, fabrication, lead time, rebalancing and installation, compared against coated carbon steel on the same scope and period. 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** - The right answer where coating preparation or start-up heat cannot be achieved. **Cons** - Alloy selection for HF and chlorides is not automatic. - Long lead time for a casing replacement. ## Insulation and Cold-Spot Control on ID Fan Housings Insulation and cold-spot control act on the cause of condensation rather than on the steel surface. ### Suited To Every ID fan, and especially casings with uninsulated access doors, flanges, drains or air in-leakage points. ### What It Is Cold-spot control keeps casing metal above the acid dew point. It combines external insulation and cladding, sealing doors and shaft seals against air in-leakage, and minimising time spent near the dew point during low-load running and shutdowns. ### Key Features - False air through seals and joints cools the steel locally and raises oxygen, creating localized corrosion zones. - Reduces condensation whatever surface protection is chosen. ### Cost Factors Insulation and cladding area, seal maintenance and monitoring of casing temperature. ### Pros and Cons **Pros** - Removes part of the cause. **Cons** - Does not stop erosion. - Cannot prevent condensation during every shutdown, so exposed steel still needs protection. ## How the 5 ID Fan Housing Options Were Chosen Five options were selected because each is used on induced draft fan housings, and all were compared on the five criteria stated before the list: temperature, gas chemistry, dust load, outage window and shell condition. Product facts come from Polylloy bulletins and case studies; the resin, wear and alloy options are described by chemistry rather than brand. Order follows fit to the acid gas, HF and dust duty, not preference. ## How to Choose ID Fan Housing Protection for Your Plant Start with the operating condition. Answer these questions before choosing: - What are the casing temperature, the excursions and the number of shutdowns a year? - Is HF present, from AF feed or other fluorine inputs? - Where is wear concentrated, and how fast is it progressing? Within 225 °C with HF and dust, FlueGard-225SQC fits. Erosion-only zones suit wear liners or WearGard-625S, and a new casing where blast and start-up heat cannot be achieved suits an alloy with HF and chloride data. Insulation and seal control apply in every case. Above 225 °C continuous, or with excursions beyond the 270 °C spike value, ask Polylloy's technical team for a project-specific grade review. Do not cite the France case as proof that FlueGard-225SQC is rated for its reported 280 °C excursions. The same logic applies upstream: see [hot flue gas duct protection](https://signals.polylloycoatings.com/use-cases/hot-flue-gas-duct-protection) and [baghouse corrosion protection options](https://signals.polylloycoatings.com/listicles/baghouse-corrosion-protection). To book a Coating Performance Review, [request a coating evaluation](https://www.polylloycoatings.com/contact) with fan type, casing area, temperature profile and excursions, gas chemistry (HF, HCl, SOx), dust load, location and outage timing. ### FAQs **What Coating is Used Inside an Induced Draft Fan Housing?** A coating rated for acid gas condensate and dust erosion at the casing temperature. FlueGard-225SQC is one option, with HF, HCl and sulfur oxide resistance listed in its bulletin and a 225 °C service limit. Glass-flake vinyl ester and novolac linings are alternatives where their published ratings cover the duty, and wear liners suit erosion-only zones. **Does FlueGard-225SQC Resist Hydrogen Fluoride?** Yes, its bulletin lists resistance to HF gas and reports one-week immersion weight gain below 1.0% in 3% HF. At a French cement plant it has served about 24 months on a fan housing exposed to HF, HCl, sulfur oxides and dust. That site's reported temperatures exceed the 225 °C rating, so confirm any hotter duty with Polylloy. **Why Do ID Fans Corrode After a New Bag Filter is Installed?** Better filtration removes the alkaline dust that used to neutralise acidic condensate, so the clean gas reaching the ID fan attacks steel directly. Lower gas temperatures from waste heat recovery narrow the margin to the acid dew point, typically 120–160 °C, and each shutdown takes the casing through it. **Which Areas of a Fan Housing Wear Fastest?** Erosion concentrates where gas changes direction and dust strikes the steel at high velocity: inlet duct sections, transition zones and flow acceleration areas. These were the erosion-prone zones targeted in the French cement plant fan housing project. Inspect them first at every outage, and consider a wear liner or wear coating where erosion, not acid attack, dominates. **Is Stainless Steel Better Than a Coating for an ID Fan Housing?** Not automatically. 316 stainless steel needs no coating, but its molybdenum gives no immunity to chloride pitting or crevice corrosion, and HF needs alloy-specific data. Protected carbon steel may offer a cost-effective alternative to stainless steel where the coating is suitable for the specified service conditions. **Can a Fan Housing Be Coated During a Short Outage?** A fan housing can be coated in a short outage if access, blasting and cure are planned before the shutdown. FlueGard-225SQC is a sprayable quick-cure system, applied at about 25 mils in the French fan housing project over an SSPC-SP10 near-white blast. Final heat cured activation is completed during start-up using process heat; an external heat source is the alternative where the schedule requires it. **How Long Does an ID Fan Housing Coating Last?** Service life depends on temperature, gas chemistry, dust load and preparation quality, so no general figure applies. The documented reference is a fan housing downstream of the bag filter at a French cement plant, about 24 months in HF-rich gas with continued suitability reported, at temperatures above the published rating. Polylloy provides project-specific performance detail.