Acid-resistant lining for desulfurization chimneys (FGD chimney acid-resistant lining) is the corrosion-protection system applied to the steel or concrete inner surface of a chimney or flue duct after a wet flue gas desulfurization (WFGD) unit is installed, to resist the acidic condensate the scrubbed gas forms on the wall: coating systems, vinyl ester glass flake mortars, fiber-reinforced plastic (FRP), acid-resistant or foam glass brick, and metal composite plate. It differs from the pre-desulfurization dry hot flue, where high-temperature oxidation and abrasion dominate, and from ordinary atmospheric corrosion protection - after WFGD the gas has been washed, humidified and cooled to near saturation, so the corrosive medium changes from a vapour phase to a continuous liquid phase, and both the attack rate and the failure mode change completely.
TL;DR — The core issue is continuous condensation below the acid dew point: SO3 hydrates to sulfuric acid vapour and, with the saturated wet gas, condenses on the liner as a film at pH 1-3. GB/T 37187 groups lining materials into coatings, glass flake mortars, FRP, acid-resistant brick and metal composite plate with quantitative thresholds (steel-substrate adhesion >=8 MPa, water-vapour permeability <=1.5x10^-5 g/(m.s.Pa), and others), and the work is organized as substrate preparation - sealer coat - lining build-up - spark (holiday) testing - off-design cycle qualification. Wall temperature, condensate pH and unit cycling decide whether a coating, glass flake mortar, FRP or titanium-clad plate is the appropriate choice.
Key data and limit anchors
- Service envelope: GB/T 37187 defines the scope of desulfurization chimney protection as flue-gas temperature 50-80 C with condensate pH 1-3. The WFGD outlet delivers saturated wet gas, and wherever the liner wall sits below the acid dew point the condensate film is continuous rather than intermittent - the direct reason one and the same chimney shows an intact hot zone and a heavily rusted cold condensation zone.
- Coating-class thresholds: steel-substrate adhesion >=8 MPa and no blistering or peeling after 20% H2SO4 immersion for 14 d. GB/T 37187-2018 has been superseded by GB/T 37187-2026; material selection and acceptance should follow the current edition.
- Vinyl ester glass flake mortar: water-vapour permeability <=1.5x10^-5 g/(m.s.Pa) and abrasion loss <=50 mg (500 g/1000 r). Platelet fillers aligned parallel to the substrate bend the diffusion path of the medium, which is the barrier core of heavy-duty linings.
- Structural linings: FRP suspended liner hoop tensile strength >=280 MPa; titanium-clad steel plate shear strength >=140 MPa, titanium layer >=1.2 mm, flatness deviation =1.0 MPa; acid-castable refractory natural drying shrinkage <=2.0%; nickel-alloy plate properties follow YB/T 5353.
- Off-design cycling: the abnormal-service check is 30 cycles of soak 1 h in 20% sulfuric acid -> dry 2 h at 150 C -> cool 1 h, watched for blistering and cracking. It represents unit start-ups and shutdowns, GGH leaks or bypass operation, when temperature and condensate load swing sharply.
- Application window (engineering-typical values): vinyl ester glass flake mortar is usually built to a total dry-film thickness of 1.5-2.5 mm in 2-3 trowel coats, each coat applied while the previous is tack-dry; DLZ 1262-2013 governs substrate grade, ambient temperature and humidity, and post-cure DC spark (holiday) testing, with test voltage set from film thickness - field experience puts the rule of thumb near 5 V/um. These are engineering-typical ranges, subject to the design file and current standard.
Corrosion mechanism: why the chimney gets worse after desulfurization
Continuous condensation below the acid dew point
Sulfur in the fuel burns to SO2 and roughly 0.5-5% of it oxidizes further to SO3; SO3 combines with moisture in the gas to form sulfuric acid vapour, which lifts the acid dew point far above the water dew point. In an unscrubbed dry flue the wall normally stays above the acid dew point, so no acid condenses and attack is chiefly high-temperature oxidation. WFGD, however, washes and cools the gas to a saturated state at 50-60 C, placing the entire liner wall below the acid dew point, so condensate runs continuously down the surface. That water, with residual SO2/SO3 and the chlorine and fluorine carried over from the scrubbing liquor, forms a dilute sulfuric acid system at pH 1-3 that actively dissolves carbon steel - attack rates can sit an order of magnitude above the dry condition.
Two attack paths: metal and concrete
Steel liners suffer hydrogen-depolarization corrosion in the low-pH condensate, and welds, anchor zones and nozzles fail first because liquid pools in the gaps; unlike controllable atmospheric rust, this cannot simply be painted over. Concrete liners are attacked by sulfuric acid into gypsum and ettringite, softening and powdering the matrix, with alternating wet-dry cycles and microbially generated acid accelerating the same family of acidic damage known from sewer concrete. A concrete substrate therefore needs full curing, a sealer coat and a genuine acid-resistant lining rather than bare concrete under a paint film. The chemical duty resins carry in desulfurization service parallels the absorption-desorption logic of coatings built for aggressive liquid cargoes, so resin-grade selection can borrow experience between the two.
Glass flake barrier and failure criteria
Vinyl ester resin already resists acid and heat better than general-purpose epoxy; adding 10-40% glass flakes by mass (an engineering-experience range) oriented parallel to the substrate forces the diffusing medium to travel around each platelet, lengthening the path and pushing water-vapour permeability toward the <=1.5x10^-5 g/(m.s.Pa) class. Failures rarely start by the resin being dissolved through; they start at interfaces - poor substrate preparation, inter-layer contamination, thin film at anchors and corners, and microcracks from the expansion mismatch between steel and lining under thermal cycling. Once a through-pore exists, condensate is heated and concentrated inside the gap and local attack accelerates sharply, which is why the standards specify adhesion, immersion and the 30-cycle service test.
Lining systems and holiday detection
Substrate preparation: the step that decides the job
Blast-clean steel to Sa2 1/2 with an anchor profile in the engineering-typical range Rz 40-70 um, free of oil and soluble salts. Concrete must be fully cured (>=28 d), moisture-controlled, laitance-free and locally neutralized where alkaline, then sealed to block capillary water and alkali migration. Once the substrate re-rusts, salt load rises or moisture exceeds the limit, every downstream lining property fails in practice - the same surface-preparation-first rule carried by ISO 12944 surface grades.
Flake mortar build-up and spark testing
Trowel the glass flake mortar over the cured sealer in 2-3 coats to the designed total dry-film thickness (usually 1.5-2.5 mm), trowel-finishing between coats and rolling while tack-dry to release air and re-align the flakes. After full cure, scan the entire surface with a DC spark (holiday) tester, with voltage set from film thickness - field experience points to roughly 5 V/um; mark every breakdown, repair and re-scan until no leak remains. In cold or humid weather extend the cure window: testing or commissioning a lining that is dry to touch but not cured invites blistering shortly after start-up.
Selection: match the system to temperature, pH and cycling
If wall temperature is stable, condensate pH stays at 2 or above and cycling is controlled, an acid-resistant coating or a thin glass flake layer suffices. For long-term near-saturation condensation with structural demands, prefer vinyl ester flake mortar or FRP. Where start-ups are frequent, a GGH fault can spike gas temperature, or abrasion is severe, move to metal composite plate (titanium or nickel alloy) or acid-resistant brick set in resin mortar, trading a heavier investment against a longer overhaul interval. The table maps service condition to system.
Service condition / lining system / testing comparison
| Corrosion service | Recommended lining system | Key quantitative thresholds | Acceptance / testing | Main risk points |
|---|---|---|---|---|
| Saturated wet gas, pH 1-3, small temperature swings | Vinyl ester glass flake mortar 1.5-2.5 mm | Water-vapour permeability =8 MPa | Full-area spark testing, Barcol hardness, adhesion sampling | Inter-layer contamination, thin film at corners |
| Liner must also carry structural load | FRP monolithic or suspended liner | Hoop tensile strength >=280 MPa | Laminate cure degree, leak testing | Resin-rich zones or bridged fibres, leaking joints |
| Strong acid with heavy abrasion | Acid-resistant brick or cast stone on resin mortar | Dense brick-mortar bond, continuous joints | Tap testing for hollows, joint integrity | Cracked joints becoming linked leak paths |
| Frequent cycling / GGH fault with hot spikes | Titanium-clad steel or nickel-alloy plate | Shear >=140 MPa, titanium layer >=1.2 mm, flatness <=15 mm/m | Weld corrosion resistance, interface shear | Sensitized-weld pitting, clad-interface disbondment |
| Off-design durability re-check | Any system (design qualification stage) | Soak 1 h in 20% H2SO4 -> dry 2 h at 150 C -> cool 1 h, 30 cycles | Blistering, cracking, bond-degradation grading | Expansion mismatch growing microcracks |
FAQ
Why did the old chimney coating fail so fast once desulfurization was added?
Because the corrosion system changed. Before WFGD the gas was dry and hot, attack was oxidation-dominated with only occasional dew. After WFGD the gas is cooled and humidified to saturation, the wall sits permanently below the acid dew point, and a continuous acidic condensate at pH 1-3 runs down the liner. A general epoxy or anticorrosive primer specified for dry service lacks both the acid resistance and the density to survive continuous liquid-phase attack, so the lining must move to the acid-resistant systems defined in GB/T 37187.
What does glass flake mortar do better than plain epoxy - can I just brush on a thin layer?
Two effects stack: the vinyl ester resin itself resists acid and heat better, and parallel-oriented glass flakes bend the diffusion path so water-vapour permeability can reach the <=1.5x10^-5 g/(m.s.Pa) class. A thin coat throws away the tortuous-path benefit - the barrier needs enough thickness to hold continuous parallel flake layers - so engineering practice builds 1.5-2.5 mm in 2-3 coats rather than one thin brush pass.
Why does the standard run 30 acid-soak / dry / cool cycles instead of a static immersion alone?
Because real failures are triggered by swings, not by steady state. Unit cycling, GGH leaks or bypass operation push the lining repeatedly between wet-cold and dry-hot; steel and resin expand differently, and the cyclic shear drives microcracks and interface degradation faster than any static bath. The 30-cycle test is an accelerated re-check of off-design durability, while the static 14 d immersion in 20% H2SO4 only proves the medium resistance of the material itself.
How do I confirm there are no hidden leaks after application?
The primary tool is full-area DC spark testing after full cure: voltage is set from the film thickness, each breakdown pinpoints a pinhole, and repairs are re-scanned until the surface passes clean. Adhesion, Barcol hardness and thickness checks at critical details (anchors, openings, corners, welds) are supplementary. Testing must wait for full cure and a dry substrate, otherwise the result is either a false pass or a missed genuine defect.
Concrete or steel chimney - does the selection logic change?
For a steel liner the priority is stopping active metal dissolution in the acidic condensate; interface adhesion and overall density carry the design, so vinyl ester flake mortar, FRP or metal composite plate all fit. For concrete, two problems must be solved at once - sulfuric acid powdering the matrix and capillary moisture blistering the lining - so the wall must be fully cured, moisture-controlled, sealed, and then lined with an acid-resistant system, with acid brick where needed. Substrate preparation is the shared red line for both; see the comparison table above.
Last updated: 2026-10-05 | Sources: GB/T 37187-2018 technical requirements for corrosion-resistant materials for desulfurization chimneys (superseded by GB/T 37187-2026), DLZ 1262-2013 guideline on corrosion protection of wet chimneys in power plants, YB/T 5353 (nickel-alloy plate), ISO 12944 surface-grade practice. Quantitative thresholds are quoted from the published requirements of these standards for each material class; construction film thickness and spark-test voltage are engineering-typical ranges, subject to the design file and current standard. Author: Kexin New Materials (Guangdong) Co., Ltd. technical team. Trade terms: EXW/FOB only.