A stack or flue duct corrodes where the gas cools below its acid dew point: sulfuric or hydrochloric acid condenses on the steel or concrete and attacks it within a few seasons. Protection relies on an internal lining resistant to that acid and to the actual temperature, most often a glass flake system (vinyl ester or novolac epoxy), applied over high-grade blasting and checked by holiday testing. The allowable temperature range is verified in the data sheet.
Boiler stacks, flue gas ducts, ducting to dust collectors and scrubbers, induced draft fans and breechings are among the most aggressive structures in a plant. They combine heat, condensed acids, abrasive ash and difficult access. This guide helps maintenance managers and project managers in Quebec and Ontario understand where and why the steel perforates, and how to choose a suitable protective coatings system.
Why do stacks and flue ducts corrode?
They corrode because flue gas contains sulfur oxides, chlorides and water vapour that form acids as soon as the wall is colder than their dew point. While the wall stays hot, the steel suffers little; once it cools, an acid film settles and eats into the metal.
Corrosion therefore concentrates in a few predictable situations:
- Cold spots. Walls exposed to wind, stack outlets, poorly insulated joints, flanges and access doors, supports acting as thermal bridges.
- Start-ups and shutdowns. At every cycle, the wall passes through the dew point. A unit that starts often corrodes faster than one that runs continuously.
- Downstream of scrubbers. Gas saturated with water after washing or desulfurization stays below its dew point permanently.
- Deposits. Ash and soot hold acid against the wall and sustain under-deposit corrosion.
On concrete, the acid dissolves the cement paste and exposes the reinforcement; on steel, it causes fast, often localized wall loss.
What is the acid dew point, and why does it decide everything?
The acid dew point is the temperature below which the acids in the gas condense on the wall. It is higher than the water dew point, because sulfuric acid condenses before water vapour. Its value depends on the fuel and on the sulfur and moisture content of the gas; it is calculated or measured for each installation.
It decides the system because it separates service regimes:
| Regime | Wall condition | Main attack | Lining direction |
|---|---|---|---|
| Hot wall, above acid dew point | Dry gas | Heat, abrasive ash | Heat-resistant system; corrosion stays limited |
| Wall crossing the dew point (start-ups, cold spots) | Intermittent condensation | Concentrated acid and thermal cycling | Chemically resistant lining, tolerant of cycling |
| Wall permanently below dew point (downstream of scrubber) | Continuous condensation | Dilute acid in near-permanent immersion | Immersion-type, thick, glass flake lining |
| Temperature excursions (bypass, upset) | Short heat peak | Thermal degradation of the lining | System whose peak rating covers the expected excursion |
The most often forgotten input is the excursion: a lining that performs perfectly in normal operation can be destroyed by a single hot gas bypass. Ask the operator for the normal range and the worst plausible excursion before choosing.
Which linings resist acid condensation?
You choose among a few proven families, depending on service regime, temperature and substrate. None covers every case: the data sheet and the installation's history decide.
- Glass flake vinyl ester. The reference for continuous or intermittent acid condensation. The glass flakes overlap in the film and lengthen the diffusion path for acids and vapour. Spray or trowel applied depending on the product.
- Novolac epoxy, filled or flake-reinforced. Good chemical and thermal resistance for moderate acids and areas subject to cycling. See the novolac epoxy entry.
- Fluoropolymer-based systems. Reserved for particular exposures, with strict application requirements.
- Mineral linings (acid-resistant brick or block, special gunite). Chosen for the highest temperatures or abrasion, in addition to or instead of an organic lining.
Our guide to industrial tank lining selection follows the same selection logic by chemical exposure and temperature.
Why is glass flake chosen so often?
It is chosen because it combines very low permeability, good resistance to thermal cycling and a thickness that leaves a margin against ash abrasion. The glass platelets, oriented parallel to the wall, form layers that slow the penetration of acid and water vapour.
That performance depends on three conditions:
- Application that orients the flakes. Cross passes, debubbling roller or finishing per the data sheet; careless application leaves randomly oriented flakes and air bubbles.
- Controlled thickness. Too thin, the barrier is insufficient; too thick, the film can crack under cycling. The range comes from the specification.
- Verified continuity. A single holiday lets acid reach the steel and the lining disbonds around it. High-voltage holiday testing, per NACE SP0188 (AMPP) or the specified method, covers the full surface.
What surface preparation does a flue duct require?
It requires high-grade abrasive blasting, usually SSPC-SP 5 or SP 10 depending on the specification, with an angular surface profile suited to the lining thickness. On acid-contaminated steel, washing and a soluble salt test are added, because sulfates and chlorides left in pits make the lining blister.
Points specific to flue ducts:
- Deposit removal. Ash, soot and corrosion products are removed before blasting; they can contain metals and are handled as controlled waste.
- Steel repair. Perforated or overly thinned areas are replaced or reinforced before lining. A lining does not replace lost steel thickness.
- Concrete. A concrete stack is prepared per SSPC-SP 13 / NACE No. 6, after removal of attacked concrete and repair.
Abrasive blasting inside a duct is done in a confined space, with ventilation and dust collection; our article on blasting in operating plants and dust control covers these measures.
How should access and containment be organized?
Organize access and containment before the final system choice, because they set the work duration and whether blasting is feasible. A stack or duct is almost always a confined space under regulation (RSST in Quebec, the confined spaces regulation in Ontario), managed under a compliant program and often per CSA Z1006.
Items to plan:
- Isolation. Dampers closed and locked out, fans stopped, other gas sources isolated, atmosphere tested before entry and continuously.
- Access. Internal scaffolding, suspended platforms or access through inspection doors depending on geometry; additional openings created and closed if needed.
- Ventilation and conditions. Dust and solvent extraction, heating and dehumidification to stay above dew point by the required margin, especially in winter.
- Rescue. A rescue plan specific to the structure, with crew and equipment on site.
Project management during shutdowns coordinates these items with the operator's schedule and the other trades.
How to inspect the work and get a quote?
Inspection works through hold points: cleanliness, profile and salts before application; ambient conditions and thickness during; holiday testing and defect repair before return to service. Coating inspection records each step in a log that will support future inspections. Our article on coating failure modes helps read a lining that blisters or disbonds at start-up.
Our crews and qualified subcontractors work from LaSalle, including the Côte-Nord and Saguenay-Lac-Saint-Jean. Send photos, dimensions, fuel, normal and excursion temperatures and the shutdown window through the request a quote form: pricing on consultation, reply within 24 business hours.
Frequently asked questions
Why does my stack corrode mostly at the top?
Because the top is often the coldest part: the gas has lost heat and the wall is exposed to wind. It drops below the acid dew point more often, and condensation concentrates there.
Is a high-temperature paint enough inside a flue duct?
Rarely. A high-temperature paint resists heat but not condensed acid. Where the wall crosses or stays below the acid dew point, a chemically resistant lining such as a glass flake system is needed.
Can a duct be lined without shutting it down?
Not for an internal lining. The inside of a duct in service is inaccessible and the wall can be neither blasted nor inspected. The work is planned during a shutdown, with isolation and a confined space procedure.
What if the steel is already perforated?
Lost steel is replaced or reinforced first, then lined. A lining does not restore steel thickness; it protects the remaining sound steel. Thickness measurement before the work defines the areas to replace.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.