Acid Dew Point Corrosion: 7 Warning Signs in Baghouses, Ducts and Stacks

Acid Dew Point Corrosion: Key Points

• Acid dew point corrosion starts when steel runs below the acid dew point of the flue gas, typically 120 to 160 °C depending on sulfur trioxide (SO3) concentration and moisture, and acid condenses on it.
• Emissions control and waste heat recovery (WHR) projects cool the gas and remove alkaline dust, so corrosion often appears after an upgrade on equipment that used to run clean.
• The clean-gas side of a baghouse lacks the alkaline dust that partly neutralizes condensate on the dirty side, so outlet plenums are a frequent first sign.
• Leaking seals, damp hopper dust, damage after shutdowns and undercutting under an existing coating each point to a different first check.
• Liquid running inside a stack, the "rain" operators see at the base door, means gas is crossing the dew point in the final stage of the flue path.
• Protection is matched by service temperature and chemistry, for example FlueGard®-225SQC to 225 °C in baghouses and StackGard®-255SQW to 255 °C in stacks.

Acid dew point corrosion starts when steel in a baghouse, duct or stack runs below the acid dew point of the flue gas, typically 120 to 160 °C, and sulfuric or hydrochloric acid condenses on it. Seven signs give it away: post-upgrade corrosion, clean-side rust, leaking seals, damp hopper dust, shutdown damage, coating undercut and stack rain.

Acid dew point corrosion is attack on steel by acid condensing from flue gas wherever the metal is colder than the gas's acid dew point. In cement plants it concentrates in ducts, baghouses and stacks downstream of the kiln.

Acid Dew Point Corrosion in Flue Gas Equipment Explained

Acid dew point corrosion in flue gas equipment begins with sulfur in the fuel and raw materials. It enters the gas mainly as sulfur dioxide (SO2), and part of it oxidizes to SO3 in the presence of oxygen and catalysts such as iron oxides and alkalis.

How the Acid Forms

SO3 reacts with water vapor to form sulfuric acid (H2SO4), present as vapor while the gas is hot and as condensate once it meets a cold enough surface. This raises the acid dew point of the gas, typically to 120 to 160 °C (248 to 320 °F) depending on SO3 concentration and moisture content.

Chlorine in the fuel adds hydrogen chloride (HCl), which condenses as hydrochloric acid on the same cold surfaces.

Why Metal Temperature Matters More Than Gas Temperature

Corrosion follows the coldest metal surface, not the bulk gas reading. A duct can show a comfortable gas temperature on the control screen while a stiffener, door frame or uninsulated corner sits below the dew point, and there sulfuric and hydrochloric acid condense.

Even a thin film of condensate accelerates attack on carbon steel. Corrosion is frequently observed in kiln inlet ducts, preheater ducts, gas conditioning systems, baghouse housings, clean air plenums and stacks. Upgrading the alloy does not remove the mechanism; the reasons are set out in a separate article on why 316 stainless corrodes in flue gas.

How the 7 Acid Dew Point Warning Signs Were Selected

Each acid dew point warning sign in this list meets all five of these criteria:

• Visible or measurable by plant staff, in operation or at an outage.
• Tied to a mechanism Polylloy has published in its technical writing on flue gas corrosion.
• Points to a specific location on the baghouse, duct or stack.
• Implies a distinct first check.
• Applies to cement plant flue gas paths and other sulfur-bearing flue gas.

The order follows the gas path and the plant timeline: the cause first (the upgrade), then the baghouse, ducts, outages, existing coatings, and the stack last.

Acid Dew Point Warning Signs at a Glance

The table summarizes the seven warning signs in the order they are described. Protection cost is project-specific for every row.

Corrosion That Begins After an Emissions Control or Heat Recovery Upgrade

The first warning sign is a change in service conditions rather than in the equipment.

What You See

Rust, pitting or leaks appear on ducts, baghouses or stacks that ran without trouble before a filter upgrade, a gas cooling change or a WHR installation.

Why It Happens

Energy-efficiency projects and WHR systems lower flue gas temperature, which narrows the margin between operating temperature and the acid dew point. Gas run close to heat recovery limits spends more time near or below it, so more steel condenses acid for longer.

Higher-efficiency filtration also cuts the alkaline dust carried downstream, removing a buffer that used to neutralize condensate. Surjit Gill, Polylloy's co-founder, has seen this pattern since plants began cooling exhaust gas to meet tighter emission limits.

Where It Shows Up

Look downstream of the WHR unit or conditioning tower, on the baghouse clean side, in outlet ducts and in the stack.

What to Do

Re-map metal temperatures against the dew point after any upgrade that changes gas temperature or dust load. Build corrosion protection into the upgrade scope rather than waiting for the next failure.

Clean-Gas Side Corrosion in the Baghouse Outlet Plenum

Clean-gas side corrosion is often the first sign inside the baghouse itself.

What You See

Scaling, pitting and rust streaks appear on the clean air plenum, the top face of the tubesheet, the roof and the outlet duct, while the dirty side looks comparatively sound.

Why It Happens

On the dirty side, cement dust deposits contain alkaline compounds that partly neutralize acidic condensate. After high-efficiency filtration the clean side largely lacks that buffering, so each condensation event attacks bare steel directly.

Where It Shows Up

Clean air plenum walls and roof, outlet transitions and door frames on the clean side are the usual locations.

What to Do

Inspect the clean side at every outage, not only the hoppers. Where protection is needed, FlueGard-225SQC is rated to 225 °C (437 °F) for baghouses, electrostatic precipitators (ESPs) and ducts, and its bulletin lists resistance to SO3, SO2, HCl and hydrogen fluoride (HF). The wider set of baghouse corrosion protection options is compared in a separate guide.

Localized Corrosion at Duct Seals, Doors and Expansion Joints

Corrosion at seals and joints marks the places where false air enters the gas path.

What You See

Rust halos and perforation appear in patches around access doors, inspection ports, rotary valves, flanges and expansion joints. Dust staining on the outside of the casing often marks the same spots.

Why It Happens

False air drawn through seal, joint or duct leaks cools the steel locally. It also raises oxygen, which can promote SO3 formation at the same time, so a small leak creates a highly localized corrosion zone.

Where It Shows Up

Duct flanges and expansion joints along the kiln-to-baghouse path, baghouse doors and hopper discharge valves are typical.

What to Do

Find cold patches with infrared thermography while the plant runs, and fix the seals first. Then protect the repaired zone; a coating cannot compensate for a leak that keeps feeding cold air onto the steel.

Damp, Caked or Bridging Dust in Baghouse Hoppers

Damp hopper dust shows that deposits are holding moisture against the steel.

What You See

Dust cakes, bridges or comes out damp at the discharge. When the hopper is opened, wall thinning and pinholes sit under retained deposits in the hopper valleys.

Why It Happens

Dust and ash concentrate soluble salts, hold moisture against the steel and create shielded spots where condensate becomes an aggressive electrolyte. Polylloy's technical paper on stainless steels and claddings also notes that fuel changes and alternative fuels alter chloride, sulfur and alkali inputs, which changes deposit chemistry.

Where It Shows Up

Hopper walls and valleys, discharge throats and stiffeners under deposits are the usual locations.

What to Do

Sample deposits and condensate, and record minimum wall temperatures, before choosing a fix. Those measurements decide whether hopper heating, insulation or a coating comes first.

Corrosion Damage That Appears After Shutdowns and Start-Ups

Damage found after outages points to corrosion during cool-down rather than steady running.

What You See

Fresh rust and wall loss are found after outages, worst on equipment that stops often. The damage appears even where steady running was dry.

Why It Happens

The sequence runs from deposition in operation to cooling at shutdown, moisture uptake or condensation, salt dissolution, and then localized corrosion beneath retained deposits. Polylloy's technical paper makes the point that dry operation does not demonstrate that shutdown exposure is benign.

Where It Shows Up

Any surface holding deposits through a cool-down is exposed, and ducts with frequent cycling are hit hardest.

What to Do

Log shutdown frequency alongside thickness data, and clean deposits before long outages where practical. Cycling-tolerant protection exists: on a FlueGard-455CHT duct at a Swiss cement plant with about 50 shutdown cycles a year, inspected areas showed no visible widespread delamination at about 17 months, although heavy dust limited the inspection. That case is covered in hot flue gas duct protection through cycling.

Blistering, Undercutting or Peeling on the Existing Coating

A failing existing coating shows where condensate is already reaching the steel.

What You See

Blisters form, rust creeps under the film from a scratch, weld or edge, and sheets of coating lift from the steel.

Why It Happens

At any defect, acidic condensate reaches the steel and spreads sideways beneath the film. Resistance to undercut corrosion decides whether a small defect stays small. Polylloy laboratory and field testing comparing FlueGard with conventional high-temperature coatings observed undercutting corrosion and delamination in the conventional coatings.

Where It Shows Up

Welds, edges, mechanical damage points and areas coated with a system rated for heat but not for acid condensate are the usual locations.

What to Do

Map the failed area and decide whether the failure is adhesion, chemistry or temperature before recoating with the same system. The way coating chemistries differ is explained in novolac, vinyl ester and hybrid coatings compared.

Rain and Acid Drips Inside the Steel Stack

Stack rain is the warning sign operators can see for themselves at the stack base.

What You See

Liquid runs down the inside of the stack and collects at the base. Surjit Gill describes opening the bottom access door and literally seeing rain inside.

Why It Happens

The stack is the final stage of the flue gas path and often the coldest. Gas keeps cooling as it rises and crosses the dew point, and lower stack temperatures with higher moisture extend the time spent condensing.

Where It Shows Up

The stack base and access door, upper sections, and shell concealed by liners or insulation are exposed; under a liner, corrosion can stay hidden until damage is significant.

What to Do

Treat stack rain as a structural warning, not a nuisance, and survey shell thickness, including concealed areas. The industrial chimney and stack corrosion options are compared separately.

Polylloy's May 2026 article in International Cement Review names StackGard-255SQW (255 °C) or CorrosionGard®-160S (160 °C, ambient cure) for stacks. Coatings can be applied inside an existing stack, on replacement sections on the ground, or on new sections before assembly.

How to Choose a Response to Acid Dew Point Corrosion Warning Signs

Choosing a response to acid dew point corrosion starts with identifying which of the seven warning signs is present, since each points to its own mechanism and first check.

Decision Path

Fix causes first where they exist: seal leaks, cold spots and deposit build-up. Then protect the steel that will still cross the dew point, matched by service temperature and gas chemistry:

• CorrosionGard-160S: to 160 °C (320 °F), ambient cure.
• FlueGard-225SQC: to 225 °C (437 °F), baghouses, ESPs and ducts.
• StackGard-255SQW: to 255 °C (491 °F), steel stacks, chimneys, ducts and fans.
• FlueGard-425S: to 425 °C, with peaks to 500 °C.
• FlueGard-455CHT: to 455 °C (850 °F).

Certified partners apply the coatings; Polylloy formulates, supplies, specifies and supervises. Grade details are on the FlueGard flue gas coatings page.

Honest Limits

Surface preparation is critical to any coating's performance. Heat-cured grades need start-up heat, because final heat cured activation is completed during start-up using process heat, and a coating does not fix wall loss below structural minimum.

To act on what you have found, book a Coating Performance Review and share equipment type, coated area, operating conditions (temperature, gas chemistry, abrasion), location and outage timing.

FAQs

What is Acid Dew Point Corrosion?

Acid dew point corrosion is attack on steel by sulfuric or hydrochloric acid that condenses from flue gas wherever the metal is colder than the gas's acid dew point. In cement plant flue gas that dew point is typically 120 to 160 °C, depending on SO3 concentration and moisture, so ducts, baghouses and stacks running near those temperatures are most exposed.

Why Did an Emissions Control Upgrade Cause Acid Condensation in Our Baghouse?

Upgrades change two things at once: gas cooling and waste heat recovery lower the flue gas temperature, so more steel runs near or below the acid dew point. Higher-efficiency filtration removes alkaline dust that used to neutralize condensate on downstream surfaces. Equipment that ran without trouble before the change can start corroding after it.

Why Do Cement Plant Baghouse Filters Fail Early?

Early baghouse failures usually trace to steel running below the acid dew point at cold spots: leaking doors and seals, hopper walls under damp dust, and the clean-gas side, which lacks alkaline buffering. Frequent shutdowns make it worse, because retained deposits absorb moisture as the housing cools and attack the steel beneath them.

What is Causing Rain Inside Our Flue Stack?

Liquid inside a stack is condensate. The stack is the last and often coldest part of the flue path, and as gas rises and cools it crosses the acid dew point, so acidic moisture forms on the shell and runs down to the base. Treat it as a corrosion warning and survey the shell thickness.

Why Do Steel Chimneys Corrode from the Inside?

Steel chimneys corrode internally because flue gas keeps cooling as it rises and acid condenses on any shell area below the dew point. Lower stack temperatures and higher moisture extend the time spent condensing. Liners or insulation can hide the shell, so the damage often goes unnoticed until wall loss is significant.

What Coating is Used for Hot Acid Dew Point Corrosion in a Dust Collector?

Choose by metal temperature and gas chemistry. FlueGard-225SQC is rated to 225 °C (437 °F) for baghouses, electrostatic precipitators and ducts and resists SO3, SO2, HCl and HF; FlueGard-425S covers hotter zones to 425 °C with peaks to 500 °C. Polylloy's certified partners apply both grades under Polylloy supervision, after an evaluation of the specific dust collector.

Does Acid Dew Point Corrosion Only Happen While the Plant is Running?

No, corrosion also develops during shutdowns. As equipment cools, deposits left on the steel absorb moisture or collect condensate, dissolve their salts and form an aggressive electrolyte. Polylloy's technical paper notes that dry operation does not prove shutdown exposure is harmless, so record shutdown frequency alongside thickness readings when diagnosing corrosion.