Steel Stack Corrosion Inspection Checklist: 10 Checks Before an Outage

Steel Stack Inspection Checklist: Key Points

• Acid condenses inside industrial steel stacks because flue gas keeps cooling as it rises, and shell metal below the acid dew point collects sulfuric or hydrochloric acid condensate.
• Before the outage, pull operating and fuel records plus past thickness, repair, lining and coating history.
• While the stack runs, map cold zones with infrared thermography, watch for stack rain and blocked drains, and walk the insulation and cladding.
• At the outage, inspect the breeching entry, base and drains, stiffener rings, flanges and welds, and the top section where gas is coldest.
• Measure wall thickness by elevation at fixed points, check any coating for blistering and undercutting, and compare readings with the engineer of record's structural minimum.
• Fix causes first, repair steel below the minimum, then protect; StackGard®-255SQW is rated to 255 °C (491 °F) and certified partners apply it under Polylloy supervision.

Acid condenses inside industrial chimneys because flue gas keeps cooling as it rises, and wherever the steel shell falls below the acid dew point, sulfuric or hydrochloric acid forms on it. A pre-outage checklist therefore finds those cold zones: records, running thermography, breeching, base, stiffeners, top section, then a thickness and coating survey.

What a Steel Stack Corrosion Inspection Checklist Covers

A steel stack corrosion inspection checklist covers industrial steel exhaust stacks and chimneys at cement, waste-to-energy, power and process plants, not residential chimneys.

Why Acid Condenses Inside Industrial Chimneys

The stack is the final stage of the flue gas path and often runs at the lowest temperature in the system. As gas rises it keeps cooling, which makes crossing the acid dew point more likely and leaves acidic condensate on the shell, as Polylloy's May 2026 International Cement Review (ICR) article explains.

In cement plant flue gas, the ICR article puts that dew point typically at 120 to 160 °C, depending on sulfur trioxide (SO3) and moisture; other fuels and processes need their own figure. The mechanism and its symptoms are covered in the acid dew point warning signs guide.

Lower stack temperatures and higher moisture extend condensation, and liners or insulation can hide the resulting corrosion until damage is significant.

Protection choices and the reline-or-replace decision are compared in industrial chimney corrosion protection options, and upstream equipment has its own baghouse corrosion inspection checklist. Wet stacks after wet flue gas desulfurization (FGD) carry continuous condensate and are noted where checks differ.

Ten Checks at a Glance

The ten checks are summarized below (UT: ultrasonic thickness; DFT: dry film thickness).

Steel Stack Records to Pull Before the Outage

Steel stack records show what changed since the last outage.

Operating and Fuel Records (Check 1)

Pull the following data for the period since the last inspection:

• Normal, minimum and peak stack inlet gas temperatures, and how much the unit ran at low load; low load and start-stop operation enlarge the cold area of the shell.
• The start-up and shutdown count, and whether the stack runs wet, after a wet FGD absorber, or dry.
• Fuel or raw material changes, including alternative fuels that can add chlorides.
• Any emissions control or heat recovery project that lowered stack gas temperature, which the ICR article notes narrows the margin to the dew point.

Thickness, Repair, Lining and Coating History (Check 2)

Collect previous ultrasonic thickness readings with their elevations and positions, repair records, and drawings showing stiffener rings, platforms, insulation and drains. For any lining or coating, gather the product and grade, surface preparation, film thickness, inspection records and any failure. Polylloy's coating evaluation asks for much of this, including any failed coating, drawings and insulation details.

Running Checks on a Steel Stack While It is on Line

Running checks on a steel stack find cold zones and leaks while their causes are present, so the outage crew arrives knowing where to look.

Shell Thermography for Cold Zones (Check 3)

Survey the shell, breeching, access doors, flanges and expansion joints with infrared thermography, and mark cold zones and insulation gaps on an elevation drawing. False air drawn through leaks cools the steel locally and raises oxygen, as the ICR article explains, so leaking doors and joints can show up as cold patches.

Stack Rain and Base Drains (Check 4)

Log any liquid collecting at the base door or drains, or droplets falling around the stack outlet, and confirm drains are open and flowing. Stack rain usually means gas is crossing the dew point; on a wet stack it can also mean liquid is not being collected and drained.

External Insulation and Cladding (Check 5)

Walk the cladding from grade and platforms. Damaged or wet insulation creates new cold zones inside and lets water reach the shell outside, where corrosion develops under insulation. Note attachments, seams and rain-cap flashing where water gets in.

Steel Stack Breeching Entry and Base Inspection at the Outage

The steel stack breeching entry and base come first at the outage, because gas impinges on one and condensate collects at the other.

Before Entry

Entry and work at height follow the site's confined space, isolation, gas-testing and access procedures; this checklist does not prescribe one. Internal visual inspection of a standing stack uses rope access or a drone.

Remove deposits from the floor, ledges and stiffener tops first, because thinning and pinholes often hide under retained deposits that absorb moisture as the stack cools. Polylloy's technical paper notes that dry operation does not demonstrate that shutdown exposure is benign.

Breeching Entry and Duct Connection (Check 6)

Inspect where the duct enters: the transition, opening reinforcement, expansion joint and the shell opposite the inlet where gas impinges. Record corrosion, perforation and erosion where dust-laden gas turns.

Stack Base, Floor, Cleanout and Drains (Check 7)

Check the floor plate, lower shell course, cleanout door frame and drain connections for wall loss and pitting under deposits. Record a photo and the deposit condition (damp or caked) for each zone.

Steel Stack Shell, Stiffener and Top Section Inspection

The steel stack shell, stiffeners and top section hold the steel that runs coolest or traps condensate.

Stiffener Rings, Flanges and Welds (Check 8)

Inspect internal faces at stiffener rings, bolted flanges, circumferential and vertical welds, and attachment points, which run cooler or trap condensate and deposits. Cross-check every cold zone on the thermography map, including the shell behind each insulation gap.

Top Section and Outlet (Check 9)

The upper shell and outlet are usually the coldest steel in the stack, so inspect the top courses, the outlet lip and the inside of any rain cap or cone. Liners can hide corrosion on the shell behind them until damage is significant; agree with the engineer of record where to open or test through a liner.

Measuring Steel Stack Wall Thickness and Coating Condition

Measuring steel stack wall thickness and coating condition turns visual findings into data comparable from outage to outage.

Thickness Survey by Elevation (Check 10)

Take ultrasonic thickness readings on a fixed grid at each elevation and zone, repeated at the same points every outage, so loss rates come from comparisons rather than single readings. Record pit depth too; Polylloy's technical paper notes that a low average rate can conceal a penetration risk.

Compare readings against the structural design minimum and corrosion allowance, with the engineer of record confirming the assessment. A coating does not restore lost wall.

Coating and Lining Condition

On a coated or lined stack, check the film two ways:

• Visual: blistering, rust creeping from welds, edges and damage points (undercutting corrosion), lifting film, and erosion at the breeching.
• Measured: dry film thickness against the specified range, and holiday (continuity) testing where the specification calls for it.

Resistance to undercut corrosion is critical in stack service, the ICR article notes. If the coating has failed, diagnose a coating failure before recoating. Polylloy's inspection and maintenance services include condition assessment and thickness and continuity verification.

Turning Steel Stack Inspection Findings Into an Outage Scope

Turning steel stack inspection findings into an outage scope follows a fixed order, so repairs are not undone by the original cause.

Order of Work

Plan the work in this order:

1. Fix causes: insulation damage, air in-leakage, blocked drains and prolonged low-load running.
2. Repair or replace steel below the structural minimum.
3. Protect the sound shell.

Relining or replacing sections is a decision for the engineer of record, set out in the chimney protection options guide linked above.

Protecting a Sound Shell

For dry or intermittently condensing stacks within 255 °C (491 °F), StackGard-255SQW is a candidate. Its bulletin lists resistance to SO3, sulfur dioxide (SO2), hydrogen chloride (HCl) and nitrogen oxides (NOx) gases and dust abrasion. Suitability for continuously wet stacks is confirmed with Polylloy's technical team.

The ICR article notes that coatings such as StackGard steel stack coating can be applied inside an existing stack, on replacement sections on the ground, or on new sections before assembly. Certified partners apply it; Polylloy formulates, supplies, specifies and supervises.

Final heat cured activation is completed during start-up using process heat. For a full recoat, follow the coating outage planning steps; for steel behind refractory, see coating a refractory-lined stack.

How Often to Inspect a Steel Stack for Corrosion

How often to inspect a steel stack is set by triggers rather than a calendar figure.

Triggers Instead of a Fixed Interval

No universal interval applies. Inspect at every planned outage, and add an inspection after a fuel change, an emissions control or heat recovery project that lowered stack temperature, a switch between wet and dry operation, or a run of low-load and start-stop operation.

Stacks with past perforations, concealed shell or visible stack rain deserve closer attention than stacks that run steadily warm. Coated stacks are still inspected; the StackGard-255SQW bulletin lists easy inspection and maintenance.

Deciding What Your Next Steel Stack Outage Needs

Deciding what the next steel stack outage needs comes down to what the inspection found:

• Cold zones and leaks only: fix the causes and re-inspect at the next outage.
• Wall loss with adequate remaining thickness: plan repair and protection matched to gas temperature, for example StackGard-255SQW within 255 °C.
• Wall below the structural minimum: repair or replace first.
• A failing coating: diagnose the failure before recoating.

A stack that runs well above the dew point with no condensation history may not need a severe-service coating at all. To book a Coating Performance Review, request a coating evaluation with stack dimensions, coated area, operating conditions (temperature, gas chemistry, abrasion), location and outage timing. Inspection photos and thickness readings help.

FAQs

Why Does Acid Condense Inside Industrial Chimneys?

Flue gas keeps cooling as it rises through the stack, which is often the coldest part of the flue gas path. Wherever the steel shell falls below the acid dew point, sulfuric or hydrochloric acid condenses on it. Low load, air in-leakage and damaged insulation enlarge the cold area, and liners can hide the damage.

What Should a Steel Stack Inspection Checklist Include?

Ten checks: operating and fuel records, past thickness and repair history, running thermography, stack rain and drains, insulation and cladding, the breeching entry, the base and floor, stiffener rings, flanges and welds, the top section, and a thickness and coating survey by elevation.

How Do You Inspect the Inside of a Steel Stack for Corrosion?

Map cold zones with infrared thermography while the stack runs, then at the outage clean deposits and inspect the breeching, base, stiffeners, welds and top section visually, using rope access or a drone. Take ultrasonic thickness readings at fixed points by elevation, record pit depth, and check any coating for blistering and undercutting.

How Often Should an Industrial Steel Stack Be Inspected for Corrosion?

No universal interval applies, so set the schedule around operation. Inspect at every planned outage, and add an inspection after a fuel change, a project that lowered stack gas temperature, a switch between wet and dry operation, or a run of low-load operation. Stacks with past perforations or hidden shell deserve closer attention.

Where Does a Steel Chimney Corrode First?

Usually at the coldest or wettest steel: the top section and outlet, the breeching entry, stiffener rings, flanges and welds, insulation gaps, and the base where condensate collects. A running thermography map tells the outage crew which zones to inspect first.

Does a Coated Steel Stack Still Need Corrosion Inspections?

Yes. Inspections confirm the coating is intact and catch mechanical damage, erosion at the breeching or new cold zones early. Check the film visually for blistering, undercutting and lifting, take dry film thickness readings, and run continuity testing where the specification calls for it.

Who Applies StackGard-255SQW After a Stack Inspection?

Certified Polylloy partners apply StackGard-255SQW, inside the standing stack or on replacement and new sections on the ground, after any lost steel is repaired and the surface is blasted to the specified standard. Polylloy formulates, supplies, specifies and supervises. Final heat cured activation is completed during start-up using process heat, so start-up forms part of the plan.