Flue Gas Composition: 7 Components and the Corrosion Each Causes

Flue Gas Composition: Key Points

• Flue gas is the exhaust a combustion process sends through ducts, filters and the stack; its bulk is nitrogen, carbon dioxide, water vapor and excess oxygen, while minor components drive most corrosion.
• Seven components matter for corrosion: water vapor, sulfur oxides (SO2 and SO3), hydrogen chloride, hydrogen fluoride, nitrogen oxides, oxygen and particulate dust.
• SO3 and water vapor form sulfuric acid and raise the acid dew point, typically 120 to 160 °C in cement plant flue gas; steel colder than that collects acid condensate.
• Hydrogen chloride and chloride-bearing deposits are a main reason stainless steel still corrodes: they can break down its passive film locally.
• Alternative fuels add sulfur, chlorine and fluorine variability; in HF-rich cement gas, FlueGard®-225SQC on a French fan housing has run about 24 months.
• Match protection to the gases present: the FlueGard-225SQC bulletin lists SO3, SO2, HCl, CO2, HF and NOx resistance; FlueGard-425S and StackGard®-255SQW list SO3, SO2, HCl and NOx.

Stainless steel still corrodes in industrial flue gas because some of its components turn into acid condensate when steel runs below the acid dew point, and chlorides break down the passive film. Flue gas is the exhaust from combustion; seven components drive corrosion: water vapor, sulfur oxides, hydrogen chloride, hydrogen fluoride, nitrogen oxides, oxygen and dust.

What Flue Gas is and Why Its Composition Decides Corrosion

Flue gas is the mixture of gases and particulate that leaves a combustion process and travels through ducts, pollution control equipment and the stack. Its composition is the set of bulk gases, acid-forming trace gases and dust it contains.

Flue Gas Defined

Flue gas comes from cement and lime kilns, boilers, furnaces, waste incinerators and smelters. Most of it is nitrogen, with carbon dioxide, water vapor and excess oxygen; nitrogen is inert for corrosion purposes and is not one of the seven components below.

Trace Components Do the Damage

Corrosion comes mainly from minor components that form acids or deposits when gas meets cooler steel. Polylloy's technical paper on stainless steels and claddings notes that bulk flue-gas chemistry may not describe the environment at the metal surface.

Think of the gas analysis as a list of ingredients: corrosion mostly happens where those ingredients meet a cold surface or a retained deposit. On stainless steel, chlorides and acid condensate break down the passive film locally; the detail is in why 316 stainless corrodes in flue gas.

How the 7 Flue Gas Components Were Selected

Each flue gas component in this list meets all four of these criteria:

• Present in industrial flue gas from kilns, boilers and incinerators.
• Plays a named role in steel corrosion: it forms acid, promotes acid formation or holds acid against the steel.
• Discussed in Polylloy's technical writing or listed in a Polylloy bulletin.
• Points to a distinct check or protection choice.

The order follows how acid attack builds up: the carrier (water vapor), the acid formers, the promoter (oxygen) and the deposits (dust). It is not a ranking.

Flue Gas Components at a Glance

The table summarizes the seven components in the order they are described. Polylloy bulletin resistance by gas is mapped in the closing section.

Water Vapor in Flue Gas: The Carrier That Turns Gases Into Acid

Water vapor comes first because it is the carrier for the acids that follow.

Where It Comes From

Water vapor comes from the combustion of hydrogen in the fuel, fuel and raw material moisture, gas conditioning and water sprays. Polylloy's May 2026 International Cement Review (ICR) article notes that decarbonization-related operating conditions can increase moisture, particularly with alternative fuels or gas conditioning.

Corrosion It Causes and Where

Water vapor does not attack steel by itself in hot gas. It is the carrier that lets SO3, HCl and HF condense as acid on cooler surfaces, and higher moisture raises the acid dew point.

During shutdowns, deposits can absorb moisture and dissolve their salts into an aggressive electrolyte. Polylloy's technical paper makes the point that dry operation does not demonstrate that shutdown exposure is benign.

Sulfur Dioxide and Sulfur Trioxide: The Source of Sulfuric Acid Dew Point Corrosion

Sulfur dioxide (SO2) and sulfur trioxide (SO3) are the source of the sulfuric acid behind most dew point corrosion.

From SO2 to Sulfuric Acid

Sulfur enters the gas mainly as SO2, and a portion oxidizes to SO3 in the presence of oxygen and catalysts such as iron oxides and alkalis. SO3 reacts with water vapor to form sulfuric acid (H2SO4), which stays as vapor or condenses depending on temperature.

This raises the acid dew point. The ICR article puts it typically at 120 to 160 °C in cement plant flue gas, depending on SO3 concentration and moisture; other fuels and processes need their own figure.

Corrosion It Causes and Where

Steel colder than the dew point collects sulfuric acid condensate in kiln inlet and preheater ducts, gas conditioning systems, baghouse housings, clean air plenums and stacks. The visible symptoms are listed in acid dew point corrosion warning signs.

On the protection side, FlueGard-225SQC showed no observed deterioration after six months' immersion in 56% sulfuric acid at 60 °C. That is Polylloy's own report, not an independent test.

Hydrogen Chloride and Chlorides: Hydrochloric Acid and Pitting of Stainless Steel

Hydrogen chloride (HCl) and chloride deposits are a main reason stainless steel still corrodes in flue gas.

Where It Comes From

Chlorine in fuels, alternative fuels and waste is released into the gas largely as hydrogen chloride. Polylloy's technical paper notes that fuel changes alter chloride, sulfur and alkali inputs, a shift traced in how alternative fuels changed kiln corrosion.

Corrosion It Causes and Where

HCl dissolves into condensate on cold surfaces as hydrochloric acid, usually at lower metal temperatures than sulfuric acid needs. Chloride-bearing deposits can concentrate salts and hold moisture against the steel.

On stainless steel, chlorides cause localized breakdown of the passive film: pitting, crevice corrosion and, under tensile stress, stress corrosion cracking. The FlueGard-225SQC bulletin reports one-week immersion weight gain below 1.0% in 37% HCl.

Hydrogen Fluoride: Attack in HF-Rich Cement Kiln Gas

Hydrogen fluoride (HF) is covered here through its fuel source and the field evidence for protecting steel against it.

Where It Comes From

The ICR article notes that alternative fuels such as refuse-derived fuel and biomass introduce variability in sulfur, chlorine, fluorine and alkali inputs. It also names HF, with HCl and SO3, among the aggressive gases that can penetrate refractory and accumulate at the steel interface, where local temperature gradients promote condensation.

Protection That Has Worked in HF-Rich Gas

One documented case: at a French cement plant, FlueGard-225SQC was installed in April 2024 on a fan housing, inlet duct sections and transition zones. It has run about 24 months in HF, HCl, sulfur oxides (SOx) and high-velocity dust service, below 250 °C continuous with short excursions approaching 280 °C, at about 25 mils (635 microns) on SSPC-SP10 steel.

The FlueGard-225SQC bulletin lists HF resistance and one-week weight gain below 1.0% in 3% HF. The FlueGard-425S and StackGard-255SQW bulletins do not list HF, so HF service for them is confirmed by Polylloy's technical team. Other options exist for HF service, and fan housings have their own comparison in ID fan housing protection for acid gas, HF and dust.

Nitrogen Oxides in Flue Gas and Their Role in Condensate

Nitrogen oxides (NOx) belong on the list, but with a short entry.

Source and Corrosion Role

NOx is part of the acid-gas mix that protection has to tolerate. In cement and power flue gas, sulfur oxides and chlorides are usually the main drivers of dew point corrosion, and NOx matters mainly as part of that mix. The FlueGard-225SQC, FlueGard-425S and StackGard-255SQW bulletins all list resistance to NOx gases.

Oxygen and False Air: Promoters of Localized Flue Gas Corrosion

Oxygen does not form acid itself, but it helps make it and marks the places where air leaks in.

Source and Corrosion Role

Excess combustion air leaves oxygen in the gas, and false air drawn through seal, joint or duct leaks adds more. Oxygen also takes part in the SO2 to SO3 step described above.

False air cools the steel locally and raises oxygen, which can promote SO3 formation at the same time, so a leak creates a highly localized corrosion zone. Typical spots are access doors, inspection ports, flanges, expansion joints and rotary valves; the first check is thermography for cold patches, then seal repair.

Particulate Dust and Ash: Buffer, Salt Carrier and Abrasive

Particulate dust and ash play three roles, one of them protective.

Buffer on One Side, Missing on the Other

On the dirty side of a cement baghouse, dust deposits contain alkaline compounds that partly neutralize acid condensate. High-efficiency filtration removes most dust before the clean air plenum, so the clean side largely lacks that buffer.

Deposits and Salts

Dust and ash can concentrate soluble salts, retain moisture and create shielded spots. Polylloy's technical paper describes the sequence: deposition in operation, cooling at shutdown, moisture uptake, salt dissolution, then localized corrosion beneath retained deposits.

Abrasion

High-velocity dust erodes protection at bends, fans and transitions. A Swiss cement plant duct carrying about 1 to 2 g/Nm³ of dust at about 13 m/s was protected with FlueGard-455CHT, as described in hot flue gas duct protection. Abrasion-led zones may favor FlueGard-455CHT (Taber 20 mg) or WearGard®-625S (Taber below 30 mg); FlueGard-425S has Taber abrasion below 150 mg.

How to Choose Protection by Flue Gas Composition

Choosing protection by flue gas composition means reading the gas analysis component by component, as this list does, against the metal temperature.

Read the Gas Analysis Against the Metal Temperature

The decision inputs are which of the seven components are present, the minimum metal temperature against the acid dew point, shutdown frequency and dust loading. Fix causes first where they exist, such as false air, insulation gaps and deposits, then protect steel that will still condense acid.

FlueGard Grades by Gas Chemistry

The FlueGard flue gas coatings are matched to gases as their bulletins list them:

For stacks, StackGard-255SQW, rated to 255 °C, lists SO3, SO2, HCl and NOx. The full range is set out in Polylloy coating grades by temperature and gas chemistry.

Certified partners apply the coatings; Polylloy formulates, supplies, specifies and supervises. Final heat cured activation is completed during start-up using process heat.

Where metal stays well above the dew point and the gas carries little acid-forming content, a conventional coating may be enough. To request a coating evaluation, share equipment type, coated area, operating conditions (temperature, gas chemistry, abrasion), location and outage timing; a recent gas analysis helps.

FAQs

What is Flue Gas?

Flue gas is the mixture of gases and particulate that leaves a combustion process, such as a cement kiln, boiler, furnace or waste incinerator, and travels through ducts, filters and the stack. Most of it is nitrogen, carbon dioxide, water vapor and excess oxygen, but minor components such as sulfur oxides and hydrogen chloride cause most corrosion.

What is Flue Gas Made Of?

Flue gas is mostly nitrogen, carbon dioxide, water vapor and excess oxygen, plus smaller amounts of sulfur oxides, hydrogen chloride, hydrogen fluoride, nitrogen oxides and particulate dust. The exact mix depends on the fuel, raw materials and process, and alternative fuels add variability in sulfur, chlorine and fluorine, so each plant needs its own gas analysis.

Why Does Stainless Steel Still Corrode in Flue Gas?

Stainless steel relies on a thin passive film that chlorides and acid condensate can break down locally. When metal runs below the acid dew point, sulfuric and hydrochloric acid condense on it, and chloride-bearing deposits hold moisture against the surface. The result is pitting, crevice corrosion and, under stress, cracking, even in 316 grades.

Which Flue Gas Components Cause the Most Corrosion?

In cement and power plant flue gas, sulfur trioxide and hydrogen chloride are usually the main concern, because they form sulfuric and hydrochloric acid condensate where steel runs below their dew points. Water vapor carries the acid, oxygen from false air promotes SO3, and dust deposits hold the condensate and salts against the steel.

What Coating Protects Steel in HF-Rich Cement Kiln Gas?

FlueGard-225SQC lists hydrogen fluoride resistance in its bulletin and has run about 24 months on a fan housing at a French cement plant exposed to HF, HCl, sulfur oxides and high-velocity dust, below 250 °C continuous. Certified partners apply it under Polylloy supervision. Other grades need HF service confirmed by Polylloy's technical team.

Does Dust in Flue Gas Protect Steel or Corrode It?

Both, depending on where it sits. Alkaline cement dust partly neutralizes acid condensate on the dirty side of a baghouse, but the clean side lacks that buffer. Deposits also concentrate soluble salts and hold moisture against the steel, which can cause corrosion beneath them during shutdowns, and high-velocity dust erodes protection at bends, fans and transitions.

How Does Flue Gas Composition Affect Coating Selection?

Match the coating to the gases present and the metal temperature. The FlueGard-225SQC bulletin lists SO3, SO2, HCl, CO2, HF and NOx resistance to 225 °C; FlueGard-425S lists SO3, SO2, HCl, CO2 and NOx to 425 °C; FlueGard-455CHT lists good resistance to acidic and alkaline condensates to 455 °C (850 °F). Dust loading and shutdown frequency narrow the choice further.