When Are Nickel Alloys Needed for Acid Dew-Point Corrosion in Flue Gas Equipment?
![Stainless steel flue gas pipes inside an industrial power plant chimney](https://emilypipe.com/wp-content/uploads/2026/07/Hastelloy-C276-tube19.png")
Image credit: Lauri Veerde, Wikimedia Commons, CC BY-SA 4.0.
Nickel alloys can be valuable in flue gas equipment exposed to persistent sulfuric acid condensation, but they are not required in every cold-end location.
The material decision should begin by confirming whether acid condensation actually occurs on the component surface.
Nickel alloys are most likely to be justified when the metal temperature repeatedly falls below the local acid dew point, sulfuric acid condensate remains on the surface, chlorides or other contaminants increase corrosivity, and thinner or difficult-to-repair components require a larger corrosion-resistance margin.
A higher nickel content alone does not prove suitability.
What Causes Acid Dew-Point Corrosion?
Sulfur in the fuel is mainly converted to SO₂ during combustion. A smaller portion may be further oxidized to SO₃.
As the flue gas cools, SO₃ reacts with water vapour to form gaseous sulfuric acid. Condensation begins when the local surface temperature falls below the local sulfuric acid dew point.
The risk is controlled by:
- SO₃ or gaseous H₂SO₄ concentration
- Water-vapour concentration
- Local metal temperature
- Flue gas composition
- Deposits
- Startup and shutdown cycles
- Air leakage
- Surface geometry
Research on H₂SO₄–H₂O condensation in flue gas shows that both acid dew point and condensate composition vary with gas conditions.
Therefore, neither a single universal dew-point temperature nor one fixed condensate concentration should be used for every plant.
Surface Temperature Matters More Than Average Flue Gas Temperature
The average gas temperature may remain above the calculated acid dew point while a local metal surface falls below it.
Possible cold locations include:
- Air-preheater cold ends
- Low-temperature economizers
- Heat-recovery tubes
- Duct walls near air leakage
- Dampers
- Expansion joints
- Stack liners
- Idle or bypassed sections
- Poorly insulated supports and transitions
The RFQ should define:
- Normal gas temperature
- Minimum gas temperature
- Normal metal temperature
- Minimum metal temperature
- Startup and shutdown conditions
- Air-leakage locations
- Duration below the acid dew point
A material test performed at the gas temperature may not represent the colder metal surface where acid actually condenses.
When Is a Nickel Alloy Usually Not Necessary?
A nickel alloy may not be justified when:
- The metal temperature remains safely above the verified acid dew point.
- Acid condensation occurs only during a controlled short-duration event.
- Deposits and air leakage are effectively controlled.
- Corrosion-resistant low-alloy steel has proven service history.
- A coating, lining or replaceable corrosion allowance provides sufficient protection.
- The component is thick, accessible and inexpensive to replace.
- The condensate chemistry is mild and supported by field data.
Material upgrading should not replace temperature control, drainage, insulation or combustion optimisation.
When Should Nickel Alloys Be Considered?
Nickel alloys become more relevant when several of the following conditions occur together:
| Condition | Why It Supports a Nickel-Alloy Review |
|---|---|
| Persistent operation below the acid dew point | Produces continuing liquid-acid exposure |
| Frequent startup and shutdown | Repeats condensation and evaporation cycles |
| Thin-wall heat exchanger tubes | Local penetration can cause early leakage |
| High chloride contamination | Increases localized-corrosion risk |
| Mixed acid condensate | Pure sulfuric-acid data may no longer apply |
| Poor drainage or crevices | Retains concentrated acid |
| Difficult access | Raises the consequence of repair |
| Long design life | Requires a lower allowable corrosion rate |
| Previous steel or stainless failure | Provides evidence that a larger material margin is needed |
| Condensate chemistry varies | Requires resistance across a wider operating envelope |
The final decision should compare nickel alloy, corrosion-resistant steel, lining, cladding and operating control—not only different nickel grades.
Which Nickel Alloys May Be Screened?
The following table provides starting candidates only.
| Alloy | Possible Starting Role | Important Limitation |
|---|---|---|
| Alloy 825 / UNS N08825 | Sulfuric-acid-dominant condensate, including selected mixed-acid conditions | Chlorides, temperature and oxidizing contaminants still require testing |
| Alloy 20 / UNS N08020 | Controlled sulfuric-acid service supported by corrosion data | Its operating window and current product specification must be confirmed |
| Alloy 625 / UNS N06625 | Tubes or components requiring strength, fabrication and broad corrosion resistance | It is not automatically the best alloy for sulfuric acid condensate |
| Alloy C-276 / UNS N10276 | Severe reducing mixed-acid condensate and chloride-containing service | Oxidizing contaminants can change its comparative performance |
| Alloy C-22 / UNS N06022 | Mixed oxidizing and reducing acid with localized-corrosion risk | Its additional cost is not justified without a severe environment |
| High-nickel cladding or overlay | Local protection of a structural substrate | Dilution, defects, minimum thickness and transition areas must be controlled |
The Nickel Institute’s sulfuric acid alloy-selection guide shows why alloy performance must be assessed against acid concentration, temperature and contaminants together.
Why Alloy 625 Is Not the Automatic Answer
Alloy 625 offers:
- Useful mechanical strength
- Good weldability
- Availability in tube and pipe
- Broad resistance to many corrosive environments
However, acid dew-point corrosion is primarily a sulfuric-acid-condensate problem.
Alloy 625 should therefore be selected only after confirming:
- Actual acid concentration
- Metal temperature
- Chlorides
- Oxidizing contaminants
- Exposure duration
- Weld condition
- Maximum acceptable corrosion rate
A general statement such as “excellent corrosion resistance” is not sufficient evidence.
When May C-22 or C-276 Be Justified?
C-22 and C-276 are more likely to be considered when the condensate contains more than sulfuric acid alone.
Examples include:
- Sulfuric acid plus chlorides
- HCl condensation
- Ferric or cupric ions
- Wet chlorine
- Mixed oxidizing and reducing species
- Severe crevice conditions
- Variable condensate composition
C-22 often provides a stronger starting position where oxidizing contaminants and localized corrosion are important.
C-276 remains an important candidate where reducing mixed acids dominate.
Neither alloy should be selected from composition alone.
Acid Dew-Point Corrosion Is Not the Same as Wet FGD Service
A cold dry-duct surface with intermittent sulfuric acid condensation is different from a wet flue gas desulfurization scrubber.
Wet FGD equipment may contain:
- Chlorides
- Limestone or lime slurry
- Fluorides
- Solids
- Oxidants
- Low pH
- Crevices and deposits
The Nickel Institute publishes separate guidance for nickel-containing materials in FGD equipment because the material-selection problem is different.
One alloy specification should not automatically cover both locations.
What Testing Is More Representative?
Immersion Testing
ASTM G31-21(2025) can support immersion testing in a representative condensate.
The test should define:
- Sulfuric acid concentration
- Chloride and other contaminants
- Temperature
- Aeration
- Duration
- Parent material
- Weld metal
- Heat-affected zone
- Maximum localized depth
Immersion testing does not reproduce acid-vapour condensation by itself.
Controlled Condensation Testing
A stronger test exposes the specimen to sulfuric acid vapour while controlling:
- Gas composition
- Water vapour
- SO₃ or H₂SO₄ vapour
- Specimen metal temperature
- Condensation rate
- Thermal cycling
- Deposits
- Gas velocity
Research on dew-point testing has used cooled corrosion probes and controlled acid-vapour environments to compare materials under actual condensation conditions.
Field Probe or Test Section
Where failure consequences are high, a field probe or test section at the proposed location can provide stronger evidence than a generic laboratory table.
The report should record:
- Gas temperature
- Specimen temperature
- Exposure duration
- Fuel and load conditions
- Deposit chemistry
- General corrosion
- Maximum local penetration
Product Standards Do Not Prove Acid Dew-Point Resistance
Relevant seamless tube standards include:
- ASTM B423-22 for Alloy 825
- ASTM B444-23 for Alloy 625
- ASTM B622-23 for C-22, C-276 and other nickel alloys
These standards verify product requirements such as:
- Chemical composition
- Heat treatment
- Mechanical properties
- Dimensions
- Hydrostatic or nondestructive testing
- Workmanship
They do not establish:
- Local acid dew point
- Condensate concentration
- Resistance to mixed flue gas contaminants
- Expected service life
- Welded-component performance
- Suitability for the complete equipment design
Application qualification remains a separate engineering responsibility.
What Buyers Should Include in the RFQ
| RFQ Category | Required Information |
|---|---|
| Fuel | Coal, oil, biomass, waste, process gas or mixed fuel |
| Sulfur data | Fuel sulfur, SO₂ and available SO₃/H₂SO₄ measurements |
| Water vapour | Normal and maximum |
| Other species | HCl, chlorides, fluorides, NOx, ash and metal contaminants |
| Acid dew point | Calculation or measured range and method |
| Metal temperature | Normal, minimum and transient |
| Gas temperature | Normal, minimum and transient |
| Operating cycle | Continuous, load-following, startup and shutdown |
| Equipment location | Air preheater, economizer, duct, damper, stack or heat exchanger |
| Condensate | Analysis, pH and expected acid concentration |
| Deposits | Chemistry, loading and cleaning method |
| Material | Exact alloy and UNS designation |
| Product form | Tube, pipe, bar, plate, lining or overlay |
| Dimensions | OD, wall, length, tolerance and quantity |
| Welding | Joint design, filler, procedure and testing |
| Corrosion test | Immersion, condensation, field probe and acceptance |
| NDT | ECT, UT, pressure test, PT or project method |
| Documentation | MTC, heat treatment, NDT and corrosion reports |
| Design authority | Equipment designer or corrosion engineer |
A request stating only:
“Need nickel alloy tubes for low-temperature flue gas.”
does not provide enough information for a technically reliable quotation.
Frequently Asked Questions
Are nickel alloys always required below the acid dew point?
No. The decision depends on condensation duration, acid chemistry, component thickness, inspection access and the effectiveness of operating controls or linings.
Which nickel alloy is best?
There is no universal best grade. Alloy 825 may be screened for sulfuric-acid-dominant service, while C-22 or C-276 may be considered for more complex mixed-acid conditions.
Is Alloy 625 suitable?
It may be a candidate, but its strength and general corrosion reputation do not automatically make it the best sulfuric-acid-condensate alloy.
Can PREN be used to rank the alloys?
Not reliably. PREN was developed mainly for chloride pitting comparisons and does not account for sulfuric acid concentration, redox condition, condensation or gas composition.
Can the acid dew point be calculated from fuel sulfur alone?
No. SO₃ or H₂SO₄ vapour, water vapour, combustion conditions and local surface temperature also matter.
Is flue gas temperature sufficient?
No. Condensation occurs on a surface when the local metal temperature falls below the local acid dew point.
Does an ASTM tube certificate prove suitability?
No. It verifies the supplied product against the ordered standard, not its service life in acid-condensing flue gas.
What information is most important for quotation?
Provide the equipment location, gas composition, measured or calculated acid dew point, minimum metal temperature, condensate chemistry, alloy, dimensions, testing and documentation requirements.
Conclusion
Nickel alloys are most useful where acid condensation is persistent, local chemistry is severe and the consequence of local penetration is high.
The selection process should answer five questions:
- Does the component surface fall below the acid dew point?
- How long does liquid acid remain on the surface?
- What is the actual condensate composition?
- Can temperature control, drainage, lining or cleaning reduce the risk?
- Which alloy has representative test or field evidence?
Alloy 825, Alloy 625, C-22 and C-276 may each be relevant in selected conditions.
None should be approved from nickel, chromium, molybdenum or PREN alone.
For nickel-alloy tube or bar enquiries, buyers should provide the exact grade, UNS designation, product standard, dimensions, supplied condition, condensate chemistry, metal temperature, corrosion testing, NDT and documentation requirements.
Emily PIPE can review whether the requested alloy tube or bar grade, dimensions, condition, surface, testing and certification scope are technically manufacturable.
Final acid-dew-point calculation, corrosion qualification, equipment design and material approval should remain with the equipment designer and responsible materials or corrosion engineer.
