In industrial flue system, the expansion joint is exposed to high temperature, high humidity, harsh flue gas environment containing acid and sulfur for a long time, and corrosion problem has always been the main factor affecting its service life. So, how to prevent the flue expansion joint from corrosion? This is a common concern of operation and maintenance personnel in many power plants, steel mills and chemical enterprises. Effective anti-corrosion treatment can not only prolong the replacement period of expansion joint, but also avoid environmental protection exceeding standard and safety accidents caused by corrosion leakage. This paper will systematically introduce the corrosion mechanism, anti-corrosion method and construction technology of flue expansion joint, and provide practical guidance for field technicians.
First, why should the flue expansion joint be prevented from corrosion?
To understand how to prevent corrosion of flue expansion joints, first of all, we need to clarify the hazards of corrosion. The types of corrosion faced by flue expansion joints mainly include:
- Acid dew point corrosion: SO₃ in flue gas combines with water vapor to form sulfuric acid, which condenses when the temperature is lower than the acid dew point, causing strong corrosion to the metal expansion joint
- Pitting and crevice corrosion: Halogen ions such as chloride ions form local corrosion pits on the surface of stainless steel
- Stress corrosion cracking: brittle cracking of metal occurs under the combined action of tensile stress and corrosive medium
- High-temperature oxidation: When exceeding 400℃, the oxide scale on the surface of ordinary stainless steel peels off
- Aging of non-metallic materials: hydrolysis, cracking, swelling of rubber or fabric layers
Expansion joints without anti-corrosion treatment may leak in just a few months under corrosive working conditions, which seriously affects the system operation. Therefore, it is of great engineering value to master how to prevent corrosion of flue expansion joint.
2. Anticorrosion method of metal expansion joint
Metal expansion joints are the most common types, and their anti-corrosion measures can be divided into two categories: material selection and surface protection.
Material selection (preservation from source)
| Operating condition | RECOMMENDED MATERIAL | Corrosion resistance |
|---|---|---|
| General acidic flue gas (temperature ≤200℃) | Stainless steel 304 | Foundation corrosion resistance |
| Wet flue gas containing SO₂ and SO₃ | Stainless steel 316L | Contains molybdenum and has excellent acid corrosion resistance |
| High chloride environment (e.g. waste incineration) | 254SMO, C276 Hastelloy | Extremely strong pitting resistance |
| High temperature oxidizing flue gas (> 400℃) | 309S, 310S | High temperature oxidation resistance |
| Seawater flue gas (coastal power plant) | Super Duplex Stainless Steel | Resistance to chloride ion stress corrosion |
Surface coating protection
When the substrate has been identified or requires additional protection, coating corrosion protection can be applied:
1. Glass flake coating
- Features: Contains glass scales, forming a labyrinth effect, hindering the penetration of corrosive media
- Thickness: 1.5~3mm
- Applicable temperature: ≤150℃
- Application Method: Spray or Hand
2. High temperature acid-resistant coatings
- Features: Silicone modified resin, can withstand 200~300℃
- Applicable working conditions: medium temperature flue expansion joint
- Application method: Air spraying or brushing
3. Enamel (enamel) coating
- Features: Inorganic vitreous coating, high hardness, excellent acid resistance
- Applicable temperature: ≤450℃
- Construction method: High temperature sintering after dipping or spraying
4. Thermal Spray Metal Coating
- Features: Arc or flame sprayed aluminum, zinc or their alloys
- Function: Cathodic protection + physical shielding
- Applicable working conditions: Low-cost anti-corrosion scheme for carbon steel expansion joint
3. Anti-corrosion design of non-metallic expansion joint
Non-metallic expansion joints themselves are manufactured with corrosion-resistant materials, but their anti-corrosion emphasis lies in structural design and material selection.
Material selection principle
| Corrosive media | Recommended flexible materials | Materials not recommended |
|---|---|---|
| SO₂-containing wet flue gas (pH 2~5) | Fluororubber, PTFE | Neoprene rubber |
| Strong oxidizing flue gas containing NOx | PTFE | EPDM rubber |
| Oily flue gas | fluororubber | Silicone rubber |
| Chlorine-containing flue gas (HCl) | PTFE, fluororubber | Ordinary rubber |
Structural anti-corrosion design
How to prevent corrosion of flue expansion joints? Among non-metallic types, structural design is crucial:
- The stainless steel guide tube extends to the inside of the expansion joint to prevent smoke from directly washing and soaking the flexible fabric layer
- Add drainage holes: Open drainage holes at the lowest part of the expansion joint to avoid condensate accumulation
- Thermal insulation layer protection: the inner ceramic fiber layer can reduce the outer surface temperature and reduce the outer wall condensation corrosion
- Flange seal optimization: Acid-resistant rubber gasket is used, and sealant is applied to prevent gap corrosion
4. Anticorrosion construction technology of flue expansion joint
The following is the most commonly used anti-corrosion construction process when making or repairing expansion joints on site, and answers the practical question of "how to prevent corrosion of flue expansion joints" for you.
Step 1: Surface Treatment (Rust Removal)
The anti-corrosion effect is 70% dependent on the quality of the surface treatment:
- Sandblasting treatment: reach the standard of Sa2.5, the surface is gray-white metallic luster, and the roughness is 40~70 μ m
- For small area repair: Angle grinder with wire brush can be used to grind to St3 grade
- Oil removal: Wipe with solvent to remove oil and salt
Step 2: Apply Primer
Primer selection according to working conditions:
- Epoxy zinc-rich primer (Zn content ≥70%): suitable for carbon steel expansion joints
- High temperature primer (e.g. inorganic zinc silicate): suitable for medium to high temperature expansion joints
- Coating thickness: 40~60 μ m
Step 3: Intermediate coating and top coating construction
- Intermediate coating: epoxy cloud iron intermediate paint, thickness 80~120 μ m, enhanced shielding effect
- Top coat: Weather or acid resistant top coat, thickness 50~70 μ m
- Total dry film thickness: ≥200 μ m in general working conditions, ≥300 μ m in severe corrosion conditions
Step 4: Quality inspection
- The wet film thickness gauge measures the thickness of each coating
- EDM leak detector detects pinhole (voltage 2000V, no alarm is qualified)
- Adhesion test: cross-grid or pull-apart
V. Anti-corrosion strategies under different working conditions
| Type of operating condition | Typical industries | Recommendation of anti-corrosion scheme |
|---|---|---|
| High temperature and low corrosion (300~500℃, low sulfur content) | Steel hot air duct | Material upgrade (310S) + uncoated or high temperature paint |
| Medium temperature acidic (120~200℃, containing SO₂) | Original flue of power plant | Material 316L + glass flake coating (optional) |
| Low temperature and high humidity (50~80℃, pH 2~3) | Clean flue after desulfurization | Non-metallic expansion joints (fluororubber) are preferred |
| High temperature with chlorine (> 200℃, with HCl) | Waste incineration | Lined with PTFE metal expansion joint or C276 alloy |
| Abrasion + Corrosion | Catalytic cracking flue gas | Wear-resistant lining + heat-resistant stainless steel double protection |
VI. Common causes and prevention of anti-corrosion failure
Even if you know how to prevent the flue expansion joint from corrosion, sometimes early failure still occurs. The main reasons include:
| Failure phenomenon | Root cause | Precautionary measures |
|---|---|---|
| Coating blistering and shedding | Surface treatment is not thorough, leaving oil or rust | Strictly blasted to grade Sa2.5, immediate coating |
| Coating pinhole corrosion | Too thin or missing coating | Increase the number of coatings, electric spark detection |
| Preferential corrosion at weld | Microstructure change in welding heat-affected zone | Passivation treatment or zinc-rich primer after welding |
| Corrosion of fluid accumulation in trough of expansion joint | No drainage hole | Open drainage holes and clean them regularly |
| Crack of outer layer of non-metallic expansion joint | UV or Ozone Aging | Install protective cover and apply protective oil regularly |
Anticorrosion during maintenance period and extension of service life
In addition to the corrosion protection of newly made expansion joints, maintenance and corrosion protection during operation are equally important:
- Regular cleaning: Remove dust, especially corrosive dust, from the surface of the expansion joint
- Repairing: If the coating is found to be scratched or peeled off, polish and repaint in time
- Drain hole inspection: dredge once a month to prevent condensate accumulation
- Bolt anti-corrosion: Apply high-temperature anti-bite agent to flange bolts to prevent rust death
- Record corrosion rate: measure expansion joint wall thickness annually to assess corrosion trend
VIII. Summary
How to prevent corrosion of flue expansion joint needs to be systematically answered from four levels: material selection, structural design, surface treatment and operation and maintenance. The core conclusions are as follows:
- Material selection is the basis: select suitable metal or non-metal materials according to flue gas temperature, acid dew point and chloride ion concentration. 316L stainless steel is suitable for most acidic wet flue gas, and fluororubber non-metallic expansion joint is the preferred flue after desulfurization
- The coating is a barrier: for metal expansion joints, sandblasting + epoxy zinc-rich primer + epoxy cloud iron mid-coat + acid-resistant top coat with a total thickness ≥200 μ m can significantly extend the service life
- Structural design is key: deflectors prevent direct flushing and drainage holes avoid accumulation of fluid — both designs are more effective than any coating
- Construction quality determines success or failure: Surface treatment should reach Sa2.5 grade, temperature and humidity should be controlled in coating process, and EDM pinhole inspection should not be omitted
- Maintenance is the guarantee: Regular cleaning, repainting and dredging of drainage holes can prolong the service life of expansion joints by 30% ~50%
Through the systematic anti-corrosion scheme, the service life of metal expansion joints can be extended from 1~2 years to 5~8 years, and that of non-metal expansion joints can reach 6~10 years in acidic wet flue gas conditions. For operation and maintenance personnel, mastering "how to prevent corrosion of flue expansion joints" is not only a technical ability, but also an important means to reduce spare parts costs and unplanned downtime.