In wet desulfurization system and chemical flue gas pipeline, how to repair the corrosion of flue gas expansion joint is the most common thorny problem encountered by operation and maintenance personnel. Whether it is the pitting and perforation of metal bellows or the corrosion and damage of non-metal expansion joint frame, if it can't be treated in time and effectively, it will lead to leakage expansion, system shutdown and even safety accidents. This paper will systematically explain how to repair the corrosion of flue gas expansion joint from corrosion type diagnosis, comparison of repair schemes to practical steps, and provide an operable technical guide for field technicians.
1. Corrosion type diagnosis: the first step before repair
The type, location and severity of flue gas expansion joint corrosion must be accurately judged before any repair operation is performed.
1. Corrosion types of metal bellows
| Type of corrosion | Appearance characteristics | Common sites | reason |
|---|---|---|---|
| Pitting corrosion | Pinhole-shaped pit, gradually expanding into a perforation | Heat Affected Zone of Bellows Wave Trough and Weld | Cl⁻ion destruction of passivation film |
| Stress corrosion cracking | Dendritic crack, transgranular or intergranular propagation | Bellows crest, cold working area | Tensile stress + corrosive medium |
| uniform corrosion | Full surface thinning, loss of metallic luster | Corrugated pipe integral | Long-term attack of acidic smoke |
| Crevice corrosion | Flange gasket, dust accumulation part | Connection between flange sealing surface and guide tube | Hypoxic environment + chloride ion concentration |
2. Non-metallic expansion joint corrosion
The "corrosion" of non-metallic expansion joints is mainly manifested as:
- Aging brittleness of skin fabric layer
- Swelling or degradation of fluororubber/silicone rubber layer
- Metal frame corrosion perforation
3. Corrosion Severity Classification
| grade | Phenomenon | Urgency | Recommended patching method |
|---|---|---|---|
| Level 1 | Slight pitting, unpenetrated | Plannable processing | Surface Passivation + Coating Protection |
| Secondary | Perforation leakage (hole diameter ≤3mm) | Higher | Metal cement leak plugging + reinforcement |
| three-level | Multiple cracks or large corrosion perforations | Emergency | Local excavation or overall replacement |
II. Scheme 1: Pitting corrosion repair of metal bellows (unperforated)
It is suitable for corrosion treatment of flue gas expansion joint with corrosion grade one.
Material Preparation
- Stainless steel acid pickling passivation paste
- Passivation test solution
- Stainless steel repair agent (polymer alloy type)
- Grinding tool (angle grinder + hundred impeller)
Operation Procedures
- Surface Cleaning: Use an angle grinder to sand corroded areas to remove corrosion products and discolored oxide layers, exposing metallic luster
- Pickling passivation: Apply pickling passivation paste, leave for 15-30 minutes, rinse with clean water
- Passivation test: Add passivation test solution dropwise and observe the color change (blue means that the passivation film is intact, red needs to be reprocessed)
- Repair agent construction: Add curing agent to stainless steel repair agent, stir evenly and apply to corroded area with a thickness of 1-2mm
- Curing maintenance: Stand and cure according to product instructions (usually 24 hours)
Effects and Cycles
- Can extend service life by 1-2 years
- Suitable for pitting depth
3. Scheme 2: Perforation and plugging of metal bellows (already penetrated)
It is suitable for corrosion repair of flue gas expansion joint with corrosion grade two.
Material Preparation
- High temperature plugging cement (temperature resistance ≥300℃)
- Stainless steel patch (same material)
- Stainless steel cable ties or clamps
- High temperature resistant sealant
Operation Procedures
- Hole reaming cleaning: Use a round file to enlarge the perforated area to a regular circular shape, and clean the corrosion around the hole
- Base surface treatment: Polish the perimeter of the hole in the range of 20mm to expose the metallic luster
- Plugging cement construction: After kneading the cement evenly, fill the perforations from the inside to the outside to form a mushroom head shape
- Patch covering: Apply high temperature sealant to the back of the stainless steel patch and cover the perforation
- Cable tie fixing: Fasten the patch to the bellows with stainless steel cable ties
- Cure Check: Check the tightness with soapy water after curing
Applicable restrictions
- Perforation diameter ≤5mm
- Working pressure ≤10kPa
- Suitable for temporary repair, should be replaced during planned maintenance
4. Scheme 3: Corrosion repair of metal frame of non-metallic expansion joint
Metal frame corrosion of non-metallic expansion joints is a common problem of flue gas expansion joint corrosion, and frame corrosion will directly affect skin fixation and overall sealability.
Material Preparation
- Angle grinder, wire brush
- Epoxy primer
- Glass flake cement (vinyl ester resin)
- Glass fiber cloth
- Stainless steel reinforcement plate (if necessary)
Operation Procedures
- Rust removal: Treat the corroded area of the frame to Sa2.5 by angle grinder or sandblasting
- Primer painting: Apply one layer of epoxy resin primer, dry film thickness ≥50 μ m
- Cement construction: scrape glass flake cement, single thickness 1-1.5mm, total construction 2-3 layers, total thickness ≥2mm
- Glass fiber reinforcement: Laying glass fiber cloth between cement layers to enhance crack resistance
- Local reinforcement: For parts with severe corrosion thinning, stainless steel reinforcing plate is welded on the back
- Curing: Curing at room temperature for 24-48 hours
Effects and Cycles
- Can be continued for 2-3 years after recoating
- Suitable for local corrosion of the frame, not for overall severe corrosion
V. Emergency temporary repair (non-stop processing)
In an emergency situation where the machine cannot be stopped, the following emergency schemes can be used to deal with flue gas expansion joint corrosion leakage.
Material Preparation
- High temperature resistant leak plugging glue stick (instant curing type)
- Stainless steel belt or throat hoop
- Acid-resistant rubber sheet
Operation Procedures
- Quick Surface Cleaning: Use a wire brush to remove floating rust around leakage points
- Glue stick sealing: After kneading and softening the leak-plugging glue stick, press it firmly at the leakage point
- Rubber mat covering: Cover acid-resistant rubber sheet over the cement
- Clamp fastening: Clamp the rubber plate to the expansion joint with stainless steel belt or throat hoop
Precautions
- Only suitable for low pressure (≤5kPa) small leakage
- Temporary measures are valid for 1-4 weeks
- Must be permanently repaired at the latest scheduled downtime
VI. Preventive Measures and Maintenance Suggestions
After the corrosion repair of flue gas expansion joint is completed, the following measures can effectively delay the re-corrosion:
1. Improvement of operating environment
- Control flue gas temperature not lower than acid dew point (usually> 80℃) to reduce condensate generation
- Purge the flue with nitrogen in time after shutdown to replace the wet flue
- Periodic neutralization with alkali wash to remove deposited acidic substance
2. Material upgrade
- Repeatedly corroded parts, upgrade the material when replaced next time:
- 304 → 316L → 2205 → Titanium
- Ordinary silicone rubber → fluorine rubber
3. Structural Optimization
- The horizontal expansion joint is provided with drainage holes to discharge the condensate in time
- Install a guide tube to prevent smoke from directly washing the bellows
- Improve heat preservation and reduce local condensation
4. Regular inspections
- Check the surface of the expansion joint monthly for signs of corrosion
- Check the acidity of the liquid discharged from the drain hole quarterly with a pH test strip
- Wall thickness sampling (metal expansion joint) is conducted once a year
sum up
How to repair the corrosion of flue gas expansion joint should choose the most suitable scheme according to the corrosion type, severity and working conditions:
- Unperforated pitting: sanding + pickling passivation + polymer repair agent, can extend the life by 1-2 years
- Small perforation leakage: plugging cement + patch + cable tie fixation, suitable for temporary emergency response
- Frame corrosion: Rust removal + glass flake heavy anti-corrosion coating to restore anti-corrosion ability
- Serious corrosion: the whole material should be replaced and upgraded, and repeated repairs should not be made
Repair is only a cure for the symptoms, and the root cause lies in:
- Improve the operating environment and reduce condensate generation
- Upgraded corrosion-resistant materials to solve problems at source
- Standardizing drainage design to eliminate accumulation conditions
- Establish a regular inspection system for early detection and early treatment
If you are facing the trouble of corrosion leakage of flue gas expansion joint, it is recommended to choose an appropriate repair plan according to the severity of corrosion, and plan to shut down and replace it in the near future. For the parts with repeated corrosion, professional organizations should be entrusted to carry out material upgrading design to thoroughly solve the problems.