1. Pre-Treatment: Thorough Surface Cleaning (Core Foundation)
Primary cleaning: Degreasing and derusting
For oil-stained surfaces: Use water-based alkaline degreaser (pH 10-12) for spray/soak cleaning (temperature 40-60℃, time 15-30min), then rinse with clean water and air-dry to remove oil, grease, and organic pollutants; avoid solvent degreasers to prevent residual organic matter.
For rust/mill scale: Adopt SA2.5 commercial sandblasting (the most suitable grade for Q355NHB) with steel grit/quartz sand as abrasive; the surface roughness after sandblasting should reach Ra 3.0-5.0 μm (optimal for patina adhesion). For small components or complex structures, use pickling passivation (10-15% dilute hydrochloric acid + corrosion inhibitor) if sandblasting is unavailable, then neutralize with 5% sodium bicarbonate solution and rinse thoroughly.
Secondary cleaning: Residue removal and drying
After sandblasting/pickling, remove abrasive dust, acid residue and surface moisture with dry compressed air (oil-free and water-free); ensure the substrate surface is completely dry (moisture content ≤5%) within 4 hours to avoid flash rust (critical in high-humidity coastal environments).
2. Key Treatment: Accelerated Uniform Patina Formation (Core Anti-Corrosion Step)
Accelerator selection: Use weathering steel special inorganic silicate-based accelerator (main components: iron salt, manganese salt, silicate, corrosion inhibitor) - avoid chloride-containing accelerators that cause pitting corrosion.
Construction method: Adopt airless spray coating (best adhesion) or roller coating for flat surfaces; the coating thickness is controlled at 30-50 μm, and spray evenly without missing coating or accumulation. For complex structural parts (e.g., welds, corners), use a brush to touch up to ensure full coverage.
Curing conditions: Place the treated steel in natural outdoor environment (temperature 5-35℃, humidity 40-80%); keep the surface slightly moist in the first 3 days (spray clean water 1-2 times a day) to promote the reaction between the accelerator and the substrate, and avoid direct rain washing or strong sunlight exposure.
Qualification standard: The surface forms a uniform, compact reddish-brown patina with no loose yellow rust, no pinholes, and the rust layer adheres firmly (pass the tape peel test: no rust layer falling off).
3. Post-Treatment: Sealed Protection (Lock Patina & Resist Salt Spray)
Sealer selection: Use weathering steel special water-based polyurethane breathable sealer or fluorocarbon sealer (solids content ≥30%); the sealer must have high chloride ion resistance, water repellency and UV resistance, and good compatibility with the patina layer (no peeling or bubbling).
Construction process: After the patina is fully stabilized (2-4 weeks after accelerator treatment), polish the surface slightly with 200-mesh sandpaper to remove loose rust particles, then clean and dry; apply 1-2 coats of sealer by spray/roller coating, with the total dry film thickness controlled at 40-60 μm (the second coat is applied after the first coat is 80% dry, interval 2-4 hours).
Curing requirement: Cure at room temperature for 7 days (full curing); avoid contact with water or salt spray during curing to ensure the sealer forms a continuous film.
4. Special Treatment for Weak Parts (Local Reinforcement)
Welds/cutting edges: After grinding and deburring (grind to R≥2mm rounded edge), reapply a thin layer of patina accelerator (10-20 μm) and an extra coat of sealer (dry film thickness +20 μm) to reinforce protection.
Bolted connections: Before assembly, apply anti-corrosion paste (copper grease or inorganic anti-corrosion paste) on the contact surface, and add a rubber or glass fiber gasket to prevent crevice corrosion; after assembly, seal the bolt head and nut with sealer to block salt spray infiltration.
Holes/notches: Use a brush to apply accelerator and sealer for full coverage, and avoid sharp angles to prevent moisture accumulation.
5. Post-Construction Maintenance & Local Repair (Long-Term Corrosion Resistance)
Regular maintenance: Conduct quarterly surface inspection and annual comprehensive maintenance; remove surface dust, industrial dust and salt deposits with clean water (low-pressure spray) every quarter, and avoid using hard brushes to scratch the sealer/patina layer.
Local repair: For damaged parts (e.g., sealer peeling, patina falling off, scratch caused by external force), follow the steps: ① Grind the damaged area to expose the clean substrate; ② Reapply patina accelerator (if the substrate is exposed); ③ Touch up with sealer to restore the protective layer, ensuring the repair area is flush with the surrounding surface and no obvious joints.
Sealer reapplication: Reapply a thin coat of sealer (20-30 μm) every 2-3 years according to the surface condition (e.g., sealer aging, gloss loss); this can effectively extend the service life of the protective layer in coastal industrial environments.
6. Adjustment of Treatment Process for Different Coastal Industrial Zones
Increase the sandblasting grade to SA3 white metal sandblasting for a cleaner substrate surface.
Increase the sealer dry film thickness to 80-100 μm (2-3 coats) and add a layer of anti-chloride ion primer between the patina and sealer.
Shorten the sealer reapplication cycle to 1-2 years and conduct monthly surface inspections.
