To restore the weather resistance of the Heat-Affected Zone (HAZ) in welded weathering steel (e.g., Q460NH, Corten), follow these technical steps:
1. Post-Weld Grinding
Purpose: Remove the HAZ's metallurgically altered layer.
Method:
Grind weld seams and HAZ to a depth of 0.5–1 mm (until uniform base metal appears).
Use 120-grit abrasive discs to achieve a surface roughness (Ra) of ≤3.2 μm.
Effect: Eliminates microstructural heterogeneity that disrupts patina formation.
2. Chemical Patina Acceleration
Solution: Apply 5% copper sulfate (CuSO₄) + 1% phosphoric acid (H₃PO₄) to the HAZ.
Procedure:
Clean the surface with acetone.
Spray the solution evenly.
Rinse after 30 minutes and expose to air.
Result: Forces rapid oxidation, forming a protective FeOOH/Cu-rich layer within 48 hours.
3. Localized Coating
For harsh environments (coastal/industrial):
Apply zinc-rich primer (e.g., ZRC Cold Galvanizing Compound) to the HAZ.
Thickness: 50–80 μm.
Rationale: Zinc sacrificially protects steel until the patina naturally reforms (6–12 months).
4. Thermal Stabilization
Process: Post-weld localized heat treatment at 250–300°C for 1 hour.
Mechanism:
Reduces residual stresses.
Promotes Cu/Cr diffusion to enhance HAZ's corrosion resistance.
5. Verification Testing
Salt Spray Test (ASTM B117):
Acceptable HAZ corrosion rate: ≤0.1 mm/year (vs. base metal's 0.05 mm/year).
Adhesion Test (ISO 4624):
Patina layer should withstand ≥3 MPa pull-off stress.
