1. Pre-Welding: Select Matching Consumables and Clean the Base Metal
Choose weathering-specific welding consumables: Never use ordinary carbon steel consumables (e.g., E6013 for SMAW). Select options with equivalent Cu, Cr, and P content to the base metal:
Shielded Metal Arc Welding (SMAW): Use E9018-GW electrodes (or JIS Z 3211 D11-T9Nb) – these contain ≥0.20% Cu, 0.30–0.80% Cr, and 0.03–0.07% P, matching SMA570W's weathering elements.
Gas Metal Arc Welding (GMAW): Opt for ER90S-GW wires to ensure the weld metal forms the same protective patina as the base metal.
Strictly clean the base metal surface: Remove all contaminants from the welding area (at least 20mm on both sides of the joint) using sandblasting, stainless steel wire brushes, or acetone. Focus on:
Rust, oxide scale, or oil: These cause weld porosity and prevent the weld metal from bonding with the base metal, disrupting patina continuity.
Salts or acidic residues (common in coastal storage): These corrode the surface alloying elements (Cu, Cr) and destroy the patina's formation foundation.
2. Welding Process: Control Heat Input to Avoid HAZ Degradation
Limit heat input strictly: For SMAW, use a current of 140–180A (for 4.0mm electrodes) and a travel speed of 15–25cm/min, keeping heat input between 25–35 kJ/cm (never exceed 40 kJ/cm).
High heat input dilutes Cu/Cr in the HAZ, making it more prone to rusting than the base metal.
Avoid excessive weaving: Keep the electrode's side-to-side movement ≤3 times the electrode diameter. Wide weaving expands the HAZ, increasing the area at risk of weathering performance loss.
Maintain interpass temperature: Keep the joint temperature between 120–180°C during multi-pass welding. Cooling below 80°C can cause cold cracking, and repairing cracks later disrupts the surface integrity needed for uniform patina.
3. Post-Welding: Clean and Protect the Surface, Avoid Improper Treatment
Remove weld residues without damaging the surface:
Use a stainless steel wire brush or fine-grit sandpaper (≥120 grit) to remove flux slag and spatter. Avoid carbon steel brushes – they leave iron particles that cause "rust spots" on the surface.
Wipe the entire welded area with a cloth dipped in neutral detergent (pH 6–8) to remove remaining flux, then rinse with clean water. Never use acidic cleaners (e.g., hydrochloric acid) – they etch the surface and dissolve Cu/Cr.
Minimize unnecessary grinding: Only grind to remove surface defects (e.g., shallow cracks), not for cosmetic purposes. Over-grinding exposes fresh metal with low weathering element content, leading to localized rust that spreads and breaks the patina's continuity.
Touch up with zinc-rich primer (if needed): If the weld or HAZ shows early rust (orange spots) after cleaning, apply a thin layer of zinc-rich primer (zinc content ≥90%). This provides temporary protection until the natural patina forms, without blocking long-term patina development.
4. Inspection: Verify Patina Uniformity and Weathering Consistency
Visual inspection: Check that the weld and HAZ have a smooth, uniform surface (no deep scratches, pits, or residual slag). These defects act as "starting points" for corrosion.
Salt spray testing (for critical projects): For structures in harsh environments (e.g., coastal bridges), conduct a salt spray test (per JIS Z 2371) on the weld metal. The annual corrosion rate should match the base metal (≤0.03 mm/year) – a higher rate indicates mismatched consumables or improper processing.
