weathering flat steel needs to be derusted before welding-surface rust, mill scale, and contaminants will directly impair weld quality, corrosion resistance, and structural strength of the joint.
Key Reasons for Derusting
Avoid Weld Defects
Rust (mainly iron oxide) and mill scale decompose into oxygen and iron during welding, creating porosity, slag inclusions, or blowholes in the weld pool. These defects weaken the joint and reduce its load-bearing capacity, which is critical for structural applications.
Ensure Weld Fusion
Contaminants like rust and oil act as a barrier between the base metal and welding electrode, preventing full fusion at the joint interface. This leads to cold laps or incomplete penetration, which can cause the weld to crack under stress.
Preserve Corrosion Resistance
Weathering steel's anti-corrosion performance relies on uniform distribution of alloy elements (Cu, Cr, Ni). If rust or scale remains in the weld zone, the alloy composition of the joint will be uneven, making it a weak point for future corrosion-even if the base metal forms a protective patina.
Recommended Derusting Methods for Welding Prep
Mechanical Cleaning: Use a wire brush (stainless steel bristles), angle grinder with a sanding disc, or sandblasting (80–120 mesh grit, 0.2–0.4 MPa pressure) to remove rust, scale, and oil from the welding zone (at least 20–30 mm on both sides of the joint line). Target surface roughness: Ra 3–5 μm for optimal weld adhesion.
Chemical Degreasing: Wipe the cleaned area with isopropyl alcohol or a neutral degreaser to remove oil residues from handling or processing-oil can also cause weld porosity.
Note
For weathering steel, only remove loose surface rust and mill scale-do not over-grind the base metal, as this will strip the thin oxide layer that helps initiate patina formation post-welding.
