The patina layer on weathering steel plates will affect welding quality if proper pre-welding preparation is not done. The patina is a dense oxide layer (mainly composed of iron oxides, along with alloy oxides of Cu, Cr, Ni) that creates several challenges during the welding process.
Porosity and Cracks in Weld Seams
The oxide layer on the steel surface contains moisture and gas trapped in its structure. During welding, high temperatures cause these oxides to decompose and release gas (e.g., oxygen, water vapor), which gets trapped in the molten weld metal and forms
porosity (small holes in the weld seam). This weakens the weld's mechanical strength and corrosion resistance.
In severe cases, the oxides can also cause
cold cracks or
hot cracks in the weld zone, as they disrupt the uniform crystallization of the molten metal.
Reduced Weld Adhesion
The patina layer acts as a physical barrier between the welding electrode and the steel substrate, preventing the molten metal from fully wetting and bonding with the base material. This leads to
incomplete fusion at the weld interface, which is a critical defect that can cause the weld to fail under stress.
Contamination of Weld Metal
Alloy elements in the patina (e.g., Cu, Cr oxides) can mix into the molten weld pool, altering the chemical composition of the weld metal. This may reduce the weld's corrosion resistance (a key property of weathering steel) and make the weld zone more prone to rusting than the base material.
To ensure high-quality welding, the patina layer must be removed from the welding area before operation. Follow these steps:
Define the Cleaning Area
Remove patina from a
20–50 mm wide zone on both sides of the welding joint (including the groove surface if a bevel is required). This ensures the electrode can directly contact the clean steel substrate.
Effective Cleaning Methods
Mechanical Cleaning: Use a wire brush, angle grinder with a grinding wheel, or sandblasting to remove the patina. This is the most common method-ensure the cleaned surface shows bright, shiny steel (no residual oxide or rust).
Chemical Cleaning: For small or precision welding areas, use a rust remover (e.g., phosphoric acid-based solutions) to dissolve the patina, then rinse thoroughly with water and dry completely to avoid re-rusting before welding. Note: Avoid acidic cleaners that leave residues, as they can also contaminate the weld.
Post-Cleaning Protection
Clean the welding area immediately before welding (within 4–8 hours) to prevent the formation of new light rust. If there is a delay, cover the cleaned area with a protective film or anti-rust oil (remove the oil before welding).
Choose Matching Welding Materials: Use welding electrodes or wires specifically designed for weathering steel (e.g., E7018-C1 for manual arc welding, ER80S-G for gas metal arc welding). These materials contain the same alloy elements (Cu, Cr, Ni) as the base steel, ensuring the weld zone has equivalent corrosion resistance.
Control Welding Parameters: Use low heat input and short arc length to reduce the oxidation of the weld pool. Avoid overheating the base material, as this can damage the patina layer in areas outside the cleaning zone.
Post-Weld Treatment: After welding, grind the weld seam to smooth it, then apply a patina accelerator to the weld area. This helps the weld zone form a uniform patina layer that matches the surrounding base steel, restoring the overall aesthetic and corrosion resistance.
