Why Corten Welding Requires Careful Filler Selection
Corten steel, also known as weathering steel, forms a protective oxide patina that gives the surface its characteristic rust-colored appearance. The alloying elements that create this patina, mainly copper, chromium, and nickel, are present in relatively small amounts, and ordinary structural steel filler metal does not contain them. If the weld metal cannot form its own protective oxide, the weld bead will corrode faster than the surrounding plate, leaving dark, recessed weld lines and premature section loss. The objective of a correct corten welding procedure is to produce weld metal that weathers at a similar rate and color to the base material.
Filler Selection for Thin Plate and Single-Pass Welds
For corten sections thinner than about 10 mm, welded with a single-pass fillet weld, a conventional mild steel filler can be acceptable in practice. With MIG welding, an ER70S-6 wire (also sold as SG2 or G3Si1) is commonly used. In a single pass, the small weld pool mixes with the base metal, and the dilution from the plate supplies enough copper and chromium to give the weld bead reasonable atmospheric corrosion resistance. This approach keeps consumables simple and cost-effective for decorative panels, edging, and light structural work.
Filler Selection for Multi-Pass and Heavy Sections
For plate over 10 mm thick, or whenever the joint is welded with multiple passes, the dilution effect is no longer sufficient and the weld metal must carry its own corrosion resistance. In this case use a filler with a composition close to the base material, typically a wire classified around ER80S-G or ER80S-W with nominally about 1% nickel and 0.5% copper. Alternatively, a 2.5% nickel steel such as ER80S-Ni2 improves toughness and corrosion resistance of the deposit. Whatever the choice, confirm the filler classification and actual chemistry against the AWS specification in use, and request the consumable certificate so the shop record matches the project requirements.
Welding Preparation and Procedure Notes
Weathering steels, especially grades with higher phosphorus such as Corten A (EN 10025-5 S355J0WP), need clean joints: remove mill scale, oil, grease, and moisture from the weld zone before starting. Use a low-hydrogen process and consumable, and keep the interpass temperature moderate to limit heat input. For sections above roughly 20-25 mm, preheating is generally recommended, and any required preheat and interpass temperature should follow the approved welding procedure specification. After welding, grinding should be limited to the weld cap only; heavy grinding removes the alloy-rich surface and creates a zone that weathers differently. Where the weld will be visible, consider matching the filler color by letting the structure weather naturally rather than painting only the weld area.
Frequently Asked Questions
Can corten steel be welded with a standard MIG machine?? Yes. Standard MIG, MAG, FCAW, and stick welding equipment is suitable. The key variables are the filler metal and the cleanliness of the joint, not the machine.
Do I need special filler for corten steel?? For thin single-pass welds on material up to about 10 mm, mild steel wire such as ER70S-6 is often sufficient because plate dilution supplies copper and chromium. For thick or multi-pass joints, use a weathering-compatible wire such as ER80S-W or ER80S-Ni2.
Why must the filler metal match the base plate?? If the weld metal cannot form its own protective oxide, the weld bead will corrode faster than the surrounding plate, leaving dark, recessed weld lines and premature section loss.
How long does corten steel take to develop its rust layer?? Outdoors, a corten object typically begins to show surface rust within a few months, and the stable patina develops over several months to a few years depending on climate and wet-dry cycling.
What joint preparation is needed before welding corten?? Remove mill scale, oil, grease, and moisture from the weld zone, use a low-hydrogen process and consumable, keep the interpass temperature moderate, and preheat sections above roughly 20-25 mm according to the approved welding procedure.
Will the weld look different from the plate?? Initially yes, but if the filler chemistry is compatible, the weld bead will oxidize with the patina and blend in over time. Avoid painting only the weld, and keep grinding light so the alloy-rich surface is preserved.
