What Is Weathering Steel (Corten)?
Weathering steel is a low-alloy steel that forms its own protective rust-like layer, called a patina, so it can be used outdoors without paint. The best-known trade name is Cor-Ten, and buyers often call the whole family “Corten steel.” Under the rust-colored surface, the alloy is engineered to resist further corrosion far better than ordinary alloy steel plate.
The patina is not a coating applied at the mill. It builds up over six to 24 months of wet and dry cycles. Once stable, it slows corrosion enough that, in the right environment, the steel can outlast painted carbon steel with no maintenance. That is why it is popular for facades, planters, bridges, railcars, containers, and outdoor structures.
The Grades Buyers See on Quotes
Corten is a brand, not a specification. Real orders are written to standards, and the grade names below are the ones that appear most often on a mill test certificate (MTC). Understanding the difference prevents paying for the wrong one.
| Standard / name | Chemistry character | Typical form | Closest equivalent | Best use |
|---|---|---|---|---|
| Cor-Ten A / ASTM A242 | Higher phosphorus, plus Cu, Cr, Ni, V | Sheet, light plate | SPA-H, S355J0WP, 09CuPCrNi-A | Architectural cladding, containers, decorative panels |
| Cor-Ten B / ASTM A588 (Gr A, B, C, K) | Lower phosphorus, Mn-Si-Cr-Ni-Cu-V | Plate, structural sections | S355J2W, Q355NH | Welded structures, bridges, thick plate |
| ASTM A606 Type 4 | Similar to Cor-Ten A | Hot and cold rolled sheet and strip | SPA-H, S355J0WP | Roofing, wall panels, thin-gauge architectural |
| ASTM A709 Grade 50W | Structural weathering, bridge grade | Bridge plate and sections | A588, S355J2W | Highway bridges |
| EN 10025-5 S355J2W / J0W / K2W | European structural weathering | Plate | A588, Q355NH | General structural, European projects |
| GB/T 4171 Q355NH / Q295NH | Chinese structural weathering | Plate | A588, S355J2W | Structural, towers, railcars |
| JIS G3125 SPA-H / SPA-C | Phosphorus-bearing weathering | Hot and cold rolled sheet | Cor-Ten A, A606 | Containers, roofing, rolling stock |
Note the split. Phosphorus grades (Cor-Ten A, A606 Type 4, SPA-H, 09CuPCrNi-A) are mainly sheet and light plate for cladding and containers, while low-phosphorus structural grades (A588, S355J2W, Q355NH, A709 50W) are used for welded structures and thick plate.
Cor-Ten A vs Cor-Ten B: The Decision That Matters
The most common buyer error is treating Cor-Ten A and Cor-Ten B as interchangeable. They are not.
- Cor-Ten A has higher phosphorus, which improves the patina but can reduce toughness and weldability in thicker sections. It is the right choice for thin sheet, architectural panels, and containers.
- Cor-Ten B (ASTM A588) has lower phosphorus and is designed to be welded into load-bearing structures. For bridge girders, thick plate, and welded assemblies, choose A588 or its EN equivalent S355J2W.
- If a project needs both the Corten look and welded structural integrity, the structural grades (A588, S355J2W, Q355NH) are the safer specification.
Where the Patina Works and Where It Fails
Weathering steel is not corrosion-proof; it is corrosion-retarding. The patina only stabilizes with alternating wet and dry cycles. Confirm the service environment before you commit, the same way you would compare galvanized and stainless steel by environment.
- Good: open atmosphere with rain and sun, architectural facades, bridge structures with good drainage.
- Bad: continuously wet areas, buried sections, water traps, low points that hold moisture, and details that trap debris.
- Marine caution: in heavy salt spray the patina may not stabilize and can perform little better than plain carbon steel. For true salt-water exposure, switch to a marine stainless grade.
What to Specify Before You Order
Because “Corten” is used loosely, lock the specification in writing. Confirm these points on the purchase order and on the MTC:
- Standard and grade (for example ASTM A588 Gr B or EN 10025-5 S355J2W), not just “Corten.” To cross-reference grades safely, compare ASTM, DIN, and JIS steel standards carefully and avoid a wrong substitution.
- Thickness and tolerance. Weathering plate is rolled to standard plate tolerances; verify plate thickness tolerance the way you would for any structural plate.
- Heat number, chemistry (Cu, Cr, Ni, P, V), and mechanical properties on the MTC.
- Surface condition: as-rolled, pickled, or pre-rusted with an accelerated patina. A pre-weathered finish costs more but gives a uniform color on day one.
- Dimensions and weight for freight. Use the steel weight calculation guide to estimate payload.
Welding and Fabrication Notes
Weathering steel is weldable, but the filler and preheat must match the grade. For A588 and S355J2W use a low-hydrogen process; to keep the joint weathering-matched, choose filler with similar chemistry rather than a standard carbon-steel rod. Preheat thick sections to avoid cracking. Expect the weld to weather at a slightly different rate than the base metal and to even out over time.
Inspection and Common Mistakes
Before release, run a pre-shipment inspection that checks dimensions, surface, MTC chemistry, and that the supplied grade matches the PO. The four mistakes buyers repeat:
- Ordering “Corten” without naming the standard, then receiving whatever the mill had in stock.
- Using a phosphorus grade (Cor-Ten A) for a welded structural application.
- Detailing water traps into the design and blaming the steel when rust streaks appear.
- Expecting a uniform patina immediately. Color stabilizes over months and varies with local climate.
Cost, Lead Time, and Next Step
Weathering steel sits between carbon structural steel and stainless in price. Against painted carbon steel it often wins on life-cycle cost because there is no repainting. Lead time and minimum order depend on thickness and pattern.
If you are sourcing A588, S355J2W, Q355NH, or A606 sheet, use our quote spec template (grade, size, quantity, incoterm) and request a quote. We will confirm availability, MTC chemistry, and lead time against your specification.