What Corten Steel Is in Standard Terms
Corten steel is the trade name that the market uses for atmospheric corrosion resistant steel, a low-alloy structural steel designed so that the rust layer on the surface becomes a protective film instead of a continuing source of metal loss. The mechanism is chemical rather than mechanical. Small additions of copper, phosphorus, chromium and nickel change the composition and density of the corrosion product, and the resulting film slows the transport of oxygen and moisture to the metal underneath. On ordinary carbon steel the same exposure leaves a porous, cracked rust layer that offers almost no barrier at all.
Under GB/T 4171-2008 the high weathering resistant structural grades carry the GNH suffix, for example Q295GNH, Q355GNH and Q460GNH, and are intended for bolted or riveted exposed structures with limited welding. GB/T 4172-2008 covers weldable weathering structural steel with the NH suffix, including Q235NH, Q295NH, Q355NH and Q460NH, where phosphorus is held low enough for reliable fusion welding. Outside China the same family appears as SPA-H in JIS G3125, as Grades A, B, K and C in ASTM A588/A588M, as Types 2 and 4 in ASTM A606/A606M, and as the W grades from S235J0W to S355K2W in EN 10025-5.
Typical Chemical Composition of Weathering Grades
The table below gives composition ranges in mass percent, following the ranges published in the standards and in mill data sheets for these grades.
| Grade and standard | C max | Si | Mn | P max | S max | Cu | Cr | Ni max |
|---|---|---|---|---|---|---|---|---|
| Q355GNH, GB/T 4171-2008 | 0.12 | 0.25-0.75 | 0.20-0.60 | 0.15 | 0.035 | 0.25-0.55 | 0.30-1.25 | 0.65 |
| Q355NH, GB/T 4172-2008 | 0.16 | 0.50 | 0.50-1.50 | 0.035 | 0.035 | 0.20-0.55 | 0.30-1.25 | 0.65 |
| SPA-H, JIS G3125 | 0.12 | 0.75 | 0.60 | 0.035 | 0.035 | 0.25-0.55 | 0.30-1.25 | 0.65 |
| A588 Grade A, ASTM A588M | 0.19 | 0.30-0.65 | 0.80-1.25 | 0.04 | 0.05 | 0.25-0.40 | 0.40-0.65 | 0.40 |
| S355J2W, EN 10025-5 | 0.16 | 0.50 | 0.50-1.50 | 0.030 | 0.030 | 0.25-0.55 | 0.30-1.25 | 0.65 |
Phosphorus is the clearest dividing line between the two families. The GNH grades deliberately carry about 0.07 to 0.15 % P because phosphorus is the most effective single element for atmospheric corrosion resistance in industrial and rural atmospheres, but that same element raises the risk of cold cracking in welded joints, which is why welded structures normally use the NH grades where P stays at 0.035 % or below. Chromium and copper are the other two workhorses and nickel is added mainly to keep toughness in thicker sections.
Mechanical Properties and Corrosion Performance
The 355 MPa class covers Q355NH, Q355GNH, SPA-H and S355J2W, all specified with a minimum yield strength of 355 MPa. ASTM A588 Grade A is defined with a minimum yield strength of 345 MPa and a minimum tensile strength of 485 MPa. For the 355 MPa weathering grades the tensile range is normally stated as 470 to 630 MPa with elongation above 20 %, which leaves enough ductility for bending, roll forming and press braking without cracking.
Atmospheric corrosion resistance is generally reported as two to eight times that of plain carbon structural steel under identical exposure. The gain depends on the environment. Weathering steel performs best where the surface is exposed and dries out regularly, so the wet film that drives the electrochemical reaction is interrupted. Under permanent wetting, heavy salt loading or high sulphur dioxide pollution the film does not stabilise and the advantage narrows sharply. Designers therefore treat the material as an exposed steel, not as a substitute for a coating system in aggressive service.
Delivery Formats, Fabrication and Quality Control
Plates are supplied in the as-rolled, normalised or thermo-mechanically rolled condition depending on grade and thickness, with hot rolled coil, cut-to-length sheet, plate and, for some grades, acid-pickled and oiled sheet for painting. Dimensional tolerances follow GB/T 709-2019 for hot rolled plate and surface condition follows GB/T 3274-2017 for carbon and low-alloy structural plate, so thickness tolerance class, flatness and edge condition should be named in the enquiry rather than left to the mill default.
Fabrication notes that matter in practice: bend radii should be opened up compared with plain carbon steel of the same strength, sheared edges are better dressed before exposure so that the patina starts evenly, and all welding, drilling and cutting should be completed before the surface is allowed to weather. Fasteners and other components in contact with the steel should be a matching corrosion class so that galvanic couples and rust staining are avoided. Each consignment is issued with a mill test certificate showing heat number, chemical analysis, tensile and impact results, and additional ultrasonic testing to GB/T 2970 or third-party inspection can be arranged for pressure and bridge applications.
Typical Applications
Bridge and highway structures, including beams, guard rails and parapet panels.
Railway rolling stock and shipping containers, where a thin protective film allows reduced plate thickness.
Transmission towers, poles and agricultural machinery exposed to open air.
Architectural cladding, landscape walls, planters, and sculpture where the patina is the visible finish.
Coal handling, cement and power plant chutes, hoppers and liners where abrasion is combined with moisture.
Frequently Asked Questions
Q: Is corten steel the same as weathering steel?
A: Corten is the commercial name used for the same family of atmospheric corrosion resistant low-alloy steels that the standards list as GNH or NH grades, SPA-H, ASTM A588 or EN 10025-5 W grades. Composition and strength requirements differ between them, so the grade number must always be confirmed on the certificate.
Q: How long does the patina take to form?
A: Under natural exposure the rust layer normally needs roughly 18 months to 4 years to stabilise in a rural or industrial atmosphere, and accelerated pre-rusting shortens that to days. Salt-rich or permanently wet conditions can prevent stabilisation altogether.
Q: Can weathering steel be welded?
A: Yes, but the welded grades with the NH suffix and P below 0.035 % should be used for primary welded structures. Low-hydrogen consumables, controlled heat input and clean, dry joints are the normal requirements.
Q: Does the material need painting?
A: The exposed, unpainted condition is the normal specification. Where appearance or chloride exposure demands a coating, the surface is usually blast cleaned first and a system suitable for low-alloy steel is applied.
Q: What is the standard plate size range for these grades?
A: Weathering plate is commonly rolled between about 3 mm and 100 mm thick, 1500 mm to 3000 mm wide and up to 12000 mm long, with thicker sections available in the GNH and A588 grades on request.
