What Corten B Weathering Steel Plate Is
Corten B is the most widely ordered grade in the weathering steel family used for load-bearing outdoor work. It is a high-strength low-alloy steel that contains copper, chromium and nickel, and it maps onto the atmospheric-corrosion-resistant grades of ASTM A588/A588M, EN 10025-5 and JIS G3114. Exposed to alternating wet and dry conditions, the surface oxidises into a dense, tightly adherent layer, normally called the patina, which seals the steel and slows further corrosion to a small fraction of the rate of unprotected carbon steel.
The key difference from a painted product is that the patina is not a consumable coating. There is no film to flake, chip or repaint, and maintenance is normally limited to inspection and cleaning, which is why the grade is specified for structures where access is difficult or repainting is uneconomic.
Chemical Composition of Corten B Plate
Corten B relies on a balanced alloy addition rather than on a single element. Copper and chromium drive the corrosion resistance, nickel supports low-temperature toughness, and the carbon, phosphorus and sulphur contents are held low enough to keep the steel weldable.
| Element | Content (%) | Contribution |
|---|---|---|
| Carbon (C) | 0.19 max | Strength; limited for weldability |
| Silicon (Si) | 0.30-0.65 | Deoxidation and oxidation resistance |
| Manganese (Mn) | 0.80-1.25 | Strength and hardenability |
| Phosphorus (P) | 0.035 max | Controlled to protect toughness |
| Sulphur (S) | 0.030 max | Controlled to protect toughness |
| Aluminium (Al) | 0.020-0.060 | Grain refinement |
| Vanadium (V) | 0.02-0.10 | Strength by precipitation |
| Copper (Cu) | 0.25-0.40 | Primary atmospheric corrosion resistance |
| Chromium (Cr) | 0.40-0.65 | Patina stability |
| Nickel (Ni) | 0.40 max | Low-temperature toughness |
Mechanical Properties and Equivalent Grades
Corten B is a structural grade, not a decorative thin-gauge product, so it carries a defined minimum yield point together with a tensile range and a minimum elongation.
| Property | Value |
|---|---|
| Minimum yield strength, ReH | 345 MPa |
| Tensile strength, Rm | 470-630 MPa |
| Minimum elongation, A | 19% |
| Material number | 1.8946 |
| Typical hardness and forming behaviour | Readily cold formed and hot formed; suitable for gas cutting and welding |
The grade is interchangeable in practice with several national designations, which is why drawings from different markets carry different names for effectively the same steel.
| Standard | Designation | Region |
|---|---|---|
| ASTM A588/A588M | Grade B | United States |
| ASTM A242/A242M | Type 1 | United States, older specification |
| EN 10025-5 | S355J0W, S355J2W | Europe |
| JIS G3114 | SMA490W | Japan |
| GB/T 4171 | Q355NH | China |
| Superseded national grades | BS 4360 WR 50 C, UNI FE 510 D2K1, NFA 35-502 E36 WB, SEW 087 WTSt 52-3 | Europe, historical |
Available Product Range
Thickness: 0.8 mm to 50 mm, covering thin strip for architectural work through to plate for structural use.
Width: 750 mm to 1500 mm in coil and strip form, with wider plate available in heavier thicknesses.
Length: up to 12000 mm for cut plate.
Forms: hot rolled coil, cut-to-length plate, and fabrications such as folded profiles, laser-cut panels and welded assemblies.
How the Patina Develops and Where It Has Limits
A freshly blasted or pickled surface begins to oxidise as soon as it is exposed. The colour moves from light orange through deep brown to a dark brown-violet tone, and the layer becomes stable normally within 18 to 36 months in a rural or moderately industrial atmosphere with regular wet and dry cycles. In favourable exposures, reported service life for weathering steel structures is a multiple of that of unpainted carbon steel, with factors of four to eight times quoted where the patina is allowed to develop freely.
Those conditions do not exist everywhere. Chloride-rich coastal air, salt spray, splash zones, permanently wet surfaces, trapped water in crevices and ponding all prevent a protective layer from forming, and steel in those locations corrodes like unprotected carbon steel. In such service the plate must be painted or otherwise protected, and the detailing should keep water moving off the surface rather than letting it sit against the steel.
Typical Applications of Corten B Plate
| Sector | Application | Reason for use |
|---|---|---|
| Architecture and sculpture | Facade cladding, screens, art panels | Formable, weldable, stable dark patina |
| Bridge and highway works | Exposed girders, parapets, gantries | Structural strength with low maintenance |
| Heavy equipment | Containers, loader and tipper bodies, skips | Resists abrasive and corrosive service |
| Transmission and lighting | Towers, masts, poles | Light weight with long service life |
| Transport and public realm | Railway wagons, crane structures, stadium and museum elements | Durability with design freedom |
| Landscape and garden | Edging, planters, retaining profiles, water features | Weathering appearance with no repainting |
Frequently Asked Questions
Q: What is Corten B weathering steel plate?
It is a high-strength low-alloy structural steel containing copper, chromium and nickel that forms a protective patina when exposed to the weather. It is supplied as hot rolled coil and plate and is used where strength and long-term corrosion resistance are needed without painting.
Q: What is the yield strength of Corten B?
The minimum yield strength is 345 MPa, with a tensile strength range of 470 to 630 MPa and a minimum elongation of 19%.
Q: Is Corten B the same as ASTM A588 Grade B?
The two grades occupy the same weathering strength class and are usually considered equivalent for structural purposes, but the specification limits are not identical. The designation stated on the drawing and purchase order controls inspection and acceptance.
Q: Does Corten B plate need to be painted?
In rural and moderately industrial exposures it is normally left unpainted so that the patina can form. Near the coast, in salt-laden or permanently damp conditions, or where water cannot drain away, a protective coating is required.
Q: How long does the patina take to develop?
In a favourable atmosphere with regular wet and dry cycles the surface typically stabilises within 18 to 36 months. Run-off staining of adjacent surfaces is normal during this period and is usually managed by detailing rather than by coating the steel.
Q: What is the difference between Corten A and Corten B?
Corten A carries a higher phosphorus content and is intended mainly for thin architectural sections and cladding, while Corten B has a lower phosphorus level and the strength and toughness needed for welded structural work, which makes it the more common choice for load-bearing applications.
