Overview of SPA-H and A588 Weather Resistant Steel Plate
Weather resistant steel plate, commonly known as corten steel plate, is a low alloy structural steel engineered to form a dense, tightly adherent oxide layer when it is exposed to alternating wet and dry conditions. That stable patina slows further atmospheric corrosion far more effectively than ordinary carbon steel, which is why exposed structures made from it can be left unpainted for decades. Two designations dominate export enquiries for hot rolled plate: JIS G3125 SPA-H and ASTM A588. Both are supplied in the hot rolled condition and both work on the same principle: a small, carefully balanced addition of copper, chromium, nickel and phosphorus changes the chemistry of the surface oxide so that it becomes compact and self renewing instead of loose and flaking.
The sections below cover the grade range, chemical composition, mechanical properties, thickness range and delivery states available for hot rolled weather resistant plate from 2 mm to 300 mm.
Grade Range by Standard
Weather resistant plate is ordered to several national and regional standards. The grades listed below are routinely rolled and stocked in plate form.
| Standard | Grades |
|---|---|
| ASTM A588 / A588M | A588 Grade A, Grade B, Grade C, Grade K |
| ASTM A242 / A242M | A242 Type 1, A242 Type 2 |
| ASTM A606 | A606 Type 4 |
| ASTM A709 | A709 Grade 50W |
| JIS G3125 | SPA-H, SPA-C |
| JIS G3114 | SMA400AW, SMA400BW, SMA400CW, SMA490AW, SMA490BW, SMA490CW, SMA570W, SMA570P |
| EN 10025-5 | S235J0W, S235J2W, S355J0W, S355J2W, S355K2W, S355J0WP, S355J2WP |
| GB/T 4171 | Q235NH, Q295NH, Q355NH, Q415NH, Q460NH, Q550NH, Q265GNH, Q295GNH, Q310GNH, Q355GNH |
| TB/T 1979 | 09CuPCrNi-A, 09CuCrNi-B, 05CuPCrNi |
| Q/BQB 340 | B460NQR, B490NQR, B480GNQR, B600GNQR |
SPA-H is the hot rolled grade of JIS G3125 and is the designation most often used for exposed bridge and building plate, while SPA-C is its cold rolled counterpart for thinner cladding, gutters and ducting. ASTM A588 Grade A is the plate grade most frequently specified for welded highway bridges and building frames, and ASTM A242 is the original weathering steel specification, still used for lighter rolled sections. For Chinese domestic projects the equivalent family is GB/T 4171 Q355NH, and for European projects EN 10025-5 S355J2W or S355J0W.
Chemical Composition and Mechanical Properties
Alloy balance is what separates weathering steel from plain carbon steel. Copper is the most important single addition, chromium and nickel refine the oxide layer, and phosphorus accelerates patina formation in the high phosphorus variants.
| Type | C max | Si | Mn | P | S max | Cr | Ni max | Cu |
|---|---|---|---|---|---|---|---|---|
| A242 (Corten A type) | 0.12 | 0.25-0.75 | 0.20-0.50 | 0.07-0.15 | 0.030 | 0.50-1.25 | 0.65 | 0.25-0.55 |
| A588 (Corten B type) | 0.16 | 0.30-0.50 | 0.80-1.25 | 0.030 | 0.030 | 0.40-0.65 | 0.40 | 0.25-0.40 |
Values are percentages by weight, balance iron. Single figures are maximum limits; ranges are the permissible window.
Typical minimum mechanical properties for plate in the usual 2 mm to 100 mm thickness range are given below.
| Type | Yield strength, MPa | Tensile strength, MPa | Elongation, % |
|---|---|---|---|
| A242 (Corten A type) | 355 min | 470-630 | 20 min |
| A588 (Corten B type) | 345 min | 470-630 | 20 min |
Thickness matters. The minima above apply to plate and rolled shapes up to about 100 mm thick. Heavier sections are permitted lower values: plate from 102 mm to 127 mm thick is normally supplied with a minimum yield strength of 320 MPa and a minimum tensile strength of 460 MPa, and plate from 127 mm to 203 mm thick with a minimum yield strength of 290 MPa and a minimum tensile strength of 430 MPa. Impact requirements, where specified, are agreed at the ordering stage because weathering grades are not automatically impact tested.
Dimensions, Delivery States and Documentation
Thickness: hot rolled 2-300 mm, with 2-100 mm the most frequently ordered range; cold rolled 0.5-2 mm for sheet and cladding products.
Width: 1000-3000 mm, with 1000-2200 mm typical for standard plate.
Length: 1000-12000 mm, or cut to the lengths required by the fabrication drawings.
Delivery states: hot rolled, controlled rolling, normalised, annealed, tempered, quenched, N+T and Q+T are all available as required.
Surface: as-rolled, blast cleaned, or primed and painted to suit the specified coating system.
Documentation: a mill test certificate is issued with each lot, typically to EN 10204 3.1, and third party inspection can be arranged before shipment.
Packaging: standard seaworthy export packing suitable for container, break bulk and rail transport.
Typical Applications
Highway and railway bridges, bridge girders and parapets.
Building facades, architectural cladding and exposed structural frames.
Transmission towers and reaction towers.
Lifting and transportation machinery, and rail wagon components.
Industrial furnace shells and boiler support structures.
Container frames, warehouse shelving and urban furniture.
Coastal and marine structures where salt exposure makes painting uneconomic.
Frequently Asked Questions
Q: What is the difference between SPA-H and A588?
SPA-H is a hot rolled weathering steel grade defined by JIS G3125, while A588 is an ASTM plate specification containing four grades (A, B, C and K). Both depend on a copper, chromium and nickel bearing chemistry to build a protective patina; the practical differences are the strength levels and which specification the project recognises.
Q: Does weathering steel plate need painting?
In most outdoor exposures it does not. The plate is left bare so the patina can form, provided the detail design allows the surface to dry between wetting cycles. Painting is still recommended where the steel is permanently wet, buried, or exposed to chloride concentrations above those of a normal marine atmosphere.
Q: How long does the patina take to develop?
Under normal atmospheric exposure the surface oxide takes about six months to settle into its protective form. Accelerated treatments can shorten this considerably, though they change the appearance of the early surface and should be agreed with the project designer.
Q: Which thicknesses are held in stock?
Hot rolled plate is normally stocked between 2 mm and 100 mm, and heavier plate up to 300 mm is rolled to order. Widths of 1000-2200 mm and lengths up to 12000 mm cover the great majority of fabrication requirements.
Q: Is a mill test certificate supplied?
Yes. A mill test certificate covering chemical composition and mechanical properties is issued for each lot, and independent third party inspection can be arranged on request before dispatch.
Q: Can SPA-H and A588 plate be welded?
Yes. Both grades are weldable with normal low hydrogen procedures for low alloy structural steel. Filler metal should match the strength of the parent plate, and stainless steel or copper bearing consumables are used where the joint must weather without rust staining.
