1. What COR-TEN Actually Means
COR-TEN, frequently written as Corten, is a trade name registered by United States Steel Corporation for a family of corrosion resistant steels. The name is a condensation of corrosion resistance and tensile strength. The original grade received the ASTM designation A242, and is commonly referred to as COR-TEN A. Later ASTM grades in the family are A588, commonly called COR-TEN B, and A606 for thin sheet and strip. These are all copper-chromium alloy steels whose composition promotes the early formation of an adhering protective rust layer when exposed to the weather, eliminating the need for painting in suitable environments.
2. SPA-H and Its Standard, JIS G3125
SPA-H is defined in JIS G3125: Hot-rolled atmospheric corrosion resisting steels for traffic, containers and similar applications. It is the higher-strength member of the SPA pair (the other is SPA-C) and is widely used for shipping containers, railway rolling stock, truck bodies and structural members. Because JIS G3125 and the ASTM A-series standards are written independently, the grades are similar in concept but not identical in chemistry or properties, which matters when a project specifies one grade and a supplier offers the other as an equivalent.
3. Chemical Composition Compared
The table below compares typical composition limits for COR-TEN A (per ASTM A242 type chemistry) and SPA-H (per JIS G3125).
| Element (max %, or range) | COR-TEN A | SPA-H |
|---|---|---|
| C | 0.12 | 0.12 |
| Si | 0.25–0.75 | 0.20–0.75 |
| Mn | 0.20–0.50 | 0.20–0.60 |
| P | 0.07–0.15 | 0.070–0.150 |
| S | 0.030 | 0.035 |
| Cr | 0.50–1.25 | 0.30–1.25 |
| Cu | 0.25–0.55 | 0.25–0.55 |
| Ni | 0.65 | 0.65 |
Both grades rely on chromium and copper for corrosion resistance. The ranges overlap broadly, which is why the two grades are often treated as close equivalents, but the exact limits differ and should be checked against the governing standard for the project.
4. Mechanical Properties Compared
For hot-rolled plate in the thinner range, both grades offer similar yield strength. Representative values are:
| Property | COR-TEN A (ASTM A242) | SPA-H (JIS G3125) |
|---|---|---|
| Min yield strength | 345 MPa (up to 19 mm) | 355 MPa (up to 16 mm) |
| Tensile strength | 480–620 MPa | 490–610 MPa |
| Elongation | 21% (50 mm gauge) | 15% min for plate up to 16 mm |
Note that the ASTM A242 tensile range and elongation are based on a 50 mm gauge length and vary with thickness, while the JIS values follow JIS G3125. Any comparison for a specific order should be confirmed against the thickness-dependent tables in the applicable standard.
5. When Can SPA-H and COR-TEN A Be Treated as Equivalent?
Equivalence in weathering steel is always approximate. A substitution of SPA-H for COR-TEN A, or vice versa, should be reviewed by the design engineer against the project specification, the applicable building code and the required corrosion performance. For non-code, atmospheric-exposure applications such as cladding, landscaping and decorative structures, the two grades behave very similarly and substitution is common in practice. For code-governed structures, the governing standard, welding requirements and inspection documentation of the specified grade should be followed. Charpy impact testing, Z-direction properties and other supplementary requirements are not automatically carried over between standards.
Frequently Asked Questions
Is COR-TEN the same as weathering steel? COR-TEN is a trade name for a family of atmospheric corrosion resistant steels, and the term corten is now used generically for weathering steel. The generic engineering term for the material class is atmospheric corrosion resisting steel.
Is ASTM A588 the same as COR-TEN B? ASTM A588 is commonly referred to as COR-TEN B in the market and is used for structural shapes and plates where higher strength and atmospheric corrosion resistance are required. The trade name itself belongs to US Steel, so the identification should be verified with the material supplier.
What is the minimum yield strength of SPA-H? Per JIS G3125, SPA-H has a minimum yield strength of 355 MPa for thicknesses up to 16 mm, decreasing for thicker material, with a tensile range of 490–610 MPa.
Can SPA-H plate be substituted for ASTM A242 plate? For many atmospheric exposure applications the two are used interchangeably, but substitution should be confirmed by the design engineer, because composition limits, mechanical property tables, thickness ranges and test requirements differ between JIS G3125 and ASTM A242.
Why does SPA-H not need painting? The alloying elements, mainly copper and chromium, promote a dense, adherent oxide layer during wet-dry cycling. This patina restricts further corrosion, so bare use is possible where the environment permits patina formation.
