What ASTM A242 Type 2 Corten Steel Plate Is
ASTM A242 Type 2 Corten steel plate is a high-strength low-alloy weathering steel produced to the ASTM A242 specification. It is a copper-bearing structural steel in which small additions of copper, chromium, nickel and phosphorus build up a dense, tightly adhering oxide layer on the surface. Once that patina has formed, the plate resists further atmospheric attack far better than ordinary carbon steel, so no paint system is needed for many exposed applications.
Type 2 identifies the plate product form supplied under the A242 specification, drawn from the same family of copper-phosphorus weathering compositions. It is selected where an unpainted, self-protecting structural steel is wanted for outdoor and industrial service, particularly for structures that will be left exposed to the weather for decades.
Chemical Composition of the A242 Weathering Family
The table below compares the A242 weathering family with the other grades commonly used for the same duty. Values are percentages by mass, and single figures are maximum limits unless a range is given.
| Element | Corten A | ASTM A588 Gr A | ASTM A242 Type 1 | IRSM 41-97 |
|---|---|---|---|---|
| Carbon, max | 0.12 | 0.19 | 0.15 | 0.10 |
| Manganese | 0.20-0.50 | 0.80-1.25 | 1.00 | 0.25-0.45 |
| Phosphorus | 0.07-0.15 | 0.04 | 0.15 | 0.07-0.11 |
| Sulphur, max | 0.030 | 0.05 | 0.05 | 0.030 |
| Silicon | 0.25-0.75 | 0.30-0.65 | 0.25-0.40 | 0.28-0.72 |
| Nickel, max | 0.65 | 0.40 | - | 0.20-0.49 |
| Chromium | 0.50-1.25 | 0.40-0.65 | - | 0.30-0.50 |
| Copper | 0.25-0.55 | 0.25-0.40 | 0.20 min | 0.30-0.39 |
| Vanadium | - | 0.02-0.10 | - | 0.050 |
| Aluminium | - | - | - | 0.030 |
Copper is the element that drives the patina, phosphorus accelerates its formation, and chromium with nickel refines the oxide structure so that it stays adherent instead of flaking away. Manganese and silicon control strength and deoxidation, and the low carbon level keeps the plates weldable without a heavy preheat schedule.
Mechanical Properties
| Grade | Tensile strength (MPa) | Minimum yield point (MPa) | Elongation, min (%) |
|---|---|---|---|
| Corten A | 470-630 | 355 | 20 |
| ASTM A588 Gr A | 485 min | 345 | 21 |
| ASTM A242 Type 1 | 480 min | 345 | 16 |
| IRSM 41-97 | 480 min | 340 | 21 |
The combination of high tensile strength, good ductility and a yield point in the 340 to 355 MPa band means a weathering plate can often be used at a thinner section than plain carbon steel, reducing the self-weight of the structure that has to carry its own dead load.
Equivalent Materials
When A242 plate is not available, ASTM A588 or ASTM A871 weathering steel can be substituted where the structural design permits, because they offer a comparable balance of strength and atmospheric corrosion resistance. ASTM A588 Grade A is the closest general-purpose plate equivalent, while ASTM A871 is normally chosen for higher-strength bridge and welded structures.
Any substitution still has to match the strength level, the thickness limitation and the toughness requirement of the original design, and the weld consumables must be selected for the actual grade being supplied rather than for the grade originally specified.
Corrosion Behaviour and Design Notes
Weathering steel depends on wet and dry cycling to develop and then maintain its protective layer. In practice this means detailing the structure so that water drains freely, avoiding pockets where water can pond, and keeping the steel away from continuously wet, buried or chloride-rich conditions where the patina cannot stabilise.
A small thickness loss allowance, typically on the order of the corrosion allowance used for the project, is usually carried in the design so that long-term section loss does not reduce the calculated capacity of the member.
Typical Applications
Bridge girders, box sections and highway structures
Transmission towers and outdoor structural frames
Building facades, cladding supports and exposed architectural steelwork
Railway rolling stock and freight container frames
Industrial plant, petrochemical and pulp and paper structures
Storage and handling structures in open-air service
Supply, Processing and Delivery
Delivery condition: as-rolled, normalised or thermo-mechanically rolled, according to thickness and toughness requirement
Plates are supplied cut to size, with flame cutting or plasma cutting to fabrication drawings
Surface condition: mill finish, shot blasted, or primed where a paint system is still specified for appearance reasons
Third-party inspection and mill test certificates covering chemistry, tensile properties and impact testing
Impact and through-thickness testing available when the joint design or the service temperature demands it
FAQ
Q: Is A242 plate the same as Corten A?
They are close relatives but not identical. Corten A and ASTM A242 Type 1 both rely on copper and phosphorus for their patina, while A242 Type 2 covers the plate product supplied under the A242 specification, so the chemistry and the acceptance data always have to be checked per heat.
Q: Can weathering steel plate be painted?
Yes. Painting is often used for appearance or for aggressive environments, and the tightly adhering patina actually improves paint adhesion compared with ordinary carbon steel.
Q: What happens if the plate stays permanently wet?
The protective layer will not stabilise, and the corrosion rate can approach that of ordinary carbon steel, so the grade should not be used in continuously damp, buried or high-chloride locations.
Q: What are the equivalent materials for ASTM A242?
ASTM A588 Grade A and ASTM A871 are the recognised weathering steel alternatives when A242 plate is unavailable.
Q: What is the minimum yield point of A242 Type 1?
345 MPa, with a minimum tensile strength of 480 MPa, which places the grade in the high-strength low-alloy structural family.
Q: How is the plate welded?
Welding uses low-hydrogen consumables matched to the copper-bearing chemistry, and the preheat requirement is set by plate thickness and the carbon equivalent of the individual heat.
