Weather-resistant and atmospheric-corrosion-resistant steel, its patina mechanism, the grade designations used in each market, and what changes when the part is a fastener rather than a plate.
A steel that protects itself, but only under the right exposure conditions.
Weathering steel — also sold as weather-resistant steel, atmospheric-corrosion-resistant steel, or under the trade names Corten and Cor-Ten — is a low-alloy structural steel that is left unpainted on purpose. Instead of a coating, it grows its own protective surface layer.
The layer only forms where the steel alternates between wet and dry. That single condition decides whether the material works: it is the right choice for exposed structures and the wrong choice for anything continuously wet, buried, or repeatedly exposed to chlorides.
The chemistry behind the patina, and why it needs the weather to work.
The alloying additions — copper, chromium, nickel and phosphorus — change what the rust becomes. On ordinary carbon steel the rust is porous and keeps flaking away. On weathering steel the alloying elements promote a dense, adherent oxide layer, largely made up of the oxyhydroxide phase known as α-FeOOH, which is what gives the surface its characteristic stable brown patina.
Weathering steel is quoted under a different designation in every market.
| Designation system | Designation |
|---|---|
| China, current | Q355GNH (GB/T 4171-2008, high-weathering structural steel) |
| China, former designation | Q345GNHL (GB/T 4171-2000) |
| China, railway grade | 09CuPCrNi-A (GB/T 4171-2008; originally TB/T 1979) |
| Japan | SPA-H (JIS G3125) |
| United States | ASTM A242 Type 1 (Corten A, UNS K11510) |
| ISO | S355WP / HSA355W1 |
| Europe | S355J0WP / S355J2WP (EN 10025-5) |
| Related US grades | Note |
|---|---|
| A242 Type 1 | Corten A. The original atmospheric-corrosion-resistant grade. |
| A242 Type 2 | Corten B. Sometimes grouped with A588. |
| A588 | The modern high-strength weathering structural grade. |
How a specifier tells a real weathering composition from a lookalike.
| Criterion | Plain carbon steel | Weathering steel |
|---|---|---|
| Alloying intent | Alloying elements are residual | Copper, chromium, nickel and phosphorus are deliberate additions |
| Rust behaviour | Porous, flakes, does not protect | Dense and adherent, protects the steel underneath |
| Surface finish | Painted or coated | Left bare to develop a patina |
| Objective measure | None required | The ASTM G101 corrosion index, an accepted way of distinguishing a true weathering composition from an ordinary steel |
| Heat treatment | Per the structural grade | Supplied as-rolled; not normally quenched and tempered |
The product form the structural standard covers is not the product form a fastener starts from.
Weathering steel is normally specified as plate, strip and structural section. Those are the product forms the structural standard covers. Fastener production is a different starting point, and the supply form is what decides the manufacturing route.
| Route | Starting form | Comment |
|---|---|---|
| Cold heading | Wire rod | The structural weathering standard does not cover wire rod, so the stock has to be sourced separately against a cold-heading specification |
| Hot forging | Bar | Bar forms are available |
| Machining | Bar | Bar forms are available |
Exposed, long-life, hard to maintain: the situations the material was developed for.
Weathering steel is chosen where a structure is exposed to the atmosphere, is expected to last for decades, and cannot reasonably be repainted: bridges and bridge bearings, outdoor steel structures and transmission towers, rail vehicles and freight containers, architectural curtain wall and roofing, and mechanical joint fasteners for water pipelines where the joint standard is AWWA C111.