CORTEN S355 J2W+N Plate

CORTEN steel (a trade name of US Steel) is a steel resistant to atmospheric corrosion under the EN 10025-5 standard. Technically it is called "weathering steel."

Its protection mechanism is based on the formation of a dense, adherent oxide patina that acts as an impermeable barrier against further oxidation. Unlike the oxide on ordinary steel, which is porous and continues to corrode, CORTEN's patina is self-limiting.

The alloying elements (copper, chromium, nickel) react with atmospheric moisture to form complex oxides that deposit on the surface. This process requires wetting and drying cycles and stabilizes after 2 to 5 years. Once formed, the patina protects the steel for decades with no maintenance required.

CORTEN patina formation occurs in three phases and depends on the environment:

• Phase 1 (0–6 months): Initial oxidation with intense orange-red colors. Oxide run-off is normal at this stage.
• Phase 2 (6 months–2 years): Progressive stabilization. The patina becomes more uniform and run-off decreases.
• Phase 3 (2–5 years): Mature patina with a uniform dark brown color. The protective barrier is fully stabilized.

In environments with frequent rain and sun cycles, the process is faster. In climates that are constantly very dry or very humid, patina formation may take longer or fail to stabilize properly.

CORTEN steel is not recommended in coastal areas within 1–5 km of the sea or in direct saline environments. Chlorine from sea spray prevents the protective patina from forming stably.

In these environments the patina does not stabilize and corrosion continues progressively as if it were ordinary steel. The result is premature degradation of the material without the protective benefits CORTEN offers under normal conditions.

For coastal projects, consider stainless steel (AISI 316) or galvanized steel with additional protection.

CORTEN A and CORTEN B are trade names from US Steel that roughly correspond to grades under the EN 10025-5 standard:

• CORTEN B ≈ S355 J2W+N: Optimal balance between mechanical strength and patina formation. It is the most widely used grade in construction and civil engineering. Better weldability than CORTEN A due to its lower phosphorus content.

For most structural applications, CORTEN B (S355 J2W+N) is the standard choice for its combination of mechanical properties, weldability and corrosion behavior.

Technically it is possible to weld CORTEN with standard consumables, but it is not recommended if visual uniformity matters.

Standard consumables (without copper or chromium) produce a weld area that does not patina in the same way as the rest of the piece. The weld remains visually distinct, with a grayish color that contrasts with the brown of the CORTEN patina.

For architectural applications where appearance is essential, consumables with a compatible composition should be used: ESAB OK 48.08, Lincoln Electric Jetwelder, Böhler ECuSi. These consumables contain Cu and Cr so that the weld area develops a matching patina.

In purely structural applications where appearance is not a concern (interiors of industrial buildings, non-visible areas), standard consumables are acceptable.

No. One of the main advantages of CORTEN steel is that once the patina has stabilized (2–5 years), it requires no maintenance. There is no need to paint, repaint or apply any protective treatment.

The mature patina forms a dense, adherent barrier that protects the base steel from atmospheric corrosion. CORTEN lasts 4 to 8 times longer than unprotected ordinary steel under normal atmospheric conditions.

The only recommended "maintenance" is a periodic visual inspection to detect any areas of abnormal deterioration (areas where water accumulates, mechanical damage that breaks the patina, etc.). If problems are found, local intervention should be carried out.

Direct contact with the ground is one of the critical limitations of CORTEN steel. The buried section, or any area in permanent contact with the ground, does not form a protective patina and corrodes like ordinary steel.

The ground keeps the surface constantly moist, without the drying cycles needed for patina formation. In addition, salts and chemical elements in the soil may accelerate corrosion.

The solution is to maintain a minimum gap of 20 mm between the CORTEN steel and the ground, using feet, supports or skirting of another material. Adequate drainage should also be ensured to prevent water from accumulating at the base.

Using CORTEN indoors is possible but comes with important considerations. In dry indoor environments, the patina does not form because the atmospheric moisture needed for the protection mechanism is missing.

If CORTEN is installed indoors, the material will remain in its initial state (orange-red tones) without evolving into the characteristic dark brown. This may be desirable aesthetically if that color is what's sought.

The potential problem is that, without the protective patina forming, the surface oxide can flake off and stain floors, walls and fabrics. It is recommended to apply an oxide fixative or sealant to stabilize the surface in indoor applications.

Yes, during the initial phase. During the first 6–18 months, while the patina is forming, CORTEN steel can produce oxide run-off that stains adjacent pavements, walls and other nearby surfaces.

These oxide stains are difficult to remove, especially on porous materials such as concrete, natural stone or ceramic tiles. Stains on concrete pavements can be permanent.

It is essential to protect nearby surfaces during the patina formation phase, or to design with appropriate clearances and drainage systems that channel the run-off. Informing the end client about this process is essential to avoid complaints later on.