CorTen

Corten is the trade name commonly used for a family of weathering steels developed in the 1930s by United States Steel. These copper-containing alloys are designed to form a stable, rust-like surface patina when exposed to weather, which protects the underlying steel from further corrosion and reduces the need for protective coatings in many environments. The patina develops through the gradual oxidation of the surface and the adhesion of iron oxides, creating a protective layer that slows deeper corrosion.

Two widely used grades are Corten A and Corten B. Corten A is optimized for architectural and thinner sections, while Corten B is stronger and intended for structural applications. Both grades include copper and other alloying elements such as chromium, nickel, and phosphorus to enhance corrosion resistance and control patina formation. They are typically specified under standards such as ASTM A242 or A588, with variations in composition and mechanical properties by grade.

Patina formation depends on climate, moisture, pollutants, and exposure conditions. Over months to years, the surface develops a corrosion layer that adheres to the metal and reduces the rate of further corrosion. In outdoor environments, the resulting appearance ranges from orange to brown and may vary with locality. Corten steels are less suitable for indoor use or highly chloride-rich environments, where the protective patina may not form effectively and staining of adjacent materials can occur.

Common applications include architectural facades, bridges, sculptures, and exterior metalwork where a low-maintenance, weathered look is desired. Design considerations include avoiding water pooling during initial weathering and, when a uniform appearance or indoor use is required, applying protective coatings.