Rust is a phenomenon that most engineers seek to avoid, but is actually desirable in weathering steel. Also known as Corten steel, weathering steel is characterized by what the American Institute of Steel Construction (AISC) describes as “useful corrosion,” in which naturally occurring rust not only lengthens the lifecycle of a weathering steel structure, but also saves the time and money associated with painting and maintenance. Rust on weathering steel sheets is not a sign of deterioration as with corrosive steel sheets; in fact, it is just the opposite.
Corten, which was trademarked by the United States Steel Corporation (USS), is the well-known brand name of weathering steel. Corten originally received the ASTM standard designation of A242, but is now recognized by the newer ASTM grade for steel sheets and coils, A606. Weathering steel was specifically developed with small amounts of copper, phosphorus, chromium, nickel and silicon. These materials allow Corten steel sheets to form a layer of rust when exposed to weather, excluding the need for paint.
Instead of ruining the metal, however, rust actually serves to protect Corten from further corrosion. Weathering steel sheets, which initially look rather unremarkable, form a protective barrier of rust when exposed to moisture and oxygen. That barrier eventually seals the exposed Corten steel sheets against the very elements that caused them to rust in the first place. According to AISC, “this stable barrier layer greatly resists further corrosion, reducing it to a low value.”
Weathering steel is also conveniently high strength, low maintenance, and cost effective. The abundant benefits of weathering steel make Corten steel sheets ideal for structural and architectural applications such as bridges, roofing and open-framed buildings. Bridges built with Corten have been known to last up to 120 years with minimal maintenance, which involves regular inspections and cleaning. Because weathering steel does not require paint, builders can also avoid the health issues concerning volatile organic compounds found in certain paints.
The popularity of weathering steel may also be explained, in part, by its attractive appearance. Structures built with Corten steel sheets take on the reddish, orange color of rust. As the rust deepens with further exposure to the environment, the steel sheets develop a deep russet patina that many people find appealing. The unusual appearance of weathering steel was featured in the New York Times a few weeks before the Barclays Center, built with Corten, opened. Weathering steel structures, the article observed, “can look suspiciously unfinished to the casual observer, [yet] has many fans in the world of art and architecture.”
Corten steel has been well-received by a community of architects and design engineers because of its considerable benefits. Weathering steel has a proven ability to withstand use over the long term, while requiring minimal upkeep. Structures built with Corten steel sheets do not require paint, which is good for the environment and the bottom line. Weathering steel sheets also develop a patina of rust that does not harm the Corten, but rather beautifies it. Beautiful and practical, weathering steel is an excellent building material.