Exterior Finishes

Cor-Ten Steel

Cor-ten steel is a type of metal used for exterior applications where a weathered and rusty appearance is desired. This material has rustic, industrial-contemporary good-looks, but is not for every application. 

Generically known as “weathering steel” this product is a family of steel alloys developed specifically to form a stable, rust-like surface appearance that eliminates the need for painting. Cor-ten is US Steel’s trademark name for its weathering steel formula.

Image of rectangular fire pit made of cor-ten steel prior to rusting
Corten steel fire pit beginning to patina

The alloys in weathering steel allow rust or surface oxidation to occur naturally over the course of about six months under normal exterior conditions. Use of chemical treatments can help speed up the process and produce the patina in as little as two hours. As the steel surface “rusts” it creates a regenerating corrosion-retarding layer that protects the main structure of the steel.

Designing for Rust

Cor-ten steel has become wildly popular for outdoor sculptures and architectural facades. The look is so attractive that architects have designed entire buildings using the material and it has a legion of residential fans, including me, around the world. This material appears unfinished and rustic at first glance, and that is what makes it so attractive.

Attractive as it is, weathering steel is not perfect for all applications. Its use should be carefully thought through before construction. Cor-ten can be challenging and even damaging to surrounding areas if used improperly. Rust bleeding or runoff from the steel surface can stain surrounding paint, stucco, stone or concrete. Avoid using this material where staining can be a problem. Also, this material is sensitive to humid climates and it is possible that in these areas the protective patina may not form properly, thus allowing the steel to continue to corrode.

Attempts to paint weathering steel have been unsuccessful because paint retards the protective patina from forming properly. When this happens, deep corrosion can occur at localized areas of oxidation like under small paint cracks or chips.  

Image of bronze looking steel outdoor sculpture by Archie Held
Stainless steel sculpture with patina by Archie Held

World class artist Archie Held likes the look so much that he developed his own formula to patina stainless steel in order to duplicate the appearance for his sculptural water features. As shown by example, if designed well and properly executed, Cor-ten steel can add an exciting and unique appearance to your projects!

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