Corten: A material where the rust is the protective coating


Cor-ten is the commercial name of weathering steels. Actually COR-TEN® is a trademarked name owned by US Steel. The material, was first developed in US in 1933, with the purpose to eliminate the need for painting. For this the ordinary steel, has been alloyed with small proportions of copper, silicon, manganese, chromium and either vanadium or phosphorus as minor constituents. 

How it works: 

Weathering steel does not arrive pre-rusted in place and if left exposed to the atmosphere it will inevitably begin to rust as any ordinary steel. However the corrosion will normally stop, since the alloying elements will cause the formation of a protective layer of rust onto the metal surface. This rust is tightly adhere to the base metal and provides the necessary protection that restrains corrosion and inhibits deeper penetration. Note that getting the material wet will increase the speed in which it will rust. After several years of exposure to the weather, the metal will get a stable, dark-brown, rust-like appearance. So, this unique look of naturally oxidizing finish is given by nature itself and it is the same as the green patina over the copper surfaces. 

The advantages:

Cor-ten, is a very strong steel alloy with a significant resistance to atmospheric corrosion. Unlike other steels, cor-ten needs no painting or other rust prevention mantainance to resist the corrosive effect of rain, ice, fog, weather, etc. Instead of painting, the alloy creates a self-protection patina that hinders further oxidation and corrosion. Painting, although it is not necessary, can be also applied. 

The aesthetic properties of corten are interesting and unique. The appearance of this naturally formed, brown rust coating could be characterized as rustic, raw, antique, warm, untreated, etc.

Issues to consider

Not- appropriate  environments: The use of weathering steels is not appropriate within marine environments, areas that have salt laden air, high rainfall, humidity or persistent fog. In humid subtropical climates it is possible that the rust will not stabilize but continue to corrode. For example, the former Omni Coliseum, built in 1972 in Atlanta, never stopped rusting and eventually large holes appeared in the structure. The structure was demolished 25 years after the construction.

What is the ideal environment for weathering steels: Cor-ten, requires alternating wet and dry cycles to form the protective patina.

Compatible metals with weathering steel: A compatible metal, that can come into contact with weathering steel is stainless steel.

Rust stains on nearby surfaces and surrounding areas: Architectural design must ensure that the rainwater run-off will not encroach on other materials and nearby surfaces. This might lead to rust stains during the first years of exposure. Particular attention, should be paid when the surrounding surfaces are concrete or glass. For example, the U.S steel tower in Pittsburgh was constructed to showcase cor-ten steel. The initial weathering of the material caused the discoloration of the surrounding sidewalks, as well as the nearby buildings.

Rain water: Rain water should not allowed to accumulate in pockets since those areas will experience higher corrosion rates.

Application in achitecture and art:

Weathering steels are used for structural and cladding applications (buildings and bridges) as well as for sculptural works of art.

Popular building and constructions

- Barclays center 

Australian center of contemporary art 

- Museum of old and new art 

-The main building of Odense university 

Monumental sculptures:

-Chicago Picasso 

-The angel of the north 


Abetxuko Bridge



Image source:

  • Pontis21, (2006), View of Abetxuko Bridge in Vitoria [ONLINE]. Available at:[Accessed 03 December 13].



materials for design

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