Ritchie Studio

Search icon

Cor-ten ®

RSC weathered sampleCOR-TEN® is a weathering steel and was developed to avoid having to paint steel for protection. This means that it has increased resistance to atmospheric corrosion when compared to unalloyed steels. It achieves this by forming a protective rust-coloured oxide layer on its surface. Its alloying with copper and chromium ensure the formation of a firmly adhering protective layer of ‘rust’ which delivers a particular aesthetic appearance and weathering performance. COR-TEN® is not rustproof in itself. COR-TEN® is a registered trademark of the United States Steel Corporation (USS).

The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. However, it should stabilise after 2-3 years. Used underground or if allowed to remain continually wet it will not form the protective patina.

It is vital to take expert advice when using COR-TEN® as an architectural element.
It is important to understand the climate where it will be used, and it presents several construction challenges – design, detailing, fabrication and erection.

– Welds have to be designed and executed to weather at the same rate as material being joined.
– Areas where water can stay may experience higher corrosion rates.
– In salt-laden air environments the protective patina may not stabilize but continue to corrode.
– Moisture should not be trapped within and adequate ventilation to ensure wet-dry cycles.

Its normal surface weathering can lead to rust stains on nearby surfaces. Basic guidelines for the use of this steel are described in EN 10025-5, and the mechanical properties of weathering steels depend on which alloy and how thick the material is.

RSC Courtyard Theatre Corten wall after one year

COR-TEN A (ASTM denotes it as A242, and the Euro Norm as S355J2WP+N)
COR-TEN A Plate is available from 1,0-12,5mm thick, and COR-TEN B plate up to 60mm thick.
COR-TEN A differs from COR-TEN B in that it has an increased Phosphorus content to help weather resistance, but , as a result is a little more limited in processing .

It is a steel alloyed primarily with additions of carbon, silicon, manganese, phosphorus, sulphur, chromium, nickel and copper, and vanadium is added to the ‘B’ grade.

The corrosion-retarding effect of the protective layer is produced by the particular distribution and concentration of alloying elements in it. The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. In other words, the steel is allowed to rust in order to form the ‘protective’ coating.

Chemical composition of COR-TEN® grades
Grade Cor-ten A Cor-ten B
%C 0.12 0.16
%Si 0.25 – 0.75 0.30 – 0.50
%Mn 0.20 – 0.50 0.80 – 1.25
%P 0.01 – 0.20 0.030
%S 0.030 0.030
%Cr 0.50 – 1.25 0.40 – 0.65
%Cu 0.25 – 0.55 0.25 – 0.40
%V 0.02 – 0.10
%Ni 0.65 0.40

Source: http://www.ajmarshall.com/corten.htm
Aluminium (soluble) is also added to COR-TEN A and B by certain manufacturers.

The mechanical properties of weathering steels depend on which alloy and how thick the material is.

Crystal Palace Concert Platform

The first use of COR-TEN® for architectural applications was the John Deere World HQ in Moline Illinois, completed in 1964 and was designed by Eero Saarinen.

Ian Ritchie Architects first used COR-TEN A for the Crystal Palace Concert Platform, London in 1996, and for the Royal Shakespeare Company’s Courtyard Theatre completed in 2006.

Ian Ritchie’s replies to questions commonly asked by students.

Why did you use COR-TEN® steel for the Crystal Palace Concert Platform project?
It concerns the relationship with nature – change, natural condition towards entropy.
This is an aesthetic position, rather than an aesthetic in purely visual terms. The site is an important landscape by Joseph Paxton (The Building (Crystal Palace) for the Great Exhibition of 1851). There has been a direction in our architecture since circa. 1990 towards making buildings from less processed materials and finding ways to allow buildings to naturally evolve with resort to heavy or constant maintenance. It has nothing to do with nostalgia.

Was it your choice to use only COR-TEN® steel? Does this refer to aesthetics of ‘the factory’?
The choice was singular. The entire external skin, including roof, was to be as if the concert platform had been shaped from a single block. The only surface not in COR-TEN A is the untreated oak stage. When the client asked us to increase the size of the stage so that it could accommodate a full orchestra (c. 80 musicians) and full choir (80), they would not fund increasing the cantilevered canopy. This left the front 2 meters of the oak stage exposed to the sky. It turned out that the oak had not been fully seasoned and has suffered as a result.

Corten and Stainless steel side by sideWhat reactions suggest the use of this material – at public level, city level, urban policies?
Generally the public associate its aesthetic with rust! That it is unfinished – “the council couldn’t afford to paint it” type of comment. The Crystal Palace Concert Platform was colloquially known as “the skip” – which I found somewhat endearing! We have also had reaction which suggests it is more than acceptable.

The RSC Courtyard Theatre in Stratford – Upon – Avon, a theatre for 1050 people, is also made of COR-TEN A, and reaction has been very positive. It is a large bold rusty shed in the middle of town – yet discreet, and blends beautifully with the red brick and roof tiles of the townscape. In urban terms, COR-TEN A may become more acceptable in the future, in much the same way that gabion walling has since we designed the Cultural Greenhouse in Terrasson-La-Villedieu in 1992. Gabion, certainly in cities in the UK has become almost the standard walling for many new developments. To have raised the aesthetic appreciation of this humble civil engineering technique to an acceptance in virtually any site and type of building is very rewarding. I find this particularly heartening; many architects sent postcards at the time thanking Ian Ritchie Architects for the idea, although the pre-concept was not necessarily understood. So the same may happen for COR-TEN A, which would mean it being set free from the civil and bridge engineering worlds.

Within the framework of your various projects, do you always think of the possible problems which could occur such as marks from run-offs of rust? Do you always take into account how it will wear with time? If so, how?
Yes. The run-off onto the concrete edge was considered and we thought it marked for the public gaze, the oxidation process that was occurring. The ferrous metal running off meant that we designed a settlement tank at the overflow exit from the small lake. This will need to be cleaned out in five years, and then never again. The reason for this was that the overflow fed into a fishing lake further down in the park. We calculated how much ferrous material may run off the building over its lifetime. At Stratford, we have not expressed this process as the building is more discreet in its setting. Here, we have a small channel around the perimeter base of the wall which collects the ferrous oxide run-off.

Concrete, Corten, Stainless Steel, Gabion

Would you consider the controlled deterioration of materials desirable within the framework of urban or other projects?
Yes. However the nature of the setting, the type of building and client will inform the particular approach and degree to which entropy is introduced in the design. In essence, we are trying to design with nature’s forces rather than resist them totally.

What do you think about the fact that this material is often associated with building in an historical sense and not an innovative one?
COR-TEN® originated in the US as a steel substitute for bridge building, where maintenance of the underside of them was particularly difficult.

One often thinks of COR-TEN ®steel as a bond between past and present. How do you consider it?
Technically, I consider COR-TEN® a 20th century development of a 19th century process – that of making steel, the aim of which was to eliminate maintenance. I do not see it as a bond between any particular human idea of history, but as a dialogue with nature and time.

What is the viability of the use of this material?
Extremely viable when used appropriately. One vitally important aspect in designing with COR-TEN® is to understand the need for it to cycle between being wet and dry.


Profiled corten structural walling for RSC Courtyard Theatre

What are the origins of your thinking towards this material?
I believe very strongly that we should seek to find ways of making buildings that are far less dependent upon sophisticated energy consuming processes and high levels of maintenance. This applies equally to the interiors of buildings. Part of the method to achieve this is raising the aesthetic value or perceived aesthetic qualities of materials and assemblies which hitherto have been considered ‘ugly’ or ‘inferior’. It is not that easy to achieve this. However, after a few projects, I am beginning to see a shift.

What does the use of such a material represent for you, especially when one knows that recent innovation in architecture is not necessarily beautiful?
I think that beauty lies in nature, and in particular, in its characteristic non-linearity – the movement of water, light upon water, clouds forming, a log burning, and of course, entropy.

RSC Courtyard Theatre Corten wall

Why does US Steel deliver a health warning with reference to its use in architecture?

“Consistent with United States Steel Corporation’s (“USS”) policy over the last two decades, U. S. Steel reiterates and reminds that COR-TEN® steel sheet products should not be sold when the intended use is for an architectural application, such as roofing and siding. U. S. Steel has consistently maintained this position because of the risk of corrosion from factors beyond the control of the COR-TEN® steel licensee (e.g. improper design, fabrication, erection and/or maintenance).”

The reason is that poor judgement that has been made. A number of building structures and cladding have failed – having become perforate. As the surface is damaged, the material rebuilds the patina and as a result the overall thickness of the plate becomes thinner and thinner.

Can COR-TEN® be painted?
Some manufacturers recommend that any vulnerable areas where full patination may not occur be painted, with high quality specification as you would for mild steel.