Advanced Metal Knowledge

Corten vs Weathering Steel

Introduction

Corten and Weathering Steel are materials heavily used in the architectural industry, but there is widespread lack of understanding around these steel alloys. In fact, there is confusion about the names themselves – Corten is simply a generic trademarked name of Weathering Steel, so they often are used interchangeably and do refer to the same material. For the purposes of this article, I will use the Corten name since that is the most widely recognised.

History of Corten

In 1930’s Industrial America, there were numerous demands for a strong steel that did not require painting and had increases resistance to atmospheric corrosion. Ideal uses for this steel would be for railway heavy haulage carriages and on the underside of steel bridges that were notoriously difficult to maintain. Therefore, scientists at US Steel discovered weathering steel which they promptly trademarked and named Corten. US Steel still hold this trademark along with ArcelorMittal. The name Corten comes from two of its key characteristics, Corrosion resistance and tensile strength. It is light (sheets are manufactured at a standard ¼ inch thickness) and exceptionally strong.

The rust-like patina was a by-product, rather than a intended facet, of this discovery. Although Corten is still used for industrial purposes (construction of containers being a great example) it has become most well-known for its incorporation in buildings and public space. It is particularly beloved by architects, who admire the uniqueness of each section after the oxidation process is complete, and the natural evolution which renders conventional environmentally harmful maintenance, such as painting, unnecessary.

Science behind Corten

Corten is a steel alloy, which means it is ‘mixed’ with a combination of chromium, copper, silicon, and phosphorus during the production phase. By varying the amounts of these elements, the mechanical properties of the resulting weathering steel are changed which is reflected in the different grades of Corten, which will be explained in more depth later in this article.

Most unalloyed steels have a porous oxide top layer, which holds moisture, resulting in rust. This rust layer will eventually flake off, exposing a new oxide layer to rust. As this process is repeated, eventually the steel with rust through. However, although Corten is not rustproof, the chemical reactions of the alloyed elements to meteorological conditions resists corrosion by forming a protective rust-coloured oxidation over the steel. This protective layer, often referred to as a patina, stops deeper rust penetration after the weathering process is complete.

The striking orange-brown colour of the patina comes from the copper content in the alloy mix, which is usually around 5%.

As the oxidation process is heavily dependent on meteorological conditions, there is no exact answer on how long the weathering process will take. As a rule of thumb, the process will take 6 months. In marine environments, the protective patina will form very quickly, but in rural environments free from pollutants the patina may take 2-3 years to form, if at all.

Where is Corten used?

Corten was first used for architectural purposes by the famous Finnish architect Eero Saarinen in 1964, when he designed John Deere’s HQ. He wanted a material that reflected the tough, earthy use of John Deere products which and Corten, post oxidisation, is perfect for that role.

Since then, Corten has become increasingly commonplace as an architectural material despite criticism from the uninformed branding it ‘unfinished’ and ‘rusty’. Alongside it’s uses in construction and heavy industry, a recent trend has seen it being adopted in a variety of domestic and furniture settings, such as planter edging and in gardens.

In the UK, Corten rose to prominence with its use as the primary material in Anthony Gormley’s 1998 Angel of the North sculpture, near Newcastle.

Corten Challenges

Take care when welding sections together. Corten can we welded but choosing the correct filler metal is vital as welds must weather at roughly the same rate as the material being joined and have similar tensile strength. Single pass joint welds can be done without using specialist Corten filler material because the heat causes the Corten to dilute its alloyed elements. These alloyed elements are picked up by the filler wire, resulting in the filler wire developing a patina almost as corrosion resistant as the base material itself. Allowable filler metals include: E70C-6 M, ER70S-3, ER70S-6 and E71T-1.
Ensure water can drain off easily. Corten is not rustproof, and if there are any water pools (such as in welded areas), these areas will experience higher corrosion rates resulting in the structural integrity being compromised.
Consider the external conditions. Corten is not suited to marine or humid environments, because the protective patina may not stabilize. Take for example the infamous Omni Coliseum, located in Atlanta, Georgia where Corten was heavily used in the structure. Due to the humid environment, the Corten did not stop rusting and developed large holes which people used to gain admission without paying. The arena lasted 25 years before being demolished, which must surely rank as one of greatest examples of non-sustainable construction.
Beware of rust-coloured runoff. During the weathering process, some run-off is inevitable. This runoff goes onto pavements and surrounding areas causing discolouration. This can be cleaned off so as soon as the patina is stable, conduct a clean on all affected areas. If any runoff is unacceptable, one option to prevent it is to artificially weather the Corten before installation, although will come with additional cost and time considerations.
Proceed with caution when painting Corten. Corten can be painted although it rather defeats the object of selecting Corten for the natural distressed finish. Approach finishing Corten the same as other steel products but be aware that painting Corten means that the oxidised layer will not form in time if scratches or cracks appear in the paint.

Corten Environmental Concerns

Corten should rate highly on the list of environmentally friendly materials because it is essentially maintenance free if used in the correct locations. But…

Due to incompetence and lack of knowledge, Corten is often used in incorrect locations. See the example of the Omni Coliseum above. By no means was this an environmentally friendly building! However, if you do your research to make sure that the atmospheric conditions of the project area suit Corten, it will have a positive environmental impact. It will be as long lasting as any other type of building material and there is no maintenance needed.

Corten Grades

The most important thing when selecting Corten is to choose the correct grade. This is a very difficult topic to understand as there as numerous different grades alongside confusion if Corten is the same as weathering steel.

The most recognised grades used for Corten are Corten A and Corten B. The differences between these two explained below:

Question	Corten A	Corten B
Where to use?	Architectural Metalwork, Aesthetically Led Projects	Structural Elements, Heavy Industry, Marine Containers
Why?	-Only available up to 12.5mm Thick -Fully Self Seals	-Available up to 40mm Thick -Increased Phosphorus content offers increased weather resistance
Phosphorous Content?	0.01 – 0.02%	0.03%
ASTM Grade	A242	A588
Euro Norm	S355JOWP	S355JOW, S355J2W