The terminology around weathering steel can often be confusing. COR-TEN, Corten, weathering steel, and preweathered steel are all used, often interchangeably, to refer to the rust-hued steel that is used today for everything from bridge structural members to world-class architecture. But there are important differences between these materials.
To help untangle the nomenclature — and to provide clarity on how these materials differ in application — it helps to understand their origins.
COR-TEN: The origins of weathering steel
Weathering steels were first developed by US Steel Corporation back in the 1930s. They were looking for a steel alloy for their ore carrying hopper carts they would use to transfer the iron ore and coal to the furnace. They needed a stronger, more durable steel alloy that had exceptional hardness and did not need to be painted. It was only later they realized this alloy had interesting corrosion resistant traits that could be marketed in other industries, so it trademarked the name COR-TEN® for this corrosion-resistant steel alloy. COR stands for corrosion resistance and TEN stands for tensile strength. (Today, while the terms Corten and COR-TEN are used interchangeably, COR-TEN® is a trademarked product of US Steel.)
The steel was not introduced as an architectural metal, however, until the 1950s. In 1964, the John Deere Headquarters, in Moline, Illinois, opened and was one of the first major architectural structures to be clad in weathering steel. Eero Saarinen designed the John Deere complex but, unfortunately, he died before seeing it completed.
John Deere World Headquarters in Moline, Illinois.
William Hewitt, then president of the John Deere company, said:
The seven buildings should be thoroughly modern in concept but should not give the effect of being especially sophisticated or glossy. Instead, they should be more ‘down to earth’ and rugged.
Saarinen chose this special weathering steel, Corten, to age and provide the earth tones and the “down to earth” feel his client desired. He knew the metal would age gracefully and create a deep, natural tone as the thickened oxide formed. The deep rich surface oxide exhibits a natural brownish-red tone that is both durable and stable.
Many artists have also incorporated weathering steel into their work, the most well-known being Richard Serra. Serra has used weathering steel plates on numerous massive sculptures found around the world.
Richard Serra’s 67-foot-tall Corten steel Vortex, installed outside the Modern Art Museum of Fort Worth in Fort Worth, Texas.
The great minimalist artist Donald Judd also used weathering steel in his works. Judd would often preweather large plates and then allow them to continue to oxidize outdoors in Marfa, Texas. He also created smaller weathering steel wall units where he worked the surface oxide to create a fine velvet-like appearance.
The Keeper of the Plains is a 44 ft Corten steel sculpture by Kiowa-Comanche artist Blackbear Bosin that stands at the confluence of the Arkansas and Little Arkansas rivers in Wichita, Kansas.
What is weathering steel?
In the context of art and architecture, steels are rarely used uncoated because of their tendency to absorb moisture and develop the porous and friable oxide commonly called rust. Coatings can consist of a sacrificial zinc, as in the galvanizing process where the steel is electroplated
or immersed into a molten bath of zinc. Other metal coatings in common use are the aluminum–zinc coatings and Galfan™, a combination of zinc and rare earth metals. These are all considered sacrificial coatings of other metals that involve the protection of the base steel by offering up their electrons as well as acting as barrier coatings.
In addition to the preweathering rust effects, there are also treatments that induce a blue or black protective coating on the steel, such as Oscura blackened steel. These involve the integration of metal salts onto the surface of the steel. There is some diffusion of the iron into these coatings to create the dark, sulfide, phosphate or selenide salt. Not as protective as the sacrificial coating of zinc in the galvanized process, these coatings are often applied to smaller features, pieces and areas.
Weathering steel, with its copper-bearing properties, forms its own distinctive oxide barrier over the surface. This barrier grows outward and inward from the surface. Unlike common rust, the oxide formed on weathering steel is a thick, impervious ferro-oxyhydroxide that will afford the base metal extensive resistance to corrosion. Below are some of the most common coatings applied when necessary for internal and external design, in art and architecture.
Surface Coatings on Steel Used in Art and Architecture
Coated with other metals – zinc, zinc–aluminum, chrome plate
Metal oxides – blackening, bluing, and phosphate treatments
Coated with paints – organic paints, polyesters, acrylics, fluorocarbons
Coated with glass – porcelain
Oxide – weathering steel
Fully-weathered Corten steel.
Protective Oxides and Applications of Weathering Steels
High-strength, low-alloy weathering steels are designed to develop a special thick iron oxide capable of slowing down the effects of atmospheric corrosion much like the green patina that develops on copper surfaces exposed to the atmosphere. This oxide grows on exposure to moisture and air.
As with all uncoated steels, the oxide develops when the surface is exposed to moisture and oxygen. For the weathering steels, however, the difference lies in the nature of the oxide that develops over time.
The oxide that gives weathering steel its protective ability is ferric oxyhydroxide, FeO(OH) an oxygenated version of ferric oxide — essentially, a mixture of oxides and hydroxide. This attractive, orange to rich purple brown oxide acts as a protective barrier to the base metal. When correctly formed, this tough layer of oxide develops into a surface that resists atmospheric degradation in outdoor environments. Once the oxide develops correctly, further changes are very slow in most environmental exposures.
Steel Surfaces, fourth in Zahner’s Architectural Metals Series, provides a comprehensive and authoritative treatment of steel applications in architecture and art. It covers everything from the history of the metal and choosing the right alloy, to detailed information on a variety of surface and chemical finishes and corrosion resistance. The book also features case studies that offer strategies for designing and executing successful projects using steel.
Weathering steel also contains copper, phosphorus, chromium, and nickel as alloying elements. The small addition of copper along with the chromium, causes a change in the oxide formation at the surface. The initial oxide develops rapidly at first, a matter of days or weeks, depending on the moisture and temperature. Both are critical for the development of this initial oxide layer. Without moisture, as in a desert climate, the weathering steel may take months or years to form, if at all.
Corten steel panels early in the weathering process.
The development of the oxide is a chemical reaction on the surface. The chemical reaction occurs as the atoms of iron and oxygen collide. The more frequent these collisions the faster the reaction. The warmer the surface of the steel the higher the number of these reactions. This thermodynamic reaction is called the activation energy and the warmer the surface the greater the energy.
The development of the iron oxide requires a surface temperature at least 10°C (50°F) along with oxygen laden moisture. The iron atoms go into the solution and collide with the oxygen. A surface that is too hot, however, will evaporate the moisture before it has time to create the ferrous ions needed to combine with the oxygen. If the temperature range is right and moisture is present, the oxide will quickly form. This rich oxygenated layer develops when the surface undergoes a series of wet and dry cycles.
Initially, a light layer of ferric oxide forms. This oxide is loosely established on the surface and is porous and brittle. It easily comes off onto one’s hands or clothing.
Different rates of weathering occur with Corten steel, as seen on the facade of the Musée Soulages in Rodez, France.
Detail view of the weathering pattern on the Corten facade of the Musée Soulages.
In a short time, if the environment is correct, it covers the entire surface with a soft, friable layer of oxide. In a way it is similar to what happens when brushing velvet. Brushing this friable oxide layer will leave a mark on the surface. This is the result of collapsing the microscopic peaks of oxide that initially forms. This initial layer of ferric oxide is responsible for the rust stain that forms below weathering steel surfaces that are allowed to form the oxide naturally. It is unavoidable during the weathering process. As the oxide develops, it initially is loosely attached and will sluff off as moisture drips down the surface, and grabs the particles only to redeposit them on adjacent materials.
The weathering steels have a rich color tone that comes across as a material of the earth. Like brick, wood, or patina copper, weathering steel has a very natural, pleasing appearance once the oxidation takes place.
The facade of the Gulf Islands Residence, designed by AA Robins, features Zahner’s Solanum Steel™.
Next: A Look at the Art and Science of Preweathering
The intrinsic beauty of weathering steel and its popularity among designers only seems to be growing, but as we’ve discussed earlier, controlling and conquering the decaying condition of rust — the weathering indicator for steel — is paramount for successful, and durable, applications of this material.
At Zahner, we’ve worked with Corten for nearly as long as it’s been around.
However, about 25 years ago we realized a major drawback, from an aesthetic point of view, was the amount of time it took for the deepest colors to appear. People did not want to wait and watch as their building corroded and stained the surrounding stonework and sidewalks with the umber hues of rust.
The idea of preweathering this type of steel seemed to be the answer. Many artists over the years pushed the metal to achieve this preweathering on their sculptures using acids and wetting the surface. In addition to being hazardous to one’s health, it’s also impractical to scale for large projects.