The Fascinating History of COR-TEN and Weathering Steel

This is part one of our three part series on weathering steels. Read about the art & science of preweathering in part 2 and the manufacture and applications of preweathered weathering steels in part 3.

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.

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

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.

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.

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.

After a bit of testing and trials, we came up with a process and preweathered high-strength, low-alloy (HSLA) steel we now call Solanum Steel™. Solanum provides even more opportunities for using weathering steel in architectural and art applications.

In our next installment of this series on weathering steel, we’ll explain the preweathering process, including the oxidizing agents, methods, and results from various applications over time.

Read Part 2 in our series on weathering steel: The Art & Science of Preweathered Weathering Steel.