Cor-Ten: a naturally patinating wonder

Are you considering using Cor-Ten? Let's take a look at the history of this material and its properties.

Cor-Ten has unique properties and is well-suited for various facade projects. Its versatile use is also aided by the fact that it is suitable for implementations that strive to contribute to sustainability. Some materials, including Cor-Ten, have an expected lifespan of over a hundred years. It is 100% recyclable, and no chemicals are used in its production. It's no wonder that it is one of the most preferred materials.

First Uses

The history of Cor-Ten® dates back to 1933 when it was innovated and patented by US Steel as an exceptionally mechanically resistant material. At that time, it was primarily used in railroad freight cars to handle heavy bulk loads.

However, it soon became known for its important advantage - controlled corrosion, for which the material is currently most renowned. The discovery of this unique property led USS to trademark Cor-Ten®.

It also made its way to facades. One of the first famous designs was created by Finnish architect Eero Saarinen. He used a steel facade in the 1960s for the John Deere headquarters in Illinois, USA, where the headquarters still resides today. This is a good example of sustainability, as the original cor-ten facade still adorns the building today.

When designing and installing the material, several characteristics need to be considered. Primarily, the longevity of the facade stems from proper designing and well-executed installation.

What You Should Definitely Know About Cor-Ten

A groundbreaking property of this material is its increased ability to resist atmospheric corrosion. A significant factor in this is its carefully developed and selected composition, certain steel quality, and the chemical composition of alloying elements such as copper, nickel, chromium, and phosphorus. The result is a layer of oxide on the surface of the steel, known as patina.

"Under normal atmospheric conditions, the patina layer remains fully impermeable and permanently protects the material beneath it. It prevents further corrosion, thus limiting the loss of steel thickness. The Finnish Technical Research Centre VTT studied the effect of patina for a whole 32 years. In that time, the loss of Cor-Ten material was only 70 µm, or 0.07 mm," clarifies Anssi Lehmonen from Ruukki.

What should you be cautious about? Primarily, overlapping and details at joints. Water and moisture can easily penetrate between overlapping steel sheets. Because corrosion in the joint quickly absorbs oxygen dissolved in water, it creates an environment with low oxygen content where contact corrosion occurs. This is a local form of corrosion, which, however, is much faster than typical corrosion. According to laboratory studies of Cor-Ten®, this speed can be up to ten times higher.


"What does this mean in practice? During a 60-year lifespan, there will be a loss of material of 1.2 mm, while the thickness of the material used in Cor-Ten® facades is typically 1 to 1.5 mm. To prevent this, it's important to separate the steel from other materials. Rubber EPDM seals, for example, work well for this," adds Anssi Lehmonen.

Only rubber seals on the screw pads are not enough. It is essential to ensure that there is a sufficiently large opening to allow movement during temperature differences and to use stainless anchoring materials. If the cladding is correctly installed, the protective oxide layer will remain intact, and rust stains will only appear minimally under the cladding.

To protect the material during delivery, Cor-Ten is delivered raw and covered with a temporary protective oil or water-based wax. This protection is especially important due to rain. If this layer is cleaned on-site before installation, the oxidation process will occur much sooner, and the steel facade will achieve its smooth, firm, and balanced appearance faster. A significant advantage of timely cleaning of the surface and removal of oil/wax is also the minimization of rusty particles that would otherwise run down the facade with the oil/wax when exposed to rain.

A Versatile Material Suitable for Interiors

The time required to create patina depends on the weather and location. Patination needs cycles in which the steel gets wet and then dries again. Achieving the final layer of patina usually takes 12 to 24 months.

In the initial phase of steel patination, there is a risk of staining other materials, as corrosion dissolves in the runoff water. It is therefore good to direct rusty wastewater away from the remaining parts of the facade. For example, stains can be easily removed from aluminum, but are very difficult to remove from concrete and other porous materials. In addition, constant contact with water accelerates material loss. Therefore, Cor-Ten is not the most suitable material for gutter systems.


Various types of weather-resistant steels can be found on the market. "At SSAB, we manufacture the original weather-resistant steel Cor-Ten, which we label based on European licensing alongside our products in our manufacturing. Other types of weather-resistant steel, like the entire range of S355W, do not relate to Cor-Ten, therefore they cannot and are not labeled as such," explains Anssi Lehmonen from Ruukki.

Cor-Ten can also be used indoors. In this case, it needs to be cleaned from the temporary protective layer and exposed to external conditions for some time, or the oxidation process can be accelerated artificially by treating it with a special patination preparation.

Ruukki and its parent company SSAB are the only manufacturers of Cor-Ten steel and facade products (Liberta panels, facade slats, and Design Cor-Ten S7 including accessories) based on the USS license in the European market.