Corten™ steel – Your questions answered

What is Corten™ steel? Where does Corten™ steel come from? What can you do with Corten™ steel?

So many questions that will be answered in this section.
Corten™ steel will hold no secrets for you and you’ll fall in love with rust as much as we do!

What is Corten™ steel?

Corten™ steel is a structural steel with exceptional resistance to weathering. Its chemical composition leads to the creation of an oxidation layer on its surface. This orange layer is constantly renewed until a stabilization phase is reached. This layer, the patina, protects the base material from deterioration and does not alter the mechanical properties of the steel.

Corten™ steel is not traditional oxidized steel. Even though they may look similar at a certain stage, they nevertheless differ greatly, especially in their lifespan. The weathering resistance of Corten™ steel is nearly 4 to 8 times that of traditional steel.

Meaning of the word: Corten

CORrosion: High resistance to corrosion

TENsile strength: High mechanical resistance.

Corten™ is actually a brand of corrosion resistant products that were developed by United States Steel. Corten™ was subsequently licensed for production by other steel producers around the world.

CAUTION: Corten™ steel is very often covered with scale and oil to protect the steel from rusting prior to factory processing. To promote the development of an oxidation layer on its surface, it is necessary to apply a sandblasting or a special treatment. Corrolux is currently developing a solution to facilitate this step.

Where does Corten™ steel come from?

Corten™ steel originated in the United States in the 1930s in the mining community as they sought a solution to use steel without the expensive treatment of paint. Starting in the 1960’s, its use became widespread in Europe and Australia due to its ease of maintenance (no need for painting/galvanizing or protection) and the underlying cost savings. Today, Corten™ steel is now popular for aesthetic reasons but also environmentally friendly (no volatile organic compounds related to the paint finish, no galvanization and 100% recyclable).


Cor-Ten® is United States Steel Corporation’s (USS) trade name for an atmospheric corrosion resistant steel it developed for use where higher strength and longer life cycle material was desired. These steels naturally form an oxidation or patina layer when exposed to various atmospheric conditions. This patina is what protects the material from corrosion.

How do you achieve a special finish with Corten™ steel?

Corten™ steel allows you to give a unique and original look to all your projects. Corten™ steel has a matte surface that reflects less light and will evolve all over the years with its surroundings.

Corten™ steel is usually sold raw with mill scale (a thin protective layer to prevent rusting in the factory). To obtain a uniform and homogeneous oxidation, the surface must be sanded.

Keep in mind that the oxidation process usually stabilizes after 2 years (variable depending on the environment).

To obtain a more industrial finish (less uniform with a mixture of gray and orange), sanding should be avoided. An effective cleaner should be used to remove the scale. Corrolux is currently developing a solution that will be proposed to you soon. In the meantime, use an industrial degreaser. To obtain an industrial effect, leave the product outside.

If necessary and depending on the use, it is also possible to apply a varnish. This stabilizes the color, prolongs the life of the product and may be relevant to prevent rust stains on textiles depending on the use of the product.

The evolution of oxidation, stains and their treatment

How the oxidation of Corten™ steel takes place

For Corten™ steel to naturally coat with an oxidation film and reveal an orange layer, certain conditions are recommended: alternating dry and wet periods as well as good ventilation. The outdoor environment with its bad weather is therefore ideal for the formation of oxidation in a few weeks. The stabilization phase can take several months to a few years depending on the environment.

When the protective layer is damaged, it reforms spontaneously, it heals! It will always protect the base steel and minimizes material loss compared to ordinary steel.

How long does it take to get a nice finish?

The rate of oxidation and the orange color can vary on the same project depending on its context and its microclimate. The oxidation period can last from several months to several years.

If you have a layer of scale, your Corten™ steel may take up to 2 years to have a nice even layer of orange (depending on dry and wet periods) as it has to penetrate the scale that covers the Corten™ steel.

On the contrary, if you remove the scale, you will have an orange layer after a few days since the steel will be bare and will not have any protection.

What is the color of Corten™ steel?

Among the unique aspects of Corten™ steel: the changes in its exposed surface over the years. It goes from an initial dark gray finish to an orange patina in a few weeks. The patina continues to evolve and reaches a definitive dark brown color after several years, depending on local weather conditions.


The rate of change and final color of the Corten™ steel depends on the surrounding atmospheric conditions. Sulfur-dense atmospheres accelerate the oxidation process and give a darker patina. Frequent cycles of wet and dry conditions accelerate oxidation. To obtain a uniform and homogeneous coloring, the surface must be sanded before being subjected to atmospheric conditions. To obtain a more industrial finish, a simple cleaning with water will be sufficient. Industrial oxidation takes much longer to create.

Will Corten™ steel stain surrounding surfaces?

During the skating process of Corten™ steel, rust stains may settle on surrounding surfaces such as glass, natural stone, pavers, aluminum, etc. This is called “bleeding” or “trickling”.

These stains can be avoided if the structure is designed so that water does not run off these surfaces but onto surfaces that are less susceptible to staining. If necessary, clean these stains by rubbing with hot water regularly. This bleed is mainly stable iron oxide which is not dangerous for the environment, vegetation or wildlife.

Oxidation stabilization

The oxidation of steel is considered to have reached its maturity when it hardly causes any dirt or marks to the touch. The oxide layer is then strong enough not to stain clothing or surrounding surfaces.

Removing a rust stain?

If the Corten™ steel skating process is not stabilized, to the touch, it may leave marks on your hands or clothing or exterior item. It can be difficult to remove some stains from fabric due to the wide range of fabrics used in manufacturing today. However, if this happens to you, simply clean the fabric or material with water and scrub. If the stain is still present, use a biodegradable powdered stain remover such as Oxy Clean and wash with hot water. Always test on a small section and follow the manufacturer’s instructions.

The more often you clean your ”bleeding” patina, the easier it will be to remove it.

Can I use certain products to activate and accelerate oxidation?

Corrolux does not recommend the use of chemicals to activate and accelerate rust formation as they are often aggressive to Corten™ steel and can cause damage if used incorrectly. We are currently working on the design of a product line for treating and finishing Corten™ steel. We will inform you when it is available.

What are the installation recommendations for Corten™ steel for custom projects?

Steel is not always easy to install. This requires good preparation, a straight floor and good working tools. All of our pieces are computer cut and therefore sound perfectly straight. For best results when assembling, take the time to place each piece perfectly straight and aligned otherwise you may have problems with the assembly.

Is Corten™ steel best used in high humidity or marine environments?

Even for Corten™ steel, excess moisture should be avoided as it disrupts its surface layer and can result in the steel peeling. Moreover, the presence of salt destabilizes the protective layer of the material. In both situations, there will be repeated deterioration of the oxide layer resulting in premature wear of the steel.

In order to address premature degradation in such a context, custom designs can be made by adapting the thickness of the Corten™ steel and thus guaranteeing an extended life.

Corrosive atmospheres, such as distances within one kilometer of coastal waters, should be avoided as high chloride concentrations will damage the patina and increase the corrosion rate.

Optimize oxidation conditions

To promote the appearance of oxidation naturally, the ideal conditions are a frequent alternation of wet/dry periods and good aeration. It should be noted that it is very difficult to oxidize Corten™ steel on the inside.


Scale is a dark gray layer generated – and very difficult to remove – on the surface of steel sheets. It occurs during its manufacture. The patina will break through this layer of scale but it is a long process. Depending on your choices, this layer can be removed with a sandblasting.

The use, treatment and maintenance of Corten™ steel.

How do you maintain Corten™ steel?

Raw Corten™ steel requires no special maintenance. A simple annual visual inspection and water flow check is sufficient. Thanks to the patina layer, Corten™ steel can be used outdoors for decades.

Can I use Corten™ steel for interior projects?

When placed indoors, Corten™ steel will not oxidize. Although it is predominantly a material for exterior use, more and more architects and designers are looking to incorporate Corten™ steel into their interior design projects.

For residential projects, it is possible to place the product outside to oxidize it until you obtain a platinum that will be to your liking. Then place it in the location of your choice. Be careful, since there is no protection, if the patina is contaminated (glass of water, wine), it may remain marked.

For commercial projects or if you want a perfect finish, Corrolux can offer you this service. For a quote, contact us via

Can Corten™ steel contaminate any other material?

Yes, this type of steel can be oxidized in electrolytic contact with copper, lead or tin. It can also cause the oxidation of aluminum, stainless or zinc. It is therefore necessary to separate each of these materials during design/installation.

Can Corten™ steel be painted?

Yes, it is possible to paint Corten™ steel. If there is a scratch on the paint, the patina will fill the hole almost instantly and prevent the steel from degrading faster in that area.

Why are screws and bolts in stainless steel in Corrolux products?

Corrolux provides stainless steel hardware (screws, bolts, etc.) as it is more durable and unbreakable. Such Corten™ steel hardware could not guarantee the same mechanical strength. However, you can paint these items if you don’t like their appearance.

Is the atmospheric steel Corten™ steel?

Yes, these are synonymous terms.

Other information

Composition of Corten™ steel

Waxable steels offer improved resistance to atmospheric corrosion due to the addition of copper during steel fabrication. Other alloying components are included to increase the tensile strength and formability of the steel.

Corten™ steel gets its properties from careful handling of alloying elements (up to 2.5%), such as copper, nickel and chromium, added to the steel during the production process.

Atmospheric corrosion index

In November 1997, ASTM modified and improved the corrosion resistance specifications by emphasizing an “atmospheric corrosion index” that can be calculated to compare the relative service life of different grades and heat batches of corrosion resistant steels. It also required each ASTM grade to meet a minimum atmospheric corrosion index of 6.0, effectively improving the quality of the weathering steels produced. Corten steel products normally have a nominal atmospheric corrosion rating of 8.0.

Note that Corten™ steel is not suitable for use in a high temperature environment (above 200 degrees)

ASTM Standard - Weathering Steel

Weathering steels are a family of ASTM specifications.

ASTM specifications are A588, A242, A606-4, A847, A871-65 and A709-50W. The main branded product is Corten™. The selection of a particular ASTM weathering steel specification depends on the type of product desired. ASTM A588 covers structural shapes, plates and bars. ASTM A242 covers the steel plate to one-half inch thickness. ASTM A606-4 covers steel sheets and coils and ASTM A847 is for weathering steel pipes and tubes. ASTM A871-65 also covers the plate, but with a minimum yield strength of 65ksi.

Are there different types of Corten™ steel?

There are essentially two types of Corten™ steel: Type A and Type B Corten™. They differ primarily in the amounts of phosphorus alloyed into the mix.

CORTEN™ A / S355J0WP STEEL: generally referred to as “Phosphorus Steel” (P), an element that primarily promotes protective oxidation. It is mainly used for architectural applications. Corten™ A steel is typically produced in sheet or coil form (1.0 mm to 12 mm) and has applications in cladding and ductwork. The increased phosphorus and copper content contributes to the production of the oxidized patina that forms on the surface of the metal, reducing its susceptibility to rust and corrosion.

CORTEN™ B / S355J2W STEEL is used for highly stressed structures. This special steel can also be supplied with low-temperature resilience. Corten™ B steel is most commonly produced in plate form (15 mm to 50 mm).

CORTEN™ STEEL A and B meet and/or exceed the requirements of ASTM A606 Type 4.