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Jerome Guth, ArcelorMittal Europe - Flat Products - Product Marketing
Georges Axmann, ArcelorMittal Europe - Long Products - Sales & Marketing

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Since the 1970s, the use of weathering steels, also known as corten or COR-TEN steels (for corrosion resistance and tensile strength), has spread through Europe. Engineers appreciate its corrosion resistance in combination with high strength steels. Weathering steels help reduce maintenance costs while lightening structures - especially useful for bridges. Architects like the expressiveness of this material (often used for facades) and its ability to fit into urban landscapes and natural environments. Additionally, the environmental footprint is greatly improved with the use of weathering steels due to the absence of coating. This article will describe the processes leading to the self-protecting behaviour of weathering steels.

Why are weathering steels corrosion resistant?

When weathering steel is exposed to the ambient atmosphere it develops an initial layer of iron oxide in the same way as carbon steel. The rate of oxidisation depends on how much oxygen, moisture, and atmospheric contaminants can access the surface of the metal. In the initial stages, a complex mix of iron oxides covers the surface to create a layer of rust. As the process progresses, the rust layer forms a barrier against the corrosive agents and the rate of corrosion slows. On a standard low alloy carbon steel, this iron oxide layer is porous. Over time, the layer detaches from the surface of the metal and the corrosion process starts again. The oxidation rate progresses in increments that depend on weather conditions and the chemical and mechanical aggressiveness of the environment. It can end with the complete destruction of the metal.

To increase the resistance of weathering steels to corrosion, alloying elements such as copper, phosphorous, nickel, or chromium are incorporated into the steel. These alloys lead to the formation of an oxide layer which remains stable and adheres to the metal's surface. A so-called 'patina' thus develops as the weathering steel is exposed to alternate wetting and drying cycles. As well as being aesthetically pleasing, the patina creates a protective barrier which impedes the further access of oxygen, moisture, and pollutants into the steel. This results in a much lower corrosion rate than that of unalloyed steels.

Conditions for long lasting protection

No constant humidity
If it is left in permanently wet or damp conditions, weathering steel will oxidise like any other unprotected carbon steel. A succession of wet and dry phases is mandatory to form a stable oxide layer on the surface. Weathering steels should not be used in:

• Sheltered locations with damp conditions
• Permanent contact with water. Welded structures require special attention to avoid the retention of water.
• Soil or covered by vegetation

Weathering steels used in these locations should be protected with paint. The paint must extend above the level of the water, soil, or vegetation.

No aggressive atmosphere
High concentrations of chloride ions negatively affect patina adherence. According to EN ISO 9223, weathering steels should not be used within two kilometres of coastal waters unless airborne chloride levels do not exceed the salinity classification S2 (Cl < 300mg/m 2/day). Direct contact between weathering steels and de-icing salts used on roads should be avoided.

No atmospheric pollution
Air-borne pollutants and industrial fumes can affect patina development. Corrosion is much higher if the metal surface is covered by solid particles such as dust or dirt. These particles can retain moisture and salts. In an industrial atmosphere, large amounts of sulphur dioxide (SO2) are detrimental to the compactness of the patina. EN ISO 9223 advises that weathering steels should not be used without protection in environments above P3 (SO2 > 200mg/m 2/day).

ArcelorMittal's weathing steel

ArcelorMittal has more than 80 years of experience in producing weathering steels. It started in the 1930s when the company Denain-Anzin created ‘InDA steel,’ the first copper-phosphorous steel. ArcelorMittal has continued the production of this type of steel, now called lndaten®. Its chemical composition has been improved to achieve better strength and corrosion resistance. lndaten® is supplied in flat formats such as heavy plates or steel coils. They are used to create facade elements, flat panels, blades, solar shading elements, profiled sheets, framing, or containers. In the 1970s, ArcelorMittal began to produce Arcorox® branded weathering steel for long products such as hot-rolled I and H sections, channels, and angles. Its use spread rapidly as the European infrastructure market began to grow.

The branded weathering steels grades produced by ArcelorMittal meet the requirements of EN 10025-5 and are CE marked.

Multiple uses and unique aesthetics

The first major work using weathering steels was realised by Eero Saarinen who designed the John Deere World Headquarters in Moline, USA in 1960. Since then, weathering steel has been used in various outdoor steel applications: bridges, buildings, railway cars, and even works of art! Today, this material continues to create enthusiasm among designers thanks to its unique aesthetic and technical benefits.

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