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

Images:
©ArcelorMittal Europe
©Gilles Martin, Courtesy of Infosteel
©Paul Kozlowski/ACAUM - Atelier BETTINGER DESPLANQUES Architectes Associés

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.

Related link(s)

The European Convention for Constructional Steelwork (ECCS) has released a new European Design Guide for the Use of Weathering Steel in Bridge Construction.

Fundamental updates

A vital element of Europe’s transport infrastructure, bridges also play a major role in connecting countries, representing cultures and functioning as landmarks. Generally in bridge construction, weathering steel provides the most cost-effective and environmental benefits with similar mechanical properties to structural steel.

This design guide updates the ECCS design guide from 2001 – The Use of Weathering Steel in Bridges. The last 20 years have seen the Eurocodes become recognised throughout Europe in addition to many other weathering steel construction standards coming into force. In addition, thanks to practical application and research projects, extensive new knowledge of weathering steel has been garnered. As such, the new European Design Guide for the Use of Weathering Steel in Bridge Construction offers a plethora of vital information. The design guide can be downloaded for free in an electronic version here.

Text:
Constructalia

Images:
ArcelorMittal Europe
Xavier Font Solà
ECCS

Related link(s)

ATRIO is an impressive mixed-use project located in the centre of Bogota, Colombia. Composed of two towers, North and South, it will offer 250 000 m² of office space and 10 000 m² of public space at ground level. It is the first high-rise project for ArcelorMittal in South America. 90% of the steel sections for the construction of the North tower were provided by ArcelorMittal.

3900 tonnes of sections from ArcelorMittal plants in Luxembourg and Spain

Constructing a high-rise building in a seismic zone and in a country where most constructions are made of concrete was a sizable challenge taken up by the teams involved in this project.

Designed by the architecture firms Rogers Stirk Harbour + Partners and El Equipo Mazzanti, the ATRIO Towers feature impressive column elements and facade trusses built to resist seismic activity and to improve the torsional behaviour of the structure - a breakthrough for local architecture and structural design.

To comply with the design and its technical requirements, ArcelorMittal International provided 3900 tonnes of steel sections, including 400 tonnes in HISTAR® grade (ASTM A913), produced by ArcelorMittal Europe – Long Products in Differdange.

Customers 100% convinced by the steel solution

ArcelorMittal Europe – Long Products’ association with the project (through our technical assistance department) began in 2013 with AECOM consultants and subsequently with ARUP engineering.

A number of reasons were behind the choice to work with ArcelorMittal:

•    HISTAR® steel offers the exact weldability properties that the client needed and does not require preheating.
•    We offer the widest range of sizes among steel producers.
•    We can provide long-length sections as well as cut-to-length sections.
•    The project required steel made with at least 50% recycled material and our sections are 100% scrap-based.

Apart from its mechanical properties, the use of steel helped to improve the design of the tower. Thanks to the jumbo steel sections embedded in concrete, column size was reduced resulting in elegant slender composite columns with sophisticated diagonal lines in the facade.

Breathing new life into the city centre

This development provides a boost to the Centro Internacional area, the city’s former business district, by bringing new business, tourism, public transportation, and culture to the area. The construction of the North Tower, measuring 200 metres (44 floors), is the first phase of the project. 50 000 m² of office space, 4 600 m² of public services, and 1800 m² of retail space are available.

The high quality of our products, the service provided, and the efficient cooperation between the teams at ArcelorMittal International and ArcelorMittal Europe – Long Products were important factors in our successful participation in this project.

The construction of the ATRIO North Tower started in 2015 and is now complete. It is one of the first major steel high-rise buildings in South America, and it certainly won’t be the last.

An award-winning tower

ATRIO North Tower attained LEED GOLD certification and has won several awards, including:

•    Council on Tall Buildings and Urban Habitat (CTBUH) 2021 Awards in the following categories: Best Tall Building 200-299 Metres 2021 Award of Excellence, Best Tall Office Building 2021 Award of Excellence, and Structural Engineering 2021 Award of Excellence.
•    ICCA ACESCO 2019 Prize in the Building category.

An international team

Architects
- Rogers Stirk Harbour + Partners (British)
- El equipo Mazzanti (Colombian)

Engineering
- ARUP (British)
- P&D (Colombian)

Steel Fabricators
HST Estructuras consortium composed of:
- TECMO S.A. (Colombian)
- HB Estructuras (Colombian)
- SUPERMETAL (Canadian)

Construction
ARPRO-EllisDon consortium

Consultants
AECOM

Management
QBO Constructores

Text:
ArcelorMittal Europe Communications
Constructalia

Images:
ArcelorMittal International

Related link(s)

Atrio (Torre Norte)