Steel-making giant ArcelorMittal believes a new form of steel it has developed has wide-ranging applications in the offshore environment
One of the world’s leading steelmakers, ArcelorMittal, believes a new type of steel it has pioneered could reduce corrosion in offshore windfarms.
Foundations and other structures such as towers for offshore wind turbines and offshore substations are exposed to extreme environmental conditions that lead to an increased risk of corrosion, potentially reducing the service life of structures and requiring the use – and maintenance – of corrosion-protection measures such as coatings to ensure the long-term performance and reliability.
The steel developed by ArcelorMittal was described in an award-winning poster presentation at Wind Europe 2024 in Bilbo, Spain in April 2024. In the poster presentation, ArcelorMittal senior project engineer Krista Van den Bergh noted the offshore environment poses unique challenges for steel structures due to the aggressive conditions induced by seawater and atmospheric chlorides.
She explained the company’s research and development efforts at its R&D centre in Ghent in Belgium have led to the creation of a cost-effective corrosion-resistant steel with a combination of good corrosion resistance, mechanical properties and weldability.
Weldability of the saline-resistant weathering steel has been tested using commercial wire, producing corrosion-resistant welds, and examples of the new type of steel have already been produced in an industrial facility in Gijon in Spain, demonstrating the feasibility and robustness of the concept, and the ability to produce it at the scale that would be required for use in the offshore wind sector.
The saline-resistant weathering steel, which has a chemical composition tailored to produce a protective patina in the offshore environment while meeting strict requirements for weld toughness, has an S420 strength level that can be comfortably achieved for plates of 20 mm thickness and, as Dr Van Den Bergh explained, this strength level is also possible for plates as thick as 50 mm.
Dr Van den Bergh said in the last couple of years, the focus of development had shifted to corrosion testing the industrially produced saline-resistant weathering grade. “Special attention is being given to atmospheric exposure testing at marine locations in moderate and highly saline environments,” she said. In these tests, the corrosion performance of the saline-resistant weathering steel had been compared to different types of steel commonly used in the marine environment.
“After testing, the corrosion performance of the saline-resistant weathering steel was up to 20% better than conventional steel,” she explained. “The performance was up to 20% better compared to S355MLO in medium saline environment C4.”
She further explained that, at the end of 2023, the new steel grade was standardised and inserted into ISO standard ISO630-5:2023 for structural steels with improved corrosion resistance, enabling ArcelorMittal to expose steel at customers’ facilities in the offshore environment.
Weathering steel was developed many years ago and has been used in many different applications, but until now it has not been suitable for use in the marine environment. 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 of this process, 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.
ArcelorMittal explains that, 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 ‘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.
So-called weathering steels have been in use since the 1970s and the use of corten or COR-TEN steels (for corrosion resistance and tensile strength), has spread through Europe. This type of steel is suitable for a wide range of applications and engineers appreciate its corrosion resistance in combination with high strength steels. Weathering steels help reduce maintenance costs while reducing the weight of structures such as bridges, and architects have regularly used the material, often for facades. A further advantage of weathering steel is that, in land-based applications, its environmental footprint is much better than conventional steel because it does not need to be used with coatings to protect it.
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