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New saline weathering grade to reduce maintenance costs of offshore structures
The interest from industry is high and fi rst projects for medium salinity are already under preparation
Krista Van den Bergh
New saline weathering grade to reduce maintenance costs of offshore structures
Although the offshore market is seeking to reduce its huge maintenance cost, current standards for offshore grades (EN 10225 and API 2W) focus on toughness and weldability, while weathering resistance is not mentioned. Therefore, a new plate steel product – with enhanced corrosion resistance to atmospheric marine/saline weathering conditions – is being developed for offshore applications. Target applications are topside painted steel structures: e.g., fixed and floating platforms, offshore wind towers, electrical substations and water ballast tanks.
The challenge of the project is to develop a corrosion-resistant steel grade that meets the stringent offshore welding and toughness requirements. The development of our metallurgical concept resulted in steel compositions with a good combination of corrosion-resistance, mechanical properties and weldability. Industrial trials in a heavy-plate mill have proven the feasibility and robustness of the concept. The results of mechanical characterisation showed that a S420 strength level is comfortably achieved for plates of 20 mm thick, and this is within reach for plates of 50 mm thick as well. High Charpy V-notch impact values were measured at -40°C, even at mid-thickness. Research is currently focusing on corrosion testing and weldability of the industrially produced saline-resistant weathering grade, with special attention to real atmospheric exposure testing at marine locations.
Due to the lack of standardised corrosion tests, a variety of accelerated corrosion tests have been defined with different test conditions in order to predict the long-term outdoor corrosion behaviour in offshore conditions. Since the enhanced corrosion resistance of weathering steel is attributed to the formation of the patina, the corrosion layer is also fully characterised. Industrial saline-resistant weathering steel panels were exposed to accelerated corrosion tests with longer exposure times in a high-chloride environment together with lab samples. These tests confirmed that the right composition was selected through conception screening. The formation of
dense and compact layers on the salineresistant weathering steel proves its better corrosion resistance, which has also been observed in lower mass loss.
Since only real offshore exposure corrosion tests can provide conclusive information concerning the corrosion performance of the steel product, multi-year exposure tests have been launched and are currently running. The corrosion resistance of the steel concept has first been studied in its bare (unpainted) state – yet, as structural steel is used in the painted state in a lot of applications, painted saline weathering steel panels were also subjected to corrosion testing. The exposure locations are specified as medium salinity (corresponding to corrosivity classes of C3-C4) and high salinity (corresponding to C5-CX). After 2 years of exposure to medium salinity, the corrosion results confirm the improved resistance (up to 22%) compared to the conventional S355G10 grade in a near-shore environment. As the corrosion rate was seen to slow down between year 1 and year 2, it is to be expected that saline weathering steel will remain below the tolerable corrosion rate of 6 µm/year. In the high-salinity atmosphere, no protective and/or adherent rust layer has yet been formed – so, longer exposure times are needed to evaluate the formation of a protective layer in this severe environment.
