Breaking the coated carbon steel cycle: North Sea data makes the case for duplex stainless
Published: 10 June, 2026
By Mia Liimatainen, Application Development Manager – Marine & Energy, and Sukanya Hägg Mameng, Senior Corrosion Engineer, Outokumpu
Oil and gas exploration has long dominated the conversation around marine structures. The sector is now joined by a new generation of offshore infrastructure: wind installations and aquaculture facilities. They are united by a shared demand for long-term reliability. Structures must be capable of performing safely and reliably for decades, in some of the most aggressive exposure conditions on earth. Material selection is therefore not only a technical decision; it can influence CAPEX, OPEX, maintenance needs, operational reliability, and lifecycle performance.
This puts a spotlight on the tradition of building with coated carbon steel. Maintenance in offshore environments is never simple. Access is difficult, weather windows are narrow, and the cost of intervention can dwarf the original material saving. When the full lifecycle is considered, the economics of coated carbon steel can look considerably less attractive than the purchase price suggests.
Duplex stainless steel offers a compelling combination of high strength, enabling lighter structures, and excellent corrosion resistance for long-term durability and reduced maintenance in aggressive marine environments. In offshore energy structures, duplex stainless steels are especially relevant for secondary steel applications, with examples including gratings and structural hollow sections used in access structures such as platforms, walkways and offshore bridges.
To achieve the intended lifecycle benefits, selecting the right stainless steel grade for the exposure conditions is essential. To support material selection for demanding offshore environments, my colleagues and I at Outokumpu designed and executed a structured two-year field test program in the North Sea.
Understanding the duplex family
The duplex family encompasses a range of grades with meaningfully different performance characteristics.
Duplex stainless steels have a mixed austenitic-ferritic microstructure, which gives them their distinctive combination of strength and corrosion resistance. They contain less nickel than austenitic grades — a practical consideration for budgeting given the metal's price volatility. Lean duplex grades such as Forta EDX 2304, an enhanced version of the standard 2304, and Forta LDX 2404 offers corrosion resistance almost equal to the standard 2205 grade. Where the marginal uplift from DX 2205 is not required, LDX 2404 offers a more cost-effective alternative. At the top sits super duplex Forta SDX 2507, engineered for the most demanding marine and offshore service.
Corrosion resistance across these grades is quantified using the pitting resistance equivalent number, or PREN, calculated as: PREN = %Cr + 3.3×%Mo + 16×%N. Higher PREN values, alongside critical pitting temperature (CPT) and critical crevice temperature (CCT), indicate greater resistance to the localized corrosion mechanisms (pitting and crevice corrosion) that represent the primary threats in chloride-rich marine conditions.
Designing the test programme
The test programme was conducted at Fraunhofer's test centre for maritime technologies on Heligoland island, off the German coast in the North Sea. Five grades were exposed across splash, tidal, and immersed corrosion zones over 24 months. These comprised the EDX, LDX, DX and SDX duplex grades, alongside Supra 316L/4404, a standard austenitic grade included as a reference point.
Rectangular test coupons measuring 400 × 90 × 3 mm were prepared from each material. Each coupon was fitted with a standard crevice former assembled at 3 Nm torque in accordance with ISO 18070:2016, allowing us to assess both free-surface and crevice corrosion performance simultaneously. Evaluation followed ASTM G46-12 and was carried out by the Outokumpu Research Centre.
The atmospheric zone sits above the splash zone and is exposed primarily to airborne chlorides. The splash zone lies just above the high tide level and is exposed to seawater splashes and wave action, while the tidal zone extends from the low tide line up to the high tide line, being submerged or exposed depending on tidal variations. The submerged zone involves continuous immersion.
What the results showed
One of the key findings is that no pitting corrosion was observed on any of the duplex grades across any of the three exposure zones over the full 24-month period. It is clear that pitting corrosion does not present an issue for any of the duplex grades across all three exposure zones. Furthermore, the presence of biofilm did not significantly affect the corrosion performance of the stainless steels, despite the formation of thick biofilm deposits on the exposed coupons in both the tidal and immersed zones.
Supra 316L/4404 told a different story. It exhibited pitting corrosion in both the splash and atmospheric zones within 12 months — a useful illustration of why the assumption that “stainless = corrosion resistance” is a mistake.
Crevice corrosion is where nuance emerges for duplex grades. It was identified as the primary challenge for duplex stainless steel in marine service, with the splash zone presenting the most severe risk. Notably, Forta SDX 2507 was the only grade that did not exhibit crevice corrosion in the splash zone when tested with standard crevice formers at 3 Nm.
However, crevice attack was observed on SDX 2507 samples where gaps had formed through the physical installation of coupons on the test rig, at contact pressures exceeding the controlled 3 Nm condition. This demonstrates that crevice geometry and contact conditions have a direct bearing on performance.
Taken together, the results position super duplex Forta SDX 2507 as the preferred grade where service conditions are most aggressive and design geometry creates unavoidable crevice conditions in the splash zone. For less demanding zones and applications where crevice risk can be managed through design, the leaner duplex grades offer strong performance at a lower total cost.
The material selection imperative
The broader lesson is that corrosion resistance should not be assumed simply because the word “stainless” appears in a specification. To achieve the intended benefits of stainless steel, the grade must be matched for example to the application, exposure conditions and design.
For offshore energy and coastal infrastructure projects, this is a commercial imperative as much as a technical one. The ability to reduce or eliminate maintenance interventions over the service life of the structure can create substantial lifecycle value. The data shows that duplex stainless steel, when properly specified, can help make this possible.
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Mia Liimatainen
