Technical Resources
Weld Microstructure and Properties of SAF2205 and SAF2507 Duplex Stainless Steel
- Forms : Plate & Forging &Pipes
- Grade : SAF2205 & SAF2507
- Weld Microstructure and Properties
Duplex stainless steels possess high strength and toughness, as well as good resistance to intergranular corrosion, stress corrosion, and corrosion fatigue. They are widely used in fields such as petrochemicals, offshore chemical product storage tanks, and seawater heat exchangers. Among duplex stainless steels, SAF2205 is the most widely used, while the super duplex stainless steel SAF2507 has gained increasing attention due to its superior corrosion resistance [1]. The mechanical properties and corrosion resistance of duplex stainless steel welded joints depend on whether an appropriate austenite-ferrite ratio can be maintained in the joint. When the proportion of austenite and ferrite is close to 50% each, the performance is optimal. If more ferrite is retained in the weld seam and heat-affected zone after welding, the tendency for intergranular corrosion and susceptibility to hydrogen-induced cracking (embrittlement) may increase [2]. This paper provides a comparative analysis of the microstructure and properties of the base metal, weld seam, and heat-affected zone of SAF2205 and SAF2507 duplex stainless steels after welding, as used in chemical containers.
-
Welding Tests and Weld Composition
Both SAF2205 and SAF2507 were welded using the shielded metal arc welding (SMAW) process. The groove was prepared in an X-shape, and multi-pass welding was employed. The electrodes used were E2209 and E2594, respectively. After welding, a visual inspection of the joints was conducted, revealing no defects such as undercut, cracks, lack of fusion, incomplete penetration, porosity, or slag inclusions. The weld reinforcement height ranged from 1 to 3 mm. The measured chemical compositions of the post-weld SAF2205 and SAF2507 weld seams are shown in Table 1.
Tab.1 Weld seam composition fo SAF2205 and SAF2507(wt%)
Elements C Si Mn Cr Ni Mo N SAF2205Weld Composition 0.03 0.54 0.2 25.4 9.4 3.98 0.24 SAF2507Weld Composition 0.02 0.51 1 23.4 8.6 2.97 0.16 -
Post-Weld Microstructure
2.1 Metallographic Structure
After welding, metallographic analysis was conducted on the weld zone, heat-affected zone, and base metal of the welded joints. The etchant used was a mixed solution of potassium ferricyanide and sodium hydroxide. The results are shown in Figure 1. The microstructures of SAF2205 and SAF2507 base metals are similar, with the austenite phase distributed in an island-like pattern within the ferrite matrix. However, under the same welding conditions, the microstructures of the weld zones and heat-affected zones of the two materials differ significantly. The weld zone and the heat-affected zone of SAF2205 are coarser compared to those of SAF2507 and exhibit Widmanstätten structure characteristics, which can easily lead to embrittlement of the joint.
2.2Phase Ratio
Duplex stainless steels exhibit better corrosion resistance, especially against chloride ion corrosion, compared to austenitic and ferritic stainless steels. The primary reason lies in their unique phase structure and phase ratio. Therefore, the phase ratio in welded joints is one of the most critical indicators for evaluating welding quality. In this study, the phase ratios in different regions of the two materials were determined using the area method via material micrography and metallographic analysis. The results are shown in Figure 2. The ferrite contents in the base metal, weld zone, and heat-affected zone of SAF2205 are 58%, 51%, and 65%, respectively, while those for SAF2507 are 47%, 47%, and 49%, respectively. The phase ratio in the weld zone of SAF2507 is the same as that of the base metal. The difference in the heat-affected zone compared to the base metal is only 2%, and it is closer to 50%. For SAF2205, the phase ratio in the weld zone is 7% lower than that of the base metal, closer to 50%, which is more favorable than the base metal ratio. The phase ratio in the heat-affected zone is 7% higher than that of the base metal, exceeding 60%. The phase ratios in the weld zones of both materials largely depend on close to 50%. The phase ratio in the heat-affected zone of SAF2205 is more significantly affected by welding, while that of SAF2507 is less influenced, resulting in a more stable phase ratio.
3. Mechanical Properties
The mechanical properties of welded SAF2205 and SAF2507 duplex stainless steels are shown in Table 2. The tensile specimens of SAF2205 fractured in the heat-affected zone, whereas those of SAF2507 fractured in the base metal. The mechanical properties of the heat-affected zone in SAF2205 are more significantly influenced by welding heating.
4. Corrosion Tests
The corrosion behavior of SAF2205 and SAF2507 duplex stainless steel welded joints was tested by immersing them in a 6 wt% FeCl3 corrosion solution. Pitting corrosion specimens with dimensions of 10mm×8mm×2mm were cut from the base metal, heat-affected zone, and weld zone of the welded joints. The specimen surfaces were smooth, and pitting corrosion tests were conducted according to the ASTM G84 standard. The test temperature was (22±1)∘C, and the test duration was 24 hours. The corrosion performance of the two materials was compared using the weight loss method. The corrosion test results are shown in Table 3. The data in the table indicate that the corrosion resistance of the base metal, heat-affected zone, and weld zone of SAF2507 is significantly better than that of the corresponding regions in SAF2205. The corrosion rates did not exceed the 10 mg/(dm2⋅d) limit specified in the ASTM G84 standard, and no pitting was observed on the surfaces. Both joints exhibited good corrosion resistance.
During the first welding pass, the softened zone is located approximately 4 mm from the weld centerline, with a width of about 1 mm and a softening magnitude of approximately 11%. After the second welding pass, heat diffuses outward, causing the softened zone to shift outward to around 6 mm, with its width increasing to 1–2 mm. The softening magnitude remains largely unchanged, while the hardness in other areas also decreases. After the third welding pass, heat continues to diffuse outward, resulting in a softened zone located between 5 mm and 9 mm from the weld centerline, with a softening magnitude of about 15%.
The tensile performance of the welded joints was tested using strip specimens as shown in Figure 1 and a CSS-44000 electronic universal testing machine. The results are presented in Table 3. It can be seen that the strength of the ER50-6 low-matching wire welded joint decreases by about 10%, and the elongation decreases by approximately 25%. The fracture occurs in the heat-affected zone. For coiled tubing subjected to plastic strain during operation, such mechanical inhomogeneity may lead to strain concentration in the softened heat-affected zone, thereby reducing the tensile and fatigue performance of the joint [7]. Therefore, when developing welding procedures, the influence of the welding thermal process on the heat-affected zone should be fully considered to achieve a more uniform strength distribution in the welded joint, thereby improving its tensile performance.
Stainless steel curtain walls contribute to building sustainability
- Forms : Plate,Square tube
- Grade : stainless steel 304/304L
- stainless steel façades
The building and construction sector accounts for nearly 39% of global energy-related CO₂ emissions. Of this amount, approximately 11% originates from embodied carbon: emissions produced during material extraction, manufacturing, transportation, and construction.¹ All the carbon emissions involved in the construction process, including the emissions from the supply chain, are effectively contained within the infrastructure once it is built. Tearing it down at the end of its life means new labor, new resources, and new emissions. The same goes for maintenance and repairs, especially if they are frequent.
In response, architecture, engineering, and construction (AEC) stakeholders are under growing pressure from regulators and clients to reduce the full life cycle emissions of materials used in the built environment. What a building will look like on the outside is now firmly intertwined with how it will perform as a physical, financial, and even ethical object.
While decarbonizing the industry will not be achieved by one solution alone, advancements in materials science and life cycle assessment have enabled practitioners to target key areas with precision. Stainless steel, when specified appropriately in demanding façade applications, offers a combination of durability, corrosion resistance, and recyclability that aligns well with growing requirements for service-life longevity and circularity.
In this article we examine how stainless steel façades can contribute to greater sustainability in the built environment, while providing realistic assessments of their role, capabilities, and limitations within the broader drive to lower the sector’s carbon footprint.
The pressure to reduce carbon across the building sector
The built environment is a key focus of new sustainability legislation around the world. In Europe, the implementation of the EU Green Deal includes the updated Energy Performance of Buildings Directive and the Corporate Sustainability Reporting Directive (CSRD), both of which bring embodied emissions into regulatory scope.
In the United States, initiatives such as Buy Clean California are enforcing carbon reporting for construction materials purchased with public funds.² In parallel, global frameworks such as the EU Taxonomy, LEED, BREEAM, and other green building certification systems can influence developer decision-making.³
All of these mechanisms direct attention not only toward operational energy use but also to the trillion-dollar question of “how long.” The longer a high-emission material remains in use, the lower its annualized impact. Short-life low-cost materials that require frequent replacement or repainting represent a hidden emissions burden. Reducing this burden requires not only low-emissions materials, but ones which will stand the test of time.
Stainless steel for façades: where does it make sense?
Stainless steel is not widely used across structural construction due to its upfront cost, as well as architects’ and fabricators’ lack of familiarity with the material compared to concrete or carbon steel. However, in targeted exterior envelope applications where mechanical and visual longevity is a requirement, stainless steel can add significant value. These are typically high-profile and high-budget projects where architectural finish retention has been deemed vital.
Using stainless steel here provides two specific sustainability-oriented benefits:
Extended service life with minimal degradation
Avoidance of protective coating systems (paints, lacquers) and their maintenance cycles
Notably, stainless steel use in façades is still reflective of a niche high-performance market rather than a broad building-level solution. Its contribution to improving the entire carbon footprint of a building is limited in percentage terms but meaningful in durability terms.
One of stainless steel’s greatest strengths is its natural resistance to rust, corrosion, and weathering, particularly in harsh urban or coastal environments if the appropriate grade has been selected. Unlike other materials that require the application of protective coatings, repainting, or frequent maintenance, stainless steel gains its properties from the microscopically thin oxide film that forms on its surface. And if scratched, this film will self-heal. This means buildings retain their original look and structural performance virtually forever, reducing both long-term costs and environmental impact.
The performance characteristics that distinguish stainless steel for façade cladding include:
Corrosion resistance
This is the defining feature of stainless steel, and varies significantly by grade. In austenitic grades such as 316L (Supra 316L/4404), added molybdenum enhances this protection in marine or urban environments.⁴ Duplex grades (e.g., Forta DX 2205) combine austenitic and ferritic microstructures for higher mechanical strength and corrosion resistance, although their use remains limited in façade applications due to forming demands.
Recyclability and recycled content
When produced with scrap, stainless steel has a high recyclability rate and does not degrade with repeated use. Outokumpu’s Circle Green stainless steel — as well as having eliminated 95% of all scope 1 and 2 CO2 emissions by using only renewable sources and low-carbon electricity for production — has an industry-leading recycled content of over 90%, and up carbon footprint as low as 7% of the global average. This is a major driver for its low Scope 3 emissions. Outokumpu’s overall carbon footprint is 75% lower than industry average.⁵
Flatness and optical uniformity
For large façades with repeating cladding elements, visual consistency is essential. Surface finishes such as Deco Linen require tight quality control to avoid visible pattern or hue variation across panels. Mechanical processing such as stretch-leveling and tension-leveling are crucial factors in producing material that meets architectural expectations.
Color stability
Unlike coated metals, stainless steel finishes do not rely on brittle paint systems that peel or chalk over time. Color or tone is achieved through surface structuring (e.g., embossing or blasting) or via physical modifications like electrochemical coloring.⁶ When properly applied, these are permanent solutions that require no reapplication over decades.
Grade selection based on environmental corrosivity
Environmental conditions should determine the grade of stainless steel specified for a façade project. ISO 9223 categorizes outdoor environments into corrosivity classes (C1 to CX), with grades recommended as follows:⁷
C1–C2 (Very low–Low): Typical grades include 1.4016 (430), 1.4301 (304)
C3 (Medium): Supra 316L/4404 becomes appropriate
C4–C5 (High–Very High): Duplex grades such as Forta DX 2205 or even Ultra 904L (1.4539) are recommended
CX (Extreme): Super-austenitic grades like Ultra 254 SMO® or Forta SDX 2507 are required
Misalignment between environment and grade can result in corrosion phenomena such as pitting and tea staining, especially in coastal regions with salt spray or in cities with poor air quality.⁸
Embodied carbon: understanding emission profiles
Embodied carbon in stainless steel varies based on input materials and production methods. At Outokumpu, production using Electric Arc Furnaces (EAF) combined with 90% low-carbon electricity of our electricity mix globally results in an average product carbon footprint of approximately 1.6 kg CO₂e per kg of stainless steel based on lifecycle assessment, well below European and global averages.⁹
Circle Green further reduces this footprint to below 1 t CO₂/t, achieved by using:
Biobased fuels (biogas, bio-coke)
100% low-carbon electricity
Up to 100% use of low-emission raw materials such as scrap
For comparison (industry averages):¹⁰
Stainless steel: 3.0–4.0 kg CO₂/kg
Aluminum: 10–12 kg CO₂/kg
Concrete: 0.5–1.0 kg CO₂/kg
While stainless steel has a higher per-kilogram footprint than concrete, its superior circularity, and higher durability — especially in exposed applications — offers better long-term amortization when considered over a service life of 75–100+ years.
Façades are not structures: a realistic impact assessment
Stainless steel, due to cost and density, is unlikely to replace concrete or carbon steel for superstructure components in most buildings. Thus, its contribution to reducing a building’s overall embodied carbon is modest, and limited primarily to the envelope.
However, façades play a role disproportionate to their volume when considering:
Thermal performance (when combined with insulation)
Aesthetic and brand value
Maintenance-related emissions
Weather resistance and building integrity (resilience)
How switching to duplex stainless steel saves weight on offshore platforms
- Forms : Plate & Coil &Pipes
- Grade : duplex stainless steel 2205/250
- offshore platforms
Walkway gratings provide safe and easy access for personnel and are an essential element of offshore structures in the oil and gas industry. Demand has also increased considerably in recent years thanks to the growth of offshore wind.
HDG gratings are the most used solution. They are available as ready-to-use, cut-to-length, products to cover the required surface area. The other candidate material in this application, glass fiber reinforced plastic (GRP), is not fire resistant, doesn’t have the same strength, and can’t withstand low temperatures.
HDG (Hot-Dipped Galvanized) gratings are the most used solution.
The challenge with using HDG gratings offshore is the constant exposure to salt spray and wide variations in temperature. These mean that they generally require frequent maintenance and replacement. This is encouraging the industry to look at stainless steel that can offer a maintenance-free solution capable of lasting the long design-life of 30 years or more expected for offshore structures.
Crevices are inherent in the design of gratings. That rules out the use of 316L as this austenitic grade is sensitive to crevice corrosion in marine environment. However, an attractive option is to use a duplex stainless steel that combines many of the beneficial properties of ferritic and austenitic stainless steels. In particular, the duplex microstructure contributes to a high mechanical strength and high resistance to stress corrosion cracking.
Forta comes to the fore
Initial projects for offshore gratings have used Outokumpu’s Forta EDX 2304. It has high strength and elongation – Rp0.2 ≥ 500 MPa, A5 ≥ 25 %. Furthermore, it has a higher chromium content in comparison to austenitic grades, giving it higher pitting and crevice corrosion resistance than 316L. An added benefit is its lower nickel content, which is an alloying element subject to significant fluctuation in pricing, so it has a much more stable price.
It should be noted we do not recommend that Forta EDX 2304 is deployed close to seawater in the splash zone. For the best corrosion performance, it needs to be around 10 to 15 meters above the seawater level in areas where there is consistent flushing by rainwater.
For more severe applications we would suggest Forta DX 2205, and if the grating is in the splash zone and potentially in contact with seawater then a step up to super duplex Forta SDX 2507 is recommended.
Suitability for welding
Two welding methods are in general use for the manufacture of walkway gratings: shielded metal arc welding (SMAW) and manual arc welding (MAW), both are carried out with stick electrodes. Duplex stainless steels have, in general, very good weldability and are compatible with most welding methods used for stainless steel grades.
Weight-saving potential
An important advantage of duplex grades is high strength, which enables light-weighting. To evaluate its potential, trials have been carried out with manufacturers to substitute lean duplex material in their existing HDG grating designs. The two critical criteria for walkway gratings are load carrying capability and deflection.
Tests performed with varied loadings over a pre-set span have confirmed that gratings manufactured in lean duplex stainless meet the load and deflection requirements with a reduced section (30 x 3mm compared with 30 x 5mm for HDG steel). This offers the potential for a weight saving of up to 40%.
The key implication of this weight saving is that duplex stainless steel presents an attractive option for modification projects. This is because a key driver for these projects is to not increase the total weight of the platform to avoid the need for rebalancing.
Furthermore, the significant potential saving in the construction of a new structure could allow the installation of additional equipment with no additional weight burden. Reduced weight could also allow the possibility for structures to be built onshore and then transported more easily to an offshore installation.
Reducing environmental impact and carbon footprint
Utilizing duplex grades in production instead of carbon steel solutions with painted or HDG surfaces will minimize pollution and hazardous waste as stainless steel does not require surface treatment. The longer life cycle, compatible with the 20-year plus life of offshore installations, makes it a superior environmental choice over carbon steel that will require regular maintenance and possible replacement. The increased life expectancy of the duplex material will also improve the carbon footprint of the project. Long life and low maintenance are also very significant factors in optimizing the total cost of ownership (TCO) of the project.
Faster installation
Avoiding the need for welding operations can generally allow faster installation for offshore projects. This can be achieved with duplex material with bolted connections and has a direct impact on the labor time required, resulting in cost savings. Furthermore, when HDG gratings are cut or modified during construction or modification the cut surfaces require treatment against corrosion, which is not needed for stainless steel.
The ideal choice for specific applications
In some offshore applications, minimizing the weight of walkways is the top priority so GRP will continue to be used. In other cases, where long life and low maintenance are not critical, then HDG will remain the natural choice. The role of duplex steel is to offer an interesting new alternative for walkways in more demanding applications. This is where its combination of high strength, light weight and corrosion resistance makes duplex steel a particularly attractive option for achieving the optimum total cost of ownership (TCO).
The choice of material for gratings can also have a significant impact on the sustainability of the project. At the end of life, GRP has no residual value, leaving operators with waste that is troublesome to process. In contrast, stainless steel is fully recyclable so the gratings will retain considerable value even after a long service life.
（Excerpt from an interview with Outokumpu）
316L Used in the Pharmaceutical Industry
- Forms : Plate & Fitting &Pipes
- Grade : Stainless steel 316L
- Pharmaceutical Equipment
Chemical and Pharmaceutical Industry
316L is usually applied in chemical reactors, tanks, and pipelines. It performs well in aggressive chemical conditions, making it ideal for use in pharmaceuticals, bioprocessing, and storage of chemicals. Moreover, alloy weldability is critical in achieving high levels of sealing in such systems.
Grade 316 Stainless Steel for the Pharmaceutical Industry:
Globally known for its superior corrosion resistance to chlorides and acids – SS 316 is the second most austenitic stainless steel with the addition of 2% molybdenum making it greatly resistant to chloride ions too.
SS 316 is specifically used to make pharmaceutical products that stay in direct contact with fluids such as system tanks, pipelines, or bulk transportation of chemicals/corrosive liquids.
Its increased resistances also help in keeping the cleaning procedure easier by enabling the use of stronger cleansers and detergents without any worries of damage or appearance changes.
Duplex 2205 vs. Super Duplex 2507
- Forms : Plate & Coil &Pipes
- Grade : Duplex Steel 2507 DIN1.4410
- Offshore Drilling
Duplex stainless steels are a class of stainless steels with a dual-phase microstructure of austenite and ferrite, combining high strength with excellent corrosion resistance. Compared with conventional austenitic grades (such as 304 and 316L), duplex steels perform significantly better in petrochemical, offshore, and desalination industries.
Among them, 2205 (UNS S32205) is the most widely used standard duplex stainless steel, while 2507 (UNS S32750), as a super duplex, demonstrates superior performance in extreme environments.
Comparison: Duplex 2205 vs. Super Duplex 2507
2.1 Chemical Composition (wt%)
| Grade | Cr | Ni | Mo | N | Others |
| 2205 | 21–23% | 4.5–6.5% | 2.5–3.5% | 0.14–0.20% | Mn ≤2% |
| 2507 | 24–26% | 6–8% | 3–5% | 0.24–0.32% | Cu ≤0.5% |
Key Difference: 2507 has higher Cr, Mo, Ni, and N → better corrosion resistance and strength compared to 2205.
2.2 PREN (Pitting Resistance Equivalent Number)
- 2205: ~35–37
- 2507: ~40–45
Conclusion: 2507 is a “super duplex” grade, offering greater reliability in seawater and chloride-rich environments.
2.3 Mechanical Properties
| Property | 2205 | 2507 |
| Yield Strength | ≥450 MPa | ≥550 MPa |
| Tensile Strength | ≥620 MPa | ≥800 MPa |
| Elongation | 25–30% | 15–25% |
Conclusion: 2507 offers higher strength, but slightly lower ductility.
2.4 Corrosion Resistance
- 2205: Good resistance to pitting and crevice corrosion, suitable for medium-to-high chloride environments.
- 2507: Can withstand long-term service in natural seawater (Cl⁻ ~19,000 ppm) with minimal risk of pitting or stress corrosion cracking (SCC).
2.5 Applications
- 2205: Petrochemical equipment, heat exchangers, desalination (freshwater side), chemical tanks, auxiliary piping on offshore platforms, pulp and paper bleaching equipment.
- 2507: Subsea pipelines and wellhead equipment, desalination brine/reject systems, marine heat exchangers and condensers, high-pressure seawater injection systems.
2.6 Cost & Fabrication
- 2205: Lower cost, better workability and weldability.
- 2507: Higher cost, more difficult to fabricate and weld; requires strict heat input control and matching filler metals.
2.7 Overall Comparison
- Strength: 2507 > 2205
- Corrosion Resistance: 2507 > 2205 (especially in seawater/high chloride environments)
- Toughness/Ductility: 2205 > 2507
- Cost & Fabrication: 2205 is more economical and easier to process
- Typical Use:
-
-
2205 → Medium-to-high corrosion environments (refining, pulp & paper, desalination
-
Why Develop 2507 (Super Duplex)?
Limitations of 2205:
- PREN ~35–37 → still at risk of pitting in long-term exposure to natural seawater.
- Increased risk of SCC in high-chloride environments at >50 °C.
Advantages of 2507:
- PREN ~40–45 → reliable in natural seawater and high-temperature, high-chloride service.
- Higher strength (yield ≥550 MPa) → suitable for subsea manifolds and high-pressure injection systems.
In other words: 2205 is the general-purpose grade, while 2507 is the safeguard grade for extreme conditions.
The Evolution of the Duplex Stainless Steel Family
Driven by diverse application needs, duplex stainless steels have expanded into different categories:
| Category | Representative Grades | PREN | Features | Applications |
| Lean Duplex | 2304 (UNS S32304) | 22–25 | Low-cost, stronger than 304/316, moderate corrosion resistance | Architecture, freshwater, mild environments |
| Standard Duplex | 2205 (UNS S32205) | 35–37 | Balanced strength & corrosion resistance; most widely used | Chemicals, petrochemicals, desalination freshwater side |
| Super Duplex | 2507 (UNS S32750), Zeron 100 | 40–45 | Excellent seawater resistance, higher strength | Offshore oil & gas, seawater heat exchangers, brine systems |
| Hyper Duplex | S32707, S33207 | 48–52 | Extreme corrosion resistance + ultra-high strength, higher cost | Deepwater (>2000 m) oil & gas, harshest conditions |
Conclusion
- 2205: General-purpose, suitable for most applications requiring both strength and corrosion resistance.
- 2507 / Zeron 100: Preferred choice for extreme seawater and chloride environments.
- 2304: Low-cost option, suitable for mild corrosion.
- Hyper Duplex: Next-generation materials, designed for ultra-harsh deepwater and high-chloride service.
Application of Super Duplex Steel 2507 in Coastal Engineering
- Forms : Plate & Forging &Pipes
- Grade : Duplex Steel 2507 DIN1.4410
- Coastal Works
Marine structures are not only critical for oil and gas exploration, they are growing in importance for the renewable energy sector. This includes floating solar schemes as well as offshore wind farms that can be sited well over 10 km from shore. Corrosion is a major issue for these structures as they are exposed to a chloride-rich environment, wet-dry cycles, high humidity, and microbiological attacks. They may also be subjected to abrasion and wear from sand, floating waste and currents. Material selection, together with the methods used to protect against corrosion protection, therefore plays an important role in determining the reliability, maintenance needs and lifetime of structures which are often expected to stay in service for 20 to 25 years or more.
raditionally, carbon steel has been protected against the corrosive marine environment by applying a coating. This adds to the initial fabrication cost, while a corrosion allowance is often added to the wall thickness, resulting in increased material costs. Furthermore, coated carbon steel structures typically require maintenance such as recoating, replacement of corroded steel plates and repair work during their service life. Switching to stainless steel, with its inherent corrosion resistance, offers the potential to reduce life cycle costs as well as the impact on health and safety and the environment.
Rather than the conventional 304L and 316L grades, using higher strength duplex stainless steel offers particular advantages for marine structures. This family of materials combines both austenitic and ferritic microstructures to offer the best properties of each main type of stainless steel: corrosion resistance and high strength. This allows the design of lighter structures based on thinner gauges.
In addition to saving weight and material cost, duplex grades offer further advantages such as extended fatigue life and high hardness that gives better resistance to wear and erosion. In addition, duplex grades contain a lower amount of nickel compared to both standard austenitic and high-performance austenitic stainless steel, which means they have less volatile pricing. The performance advantages of higher strength duplex grades are illustrated in Figure 1.
Figure 1 – Comparison of strength and corrosion resistance of stainless steel grades – the Forta grades are duplex stainless steels. CPT is the critical pitting temperature in degrees Celsius at which corrosion starts, it is a useful measure that helps design engineers compare the likely performance of different materials.
Since the corrosion resistance of duplex stainless steel doesn’t depend on an external coating, the risk of failures caused by possible damage to the coating can be avoided. This is important, especially in offshore structures where limited access makes inspections and repair work hard to carry out.
A cost-effective solution
Duplex stainless steel has a higher initial cost compared to carbon steel, so will not seem appealing to a designer that considers only the material price per tonne. However, that does not show the true story as using duplex grades will make a significant impact over the life of a structure through reduced weight, longer lifetime and less maintenance. This is illustrated in Figure 2.
Figure 2 – It is important to compare duplex stainless steel and carbon steel on a total Life Cycle Cost (LCC) basis
It is important to make comparisons on a case-by-case basis. Even so, our experience is that the potential weight saving from switching to lean duplex stainless steel can offer savings of between 30 to 40%. That is before the LLC costs are taken into consideration.
Welding duplex stainless steels – not difficult, but different
The welding of duplex stainless steel is not particularly difficult. But it is different to other steels. In fact, the weldability and welding characteristics of duplex stainless steels are better than those of ferritic steels, although not generally as good as austenitic steels. The properties of a duplex weldment are strongly affected by the welding parameters such as heat input. Therefore, it is vital to establish the correct welding procedures to obtain a structure that delivers the required strength and corrosion-resistance.
Galvanic corrosion is not a major concern
There are concerns about galvanic corrosion when stainless steels are in contact with other metals such as carbon steel, galvanized steel, copper and brass. However, duplex stainless steel is not typically attacked in this way since it is usually the most noble material in the galvanic couple. Galvanic corrosion can usually be prevented with proper design and by electrically insulating dissimilar metals.
Different grades for different zones
Offshore structures can be exposed to different corrosion zones that require different material solutions and corrosion protection methods. These zones are defined in Figure 3.
Figure 3 – Schematic representation of marine corrosion zones
Atmospheric zone
In the atmospheric zone, marine structures are exposed to airborne chlorides and therefore at risk of chloride-induced atmospheric corrosion. The marine atmosphere is a demanding environment in which factors such as atmospheric salt content, temperature, and relative humidity affect the corrosivity. These factors are mainly dependent on geographical location. Sheltered conditions where rainwater is not able to naturally clean the surfaces of the structures will lead to a more severe situation.
In the atmospheric zone, corrosion is commonly controlled by using corrosion-resistant alloys or protective coatings. A variety of stainless steels can be used in this zone depending on the corrosivity of the environment and the design of the structures. It is also important to note that the surface roughness of stainless steels can have a significant impact on their performance since a smoother surface typically offers higher corrosion resistance to atmospheric corrosion.
In less aggressive marine atmospheres, lean duplex stainless steels like Forta LDX 2101, Forta DX 2304 and Forta EDX 2304 may be the most cost-effective options. They can be expected to perform well if there is natural cleaning by rainwater and the design does not contain severe crevices. Otherwise, a higher corrosion-resistant duplex stainless steel like Forta DX 2205 may be better.
Splash zone
The splash zone is an extremely aggressive environment. Designing structures in this zone is recognized as a critical challenge. For carbon steel, the highest corrosion rates occur in this zone, but this is not necessarily the case for stainless steel.
In the splash zone, the structures are exposed to wetting by oxygen-rich seawater, wet-dry cycles and UV radiation. The wet-dry cycles can increase chloride contents as the water evaporates, leading to highly severe conditions. Furthermore, structures may be subjected to erosion due to spray, waves and tidal actions, as well as mechanical stresses due to ice drifting, collisions, and floating debris.
Corrosion protection of carbon steel structures in the splash zone requires special consideration. The most common way to protect carbon steel against corrosion is to combine coating with a corrosion allowance. In cases where protective coatings are applied, the risk of mechanical damage, such as wear or scratching of the coating must be considered. Once the coating is damaged, a process of preferential corrosion in the exposed area of the base metal can be accelerated by the galvanic coupling formed. Cathodic protection is not reliable in the splash zone due to the lack of continuous contact with the electrolyte (seawater). In this zone, high-performance stainless steels are reliable alternatives to carbon steel. Depending on how aggressive the conditions are, Forta DX 2205 and Forta SDX 2507 may be options
（Excerpt from an interview with Outokumpu）
QUOTE, BUY, TRACK
We make it easy to get instant pricing and purchase your metal at the click of a button. Track your order progress, get notified when it ships, and follow your shipment online until it’s delivered. It’s that easy!