The engineer who we recently talked to was quite clear: he needed a pipe that could stay intact in seawater for 20 years, without costs of maintenance, and at a reasonable price. And specifically to solve this problem, duplex stainless steel grades were invented. While standard austenitic grades such as 304 and 316L can be used effectively in many situations, when it comes to aggressive chlorides and high pressures, or when the failure of the pipe brings larger expenses than this material costs itself, something more advanced is needed. This is what duplex stainless steel brings: a physical structure that consists of 50% ferrite and 50% austenite, enabling you to achieve the properties of both classes of steel in one material. In the following section, we will discuss the most popular duplex stainless steel grades, 2205 and 2507, in terms of their composition, mechanical properties, corrosion resistance, and suitability for various applications.
If you know that stainless steel is the appropriate material to use and just need to choose a typical austenitic grade, refer to our Stainless Steel Pipe Grades Charts for a comparison of 304, 304L, 316, 316L, 321 and 347 grades.
What Is Duplex Stainless Steel?
It is called duplex stainless steel due to the dual-phase microstructure that it possesses, which consists of approximately fifty per cent ferrite and fifty per cent austenite, unlike the fully austenitic structure attained in materials that are also known as austenitic steel, such as 316 and 304 steel. The importance of this balance is seen since the property of being ferritic contributes to the lengthening and stress corrosion resistance property, while the fact that it is also austenitic allows it to possess toughness and ductility. In a way, the combination of these phases allows the duplex grade to remain twice stronger than a typical austenitic grade with similar or better corrosion resistance rates.
2205 and 2507: UNS, EN and ASTM Equivalents
The table below shows the UNS designation, the European EN number, and the governing ASTM standards for both grades covered in this guide.
| Grade | UNS Number | EN Number | Governing ASTM Standard |
| 2205 (Duplex) | S32205 / S31803 | 1.4462 | ASTM A240 plate, ASTM A790 pipe, ASTM A182 fittings |
| 2507 (Super Duplex) | S32750 | 1.4410 | ASTM A240 plate, ASTM A790 pipe, ASTM A182 fittings |
Duplex 2205 vs Super Duplex 2507: Chemical Composition Chart
Composition ranges below follow ASTM A240 and ASTM A276 for duplex and super duplex stainless steel.
| Grade | Carbon (max) | Chromium | Nickel | Molybdenum | Nitrogen |
| 2205 (Duplex) | 0.030% | 22.0% to 23.0% | 4.5% to 6.5% | 3.0% to 3.5% | 0.14% to 0.20% |
| 2507 (Super Duplex) | 0.030% | 24.0% to 26.0% | 6.0% to 8.0% | 3.0% to 5.0% | 0.24% to 0.32% |
Mechanical Properties Comparison
The table below includes 304 and 316L as reference grades, so you can see exactly how much stronger duplex and super duplex actually are, rather than taking that claim on faith.
| Grade | Tensile Strength (min) | Yield Strength (min) | Elongation (min) | Approximate PREN |
| 2205 (Duplex) | 620 MPa | 450 MPa | 25% | 35 |
| 2507 (Super Duplex) | 795 MPa | 550 MPa | 15% | 43 |
| 304 (reference grade) | 515 MPa | 205 MPa | 35% | 18 |
| 316L (reference grade) | 485 MPa | 170 MPa | 35% | 24 |
PREN and Corrosion Resistance: Why 2507 Costs More
PREN, the Pitting Resistance Equivalent Number, is a simple way to rank how well a stainless steel resists pitting and crevice corrosion. It is calculated from the chromium, molybdenum and nitrogen content of the alloy, and a higher number means better resistance. Looking at the table above, 2205 sits around a PREN of 35, already well ahead of 316L at roughly 24. 2507 pushes that further to around 43, which comes from its extra chromium, molybdenum and nitrogen compared with 2205. That jump is exactly why 2507 costs more and is exactly why it is worth paying for once you are dealing with seawater, brine or other severe chloride environments where 2205 would eventually start pitting and 2507 will not.
Applications: Where 2205 Fits, Where 2507 Fits
Duplex 2205
- Pressure vessels: Suitable for chemical and process vessels requiring higher strength and chloride resistance.
- Heat exchangers: Used in systems exposed to chloride-containing process fluids.
- Storage tanks: Suitable for chemical storage where corrosion resistance and structural strength are required.
- Piping systems: Commonly used for chemical processing and other corrosive service conditions.
- Flue gas desulfurisation equipment: Used in systems exposed to chlorides and corrosive flue gas environments.
- General chemical processing: A practical choice when higher strength and better chloride resistance than 316L are required without moving to a super duplex grade.
Super Duplex 2507
- Offshore platforms: Suitable for equipment and piping exposed to prolonged seawater contact and chloride-rich conditions.
- Subsea piping: Used where high strength and resistance to seawater corrosion are required.
- Desalination plants: Suitable for seawater intake, processing and other high-chloride service conditions.
- Petrochemical equipment: Used for aggressive process streams, including high-chloride and sour service environments.
- High-temperature chloride service: Selected where elevated temperatures combine with chloride exposure.
- Critical equipment: Appropriate for applications where corrosion-related failure would result in difficult or costly inspection, access or repair.
Standards Referenced
- ASTM A240 covers plate, sheet and strip for duplex and super duplex stainless steel.
- ASTM A790 covers seamless and welded duplex and super duplex stainless steel pipe.
- ASTM A182 covers forged duplex and super duplex fittings and flanges.
Frequently Asked Questions
2507 contains higher chromium, molybdenum and nitrogen than 2205, giving it a higher PREN of around 43 versus 35, along with greater strength and chloride corrosion resistance.
PREN stands for Pitting Resistance Equivalent Number. It provides a quick comparison of stainless steel resistance to pitting and crevice corrosion, particularly in chloride environments.
Yes. Duplex 2205 has a minimum yield strength of about 450 MPa, compared with roughly 170 to 205 MPa for common 316/316L grades.
2507 is commonly used for offshore platforms, subsea piping, desalination equipment and petrochemical applications involving high chloride or sour service conditions.
Yes. 2205 is widely used offshore and in marine equipment. More severe seawater exposure may require 2507 for its higher corrosion resistance.
Yes. Its higher chromium, molybdenum, and nitrogen content increases material cost while providing greater strength and corrosion resistance.
Yes, both 2205 and 2507 are weldable, but welding procedures need careful control of heat input and cooling rate to preserve the correct balance between ferrite and austenite in the weld zone. Matching filler metal and qualified welding procedures are essential.
Yes, unlike fully austenitic grades such as 304 and 316, duplex stainless steel is mildly magnetic because of its ferrite content.
ASTM A790 covers seamless and welded duplex and super duplex stainless steel pipe, while ASTM A240 covers plate and sheet and ASTM A182 covers forged fittings and flanges.
Only if your environment does not involve severe or prolonged chloride exposure. In moderate environments, 2205 typically performs well and costs less. In severe environments such as continuous seawater contact, substituting 2205 for 2507 risks premature pitting failure.
Why Source Duplex Stainless Steel From Metinox Overseas
Metinox Overseas supplies ASTM and ASME certified duplex and super duplex stainless steel, including 2205 and 2507, with ISO 9001 certified processes and Mill Test Certificates for every order. With over 45 years of experience, we serve international markets across oil and gas, petrochemical, offshore and heavy engineering industries.
Related Guides
Stainless Steel Pipe Grades Chart: 304, 304L, 316, 316L, 321 and 347 Compared
Stainless Steel Pipe Weight Chart: Weight Per Meter by Size and Schedule
Stainless Steel vs Carbon Steel: Differences, Properties and Applications
