Duplex 2205 and super duplex: where they are needed
Duplex 2205 is chosen where ordinary austenitic stainless steel does not hold up: seawater, brines, chlorides under stress, media with abrasives. It is roughly twice as strong as 304 and more resistant than 316 to pitting and crevice corrosion, while containing less nickel.
Super duplex goes even further in resistance, but it is more expensive and more difficult to weld.
Let us look at where these properties come from, how duplex differs from 304 and 316, where its capabilities end, and what to take into account in an order.
What duplex is
Duplex is a ferritic-austenitic steel. Its structure is roughly half ferrite and half austenite, and this is not a random mixture but the result of the chemical composition and heat treatment. Ferrite provides high strength and resistance to chloride cracking, austenite provides toughness and workability in welding.
The composition is built around three elements. Chromium creates the passive film, molybdenum and nitrogen strengthen the resistance to pitting and crevice corrosion, and nickel stabilises the austenitic part. In 2205, chromium is about 22%, molybdenum about 3%, nickel noticeably less than in 316L, and nitrogen is present. The exact composition and mechanical properties are confirmed by the batch certificate.
The key consequence is a yield strength roughly twice as high as that of 304 and 316. This makes it possible to reduce the wall thickness or the section and obtain the same load-bearing capacity at a lower weight. Hence the interest in duplex for tanks, pipelines and structures where mass matters.
2205 versus 304 and 316
| Feature | 304 | 316L | Duplex 2205 |
|---|---|---|---|
| Structure | austenite | austenite | ferrite + austenite |
| Strength | baseline | baseline | roughly twice as high |
| Chloride resistance | low | medium | high |
| Chloride cracking | prone | prone | resistant |
| Magnetism | weak | weak | noticeable |
| Welding | simple | simple | requires control |
| Nickel content | 8–10% | 10–14% | lower than in 316L |
The difference is visible in practice. In seawater and brines, 304 quickly develops pitting, 316L holds out longer, but at elevated temperature and stress it may crack. Duplex works longer in these conditions because it combines molybdenum with nitrogen and a ferritic component. What you pay for this is workability: duplex is more difficult to weld, and after welding the properties of the seam must be controlled.
Magnetism is a frequent cause of confusion. Duplex is noticeably more magnetic than austenitic grades, because there is ferrite in the structure. A magnet is not a sign of a defect and not a way to distinguish 2205 from 430: the grade is confirmed by a document and by analysis.
Super duplex
Super duplex is the next class. Its typical representative, 2507, has chromium around 25%, molybdenum around 4%, and a higher content of nitrogen and nickel. This gives even higher resistance to pitting and crevice corrosion in seawater and aggressive chemicals.
It is used where 2205 does not meet the requirements of the environment: desalination, offshore platforms, water injection systems, pipelines in oil and gas, chemical reactors, equipment for sulphuric and phosphoric acid. What you pay for the resistance is the price and the demands on welding: super duplex is sensitive to heat input, and incorrect heating causes precipitation of secondary phases and a loss of toughness.
Intermediate and special variants differ in the balance of alloying for a specific environment. The selection is made not "by name" but by the composition of the medium, the temperature and the stress: sometimes 2205 is enough, sometimes super duplex is needed, and sometimes it is more profitable to stay with 316L.
Where they are used
Oil and gas and offshore facilities: pipelines, manifolds, pump units, platform parts, seawater firefighting systems. Here both chloride resistance and strength are valued.
Chemicals and petrochemicals: reactors, heat exchangers, tanks, pipework, work with acids and brines. Duplex withstands media in which austenitic grades corrode faster.
Desalination and water treatment: housings of membrane units, pumps, valves, pipelines. Chlorides and constant contact with water are a typical task for 2205.
Pulp and paper: digesters, tanks, washing lines, where an aggressive medium, temperature and abrasive are combined.
Structures with weight savings: bridges, platforms, load-bearing elements, where the smaller thickness of duplex reduces mass and installation cost. Here it is not corrosion that decides but strength.
Pools and seawater systems: parts that cannot be protected with a coating and where 316L does not hold up. For chlorinated pools, the choice of grade is calculated separately.
Limitations and welding
Duplex has an upper temperature limit. With prolonged heating at about 300 °C and above, the sigma phase precipitates in the structure, and the metal loses toughness and resistance. That is why duplex is not chosen for hot media and is not used where prolonged overheating is possible.
Welding requires discipline. The right filler material is needed, usually more alloyed than the base metal, along with control of heat input and interpass temperature. After welding it is important to clean the seam, remove scale and heat tint, and carry out passivation if necessary. If heat treatment after welding cannot be done, the properties of the seam are checked separately.
Duplex is not listed in the GCX catalogue: the exchange's base grades are 201, 304, 316L and 430. If the task requires 2205 or super duplex, that is a separate made-to-order item, and for most environments 316L is sufficient. Before replacing an austenitic grade with duplex, the economics are calculated: smaller thickness and longer service life against the price of the metal and the cost of welding.
Frequently asked questions
What is duplex 2205?
A ferritic-austenitic stainless steel with roughly equal proportions of ferrite and austenite, chromium around 22%, molybdenum around 3% and nitrogen. It combines high strength with resistance to chlorides.
How does duplex differ from 316?
Duplex is roughly twice as strong and withstands chlorides better, especially at elevated temperature and stress. But it is more difficult to weld, is more magnetic and is limited in operating temperature.
Is duplex magnetic?
Yes, noticeably. Its structure contains a ferritic component, so its magnetic behaviour is closer to ferritic grades than to austenitic ones. A magnet does not confirm the grade.
Can 316 be replaced with duplex?
Yes, if the environment is more aggressive than 316L can withstand: seawater, brines, chlorides under stress. In that case the temperature, weldability and economics are checked, because duplex is more expensive and more demanding in processing.
What is super duplex?
A class of duplex steels with higher alloying: chromium around 25%, molybdenum around 4%, more nitrogen and nickel. They are used in the most aggressive environments — seawater, oil and gas, chemicals.
Is duplex available in the GCX catalogue?
There is no duplex in the exchange's base catalogue: sheet, pipes, round bar and angle made of 201, 304, 316L and 430 are traded. Duplex and super duplex are obtained as a separate item for the task.
Choosing a grade for the environment
Look at the base product range, compare 316L and duplex by environment, temperature and weldability, and check prices on a separate page.