Why does duplex stainless steel resist corrosion? Duplex stainless steel resists corrosion because of its high chromium and molybdenum content and its two-phase (austenite plus ferrite) microstructure. The chromium and molybdenum give a high Pitting Resistance Equivalent Number (PREN), which strongly resists pitting and crevice corrosion in chlorides, while the balanced microstructure gives excellent resistance to chloride stress corrosion cracking, a weakness of standard austenitic grades.
Why duplex exists between standard and super-alloy grades
Standard austenitic stainless steels such as 304 and 316 are workhorses, but they have two well-known weaknesses in aggressive service: limited resistance to pitting and crevice corrosion in high-chloride environments, and vulnerability to chloride stress corrosion cracking at elevated temperature. Duplex stainless steels were developed to close these gaps while also offering higher strength, which allows thinner, lighter sections. They sit between the common austenitics and the expensive high-nickel super-alloys in both performance and cost.
For plants in the Netherlands and along the North Sea and estuarine coast, where chloride-bearing and brackish conditions are common, this combination of chloride resistance and strength is exactly why duplex is specified. GreyTec provides duplex and other corrosion-resistant process equipment for demanding EU chemical and petrochemical service.
What is PREN and why does it matter?
Answer: PREN (Pitting Resistance Equivalent Number) is a number that estimates a stainless steel’s resistance to pitting corrosion from its chromium, molybdenum, and nitrogen content, using the formula PREN equals percent chromium plus 3.3 times percent molybdenum plus 16 times percent nitrogen. A higher PREN means better resistance to pitting in chloride environments.
PREN lets engineers rank grades quickly. Standard 316 sits well below the common 2205 duplex grade, and super duplex grades sit higher again. PREN is a guide rather than an absolute guarantee, because temperature, crevices, and the specific environment all matter, but it is the standard first-pass comparison for chloride pitting resistance.
Indicative PREN ranges (confirm exact composition for the specific product):
- Type 304: approximately 18 to 20
- Type 316: approximately 24 to 26
- Duplex 2205: approximately 34 to 38
- Super duplex 2507: approximately 40 and above
(Source: PREN concept per stainless steel metallurgy references, for example IMOA and the Nickel Institute; confirm the exact PREN from the certified composition of the specific grade.)
How does the dual-phase microstructure help?
Answer: Duplex steel has roughly equal amounts of austenite and ferrite. This balanced two-phase microstructure gives it about twice the strength of standard austenitic stainless and, critically, strong resistance to chloride stress corrosion cracking, which single-phase austenitic grades such as 304 and 316 are prone to at elevated temperature in chlorides.
The microstructure is the reason duplex resists a failure mode that can crack 316 in the right conditions. It also delivers higher yield strength, which allows lighter, thinner-walled equipment for the same duty, saving weight and sometimes cost despite the higher price per kilogram.
Which corrosion types does duplex resist?
Answer: Duplex stainless steel resists pitting corrosion and crevice corrosion in chlorides (through high PREN), chloride stress corrosion cracking (through its two-phase microstructure), and general corrosion in many acids and process environments. It is widely used in seawater, brackish water, and chloride-bearing chemical service where 304 and 316 struggle.
Corrosion types and duplex behaviour:
- Pitting: strongly resisted, rising with PREN.
- Crevice corrosion: well resisted, though crevices still need good design.
- Chloride stress corrosion cracking: a key strength versus austenitic grades.
- General corrosion: good across many process media.
- Erosion-corrosion: aided by higher strength and hardness.
Numbered factors that affect duplex corrosion resistance in service
- Grade and PREN: higher chromium, molybdenum, and nitrogen improve pitting resistance.
- Temperature: higher temperature can reduce pitting resistance and, at the extremes, affect the phase balance.
- Chloride concentration: more chloride raises the pitting and crevice risk.
- Crevice geometry: poor design creates crevices that concentrate attack.
- Fabrication and welding: incorrect welding can unbalance the austenite-ferrite ratio and harm resistance.
- Surface condition: pickling and passivation restore the protective film after fabrication.
- Deposits and stagnation: under-deposit and stagnant conditions worsen local corrosion.
Duplex vs austenitic stainless: corrosion and strength at a glance
| Property | 304 | 316 | Duplex 2205 | Super duplex 2507 |
|---|---|---|---|---|
| Indicative PREN | 18 to 20 | 24 to 26 | 34 to 38 | 40 and above |
| Chloride pitting resistance | Low | Moderate | High | Very high |
| Chloride stress corrosion cracking resistance | Low | Low to moderate | High | Very high |
| Relative yield strength | Baseline | Similar to 304 | About double | About double |
| Relative cost | Lowest | Low to medium | Medium | Highest |
Values are indicative. Confirm against certified composition and the specific service environment.
Checklist: is duplex the right choice for your service?
- ☐ The environment is chloride-bearing or otherwise aggressive to 304/316.
- ☐ Chloride stress corrosion cracking is a credible risk at the service temperature.
- ☐ The required PREN is met by the selected grade.
- ☐ The service temperature is within duplex’s suitable range.
- ☐ Higher strength offers a weight or thickness benefit.
- ☐ Fabrication and welding will be done to preserve phase balance.
- ☐ Post-fabrication pickling and passivation are specified.
Myth: duplex is corrosion-proof
Is duplex stainless steel immune to corrosion? No. Duplex has excellent resistance in chloride environments, but it is not corrosion-proof. Its pitting resistance falls at high temperature, it can be attacked in very severe chloride conditions beyond its PREN, and poor welding can unbalance the microstructure and reduce resistance. It is a high-performance material, not an invincible one.
Duplex also has a practical upper temperature limit for long-term service, above which the microstructure can be affected. Correct grade selection, good design, and proper fabrication are what deliver its corrosion performance in practice.
More questions (FAQ)
Is duplex better than 316 for seawater? Generally yes for pitting and stress corrosion cracking resistance, with super duplex used for more severe seawater service. The exact choice depends on temperature, chloride level, and crevice risk.
Does duplex need special welding? Yes. Welding must control heat input and use correct consumables to keep the austenite-ferrite balance, followed by pickling and passivation. Poor welding reduces corrosion resistance.
What is super duplex? Super duplex grades (for example 2507) have higher chromium, molybdenum, and nitrogen, giving a PREN of 40 and above and superior resistance for the most demanding chloride service.
Is duplex magnetic? Yes, duplex is partly ferritic and therefore magnetic, unlike fully austenitic 304 and 316. This does not affect its corrosion performance.
Can duplex be used at high temperature? Duplex has a practical upper limit for sustained service because prolonged high temperature can embrittle the microstructure. For very high temperatures, other materials may be more suitable.
What are the different families of duplex stainless steel?
Answer: Duplex stainless steels form a family: lean duplex has lower alloying and cost with moderate corrosion resistance, standard duplex (typically 2205) is the most widely used with strong chloride resistance, super duplex (such as 2507) has higher chromium, molybdenum, and nitrogen for severe chloride and seawater service, and hyper duplex pushes performance higher still for the most demanding duties.
This family structure lets engineers match cost to requirement. Lean duplex can replace 304 or 316 in some structural and moderate-corrosion uses while adding strength. Standard 2205 covers most aggressive chemical and chloride service. Super and hyper duplex are reserved for the harshest seawater and high-chloride environments where even 2205 would be at its limit. Selecting the right family member is what makes duplex cost-effective rather than simply expensive.
What are the practical temperature limits of duplex?
Answer: Duplex has a practical upper temperature limit for sustained service, because prolonged exposure to high temperature can precipitate embrittling phases and reduce toughness and corrosion resistance. It is therefore generally used within a defined temperature band rather than at very high temperatures, where austenitic grades or other materials may be more suitable.
This limit is one of the honest trade-offs of duplex. Its microstructure delivers excellent strength and chloride resistance in its working range, but that same microstructure is sensitive to prolonged heat, which can affect the phase balance over time. Correct grade selection, awareness of the service temperature, and proper fabrication that avoids harmful heat exposure are what keep duplex performing as intended.
More expert questions
In which industries is duplex most used? Duplex is widely used in chemical processing, petrochemical and offshore oil and gas, desalination and seawater handling, pulp and paper, and storage of chloride-bearing media, all of which combine corrosion challenge with a need for strength.
Does duplex resist acids as well as chlorides? Duplex offers good resistance across many acids in addition to its chloride performance, but resistance is medium-specific, so the exact acid, concentration, and temperature must be checked against data for the grade.
Why is nitrogen added to duplex? Nitrogen improves pitting resistance (it appears in the PREN formula), stabilises the austenite phase, and increases strength, which is why modern duplex grades are nitrogen-alloyed.
Can duplex be used for storage tanks and vessels? Yes. Its combination of chloride resistance and high strength makes it well suited to tanks and pressure vessels handling aggressive media, often allowing thinner walls than austenitic grades.
How does fabrication affect duplex performance in service?
Answer: Fabrication has a direct effect on duplex corrosion resistance because welding and forming can shift the austenite-ferrite balance and precipitate harmful phases if heat input is not controlled. Correct welding procedures, suitable filler metals, controlled heat input, and post-fabrication pickling and passivation are what preserve the corrosion resistance that the grade offers on paper.
This is why duplex is specified with fabrication requirements, not just a grade name. A perfectly chosen grade can underperform if welded poorly, because the weld and heat-affected zone may lose the balanced microstructure that gives duplex its resistance. Treating fabrication quality as part of the specification, and verifying it, is essential to realising duplex’s benefits in the finished equipment.
Even more questions
Does duplex need passivation after fabrication? Yes. Pickling and passivation remove weld heat tint and restore the protective chromium-rich film, which is important for full corrosion resistance after welding.
Is duplex suitable for buried or immersed service? It can be, depending on the environment, but chloride level, oxygen, and crevice conditions must be assessed, and for severe seawater immersion, super duplex or other materials may be required.
Key facts at a glance
- Duplex resists corrosion through high chromium and molybdenum content and a two-phase microstructure.
- PREN equals percent chromium plus 3.3 times percent molybdenum plus 16 times percent nitrogen.
- 2205 duplex has a much higher PREN than 316, giving far better chloride pitting resistance.
- Its microstructure gives strong resistance to chloride stress corrosion cracking.
- It is not corrosion-proof and requires correct grade selection and fabrication (Source: duplex metallurgy per IMOA and the Nickel Institute; confirm exact PREN from certified composition).
Specify duplex correctly for aggressive EU service
Duplex delivers its corrosion performance only when the grade, design, and fabrication are right. GreyTec supplies duplex and other corrosion-resistant process equipment for chemical and petrochemical plants across the Netherlands and the EU, matched to your chloride level, temperature, and strength requirements.
See our engineering solutions for petrochemical plants, or call for a materials review.
Related reading: Duplex stainless steel vs 316: which to specify? and duplex stainless steel vs 304: corrosion and cost compared.