Duplex stainless steel resists chloride pitting, crevice corrosion, and stress-corrosion cracking far better than standard austenitic grades, but only within defined thresholds of temperature, chemistry, and microstructure. A 316L pipe has a critical pitting temperature of roughly 10 to 15°C. Run it in warm seawater and it will pit, not because it is “bad” steel, but because every grade has a corrosion limit. Duplex simply pushes those limits much higher.
“Corrosion resistant” is not a binary label. It is a set of measurable thresholds, and for duplex the two-phase microstructure behind those thresholds is what most guides skip.
This duplex stainless steel corrosion resistance guide explains how and why duplex resists pitting, crevice corrosion, and chloride stress-corrosion cracking, the PREN, CPT, and CCT numbers that matter, and how a buyer verifies that the delivered material actually meets them. It is written from a manufacturer’s perspective, because corrosion resistance is manufactured into the alloy and can be welded or heat-treated away.
Key Takeaways
- Duplex’s roughly 50/50 ferrite-austenite structure combines high strength with superior resistance to chloride pitting, crevice corrosion, and stress-corrosion cracking.
- PREN is a screening tool, not a guarantee: 2205 calculates to roughly 35, super duplex 2507 exceeds 40, and 316L lands around 24 to 26. In duplex, the weaker of the two phases sets the real limit.
- Quantitative benchmarks: duplex 2205 CPT is roughly 35 to 40°C versus 10 to 15°C for 316L, and 2205 resists chloride SCC to about 150°C versus roughly 60°C for austenitic grades.
- Sigma phase, 475°C embrittlement, and poor welding destroy corrosion resistance, which is why phase balance and heat treatment are manufacturer responsibilities.
- Verify delivery with ASTM G48 test reports, ASTM A923 intermetallic testing, ferrite measurement, and MTR chemistry before installation.
What Makes Duplex Stainless Steel Corrosion Resistant?
The two-phase microstructure
Duplex stainless steel is roughly 50% ferrite and 50% austenite after solution annealing and rapid water quenching. That split is not decorative. The two phases partition alloying elements differently, and corrosion resistance depends on both.
Chromium and molybdenum concentrate in the ferrite. Nitrogen and nickel concentrate in the austenite. Ferrite supplies the chromium and molybdenum that build the passive film and resist pitting. Nitrogen in the austenite boosts pitting resistance where local breakdown tries to start.
The phases do the job together. That is why the phase balance matters more for duplex than for any single-phase grade.
Phase-specific PREN: the duplex caveat
Here is the nuance most guides miss. A duplex pipe has a bulk PREN, but each phase has its own PREN. Chromium and molybdenum partition to ferrite, so the ferrite may show a higher PREN than the austenite in the same heat. Pitting typically initiates at the weaker phase or at the ferrite-austenite phase boundary, not at some averaged composition.
Two pipes both labelled S32205 can therefore behave differently in service if their heat treatment left one with an unbalanced phase ratio. This is the concrete reason manufacturer heat treatment and phase-balance control matter for duplex in a way they do not for 316L. For the wider grade background, see our complete duplex stainless steel guide.
The passive film
Corrosion resistance starts with the thin chromium oxide passive film on the surface, reinforced by molybdenum and nitrogen. In oxygenated service the film self-repairs. It fails where it is starved of oxygen or locally damaged, which is how pitting and crevice corrosion begin. Duplex is not immune to this failure mode; it simply resists the initiation of it at much higher chloride severity and temperature.
Specifying a duplex grade? Send us the fluid chemistry, operating temperature, pressure, and service conditions with your seamless stainless steel pipe enquiry, and our technical team can confirm the grade and required documentation.
PREN in Duplex Stainless Steel
The formula
The Pitting Resistance Equivalent Number, or PREN, is a composition-based formula used to compare pitting resistance across stainless grades:
PREN = %Cr + 3.3(%Mo) + 16(%N)Tungsten-bearing grades use a modified variant, because tungsten behaves like molybdenum in chloride pitting:
PREN = %Cr + 3.3(Mo + 0.5W) + 16(%N)The British Stainless Steel Association’s PREN article is a useful reference for the calculation and its limits.
PREN values by grade
| Grade | UNS designation | PREN (approx.) |
|---|---|---|
| 304 | S30400 | ~19 |
| 316L | S31603 | ~24 to 26 |
| Duplex 2205 | S31803 / S32205 | ~34 to 36 |
| Super Duplex 2507 | S32750 | >40 |
The gap between 316L and 2205 is the corrosion margin buyers are paying for. The gap between 2205 and 2507 is the margin for heated, creviced, or higher-chloride service.
Why PREN is a screening tool, not a guarantee
PREN ranks alloys. It cannot predict service life. Pitting depends on temperature, pH, oxygen, crevices, deposits, flow, and weld condition, none of which appear in the formula. Treat PREN as a first-pass material-selection filter, then confirm the grade against the actual service and a corrosion assessment.
Pitting Corrosion Resistance
What pitting is
Pitting is localized breakdown of the passive film in chloride environments. It is dangerous because a small pit can hide significant metal loss, and a single perforation can force a full line replacement. Chloride ions attack weak points in the film, and once a pit starts, the local chemistry inside it becomes more aggressive.
Critical pitting temperature (CPT)
The critical pitting temperature is the temperature above which a grade is vulnerable to pit initiation in a given chloride test. Duplex 2205 has a CPT of roughly 35 to 40°C. 316L is typically around 10 to 15°C. That is why 316L pits in warm seawater while 2205 does not.
For the full grade-by-grade corrosion comparison, see our duplex 2205 vs 316L analysis.
Illustrative field scenario: A Gulf plant specified 316L for a seawater cooling line running at roughly 38°C. Within 18 months, pitting appeared under biofouling and at weld seams. The material’s PREN was near 25, above the line for ambient seawater but below what warm seawater demands. Replacing the line in UNS S32205 duplex, with a PREN near 35, eliminated the pitting and removed the need for continuous biocide dosing.
The critical pitting temperature of duplex 2205 is a benchmark for material comparison, not a universal service limit. The actual value shifts with test method, chloride concentration, and surface condition.
ASTM G48 testing
ASTM G48 is the standard used to verify pitting and crevice corrosion resistance. Method A is the ferric chloride pitting test. Method E measures critical pitting temperature.
Duplex 2205 is commonly tested at 25°C, and super duplex at 50°C. Weld qualification frequently applies NORSOK-style weight-loss acceptance criteria in the range of roughly 1 to 4 g/m².
Crevice Corrosion Resistance
Why crevices are more aggressive
Crevice corrosion initiates at lower temperatures than pitting. Stagnant zones under flanges, gaskets, deposits, and weld seams deplete oxygen and lower pH, which attacks the passive film from a protected, hidden location. A flange face or a support saddle can be the weakest point in an otherwise sound system.
Critical crevice temperature (CCT)
The critical crevice temperature follows the same logic as CPT but under a crevice assembly. Duplex 2205 has a CCT of roughly 20 to 30°C. Super duplex 2507 resists crevice attack to roughly 40°C. 316L becomes susceptible above about 10°C.
ASTM G48 Method B tests crevice corrosion, and Method F measures critical crevice temperature.
Design guidance
Full-penetration welds and designs that avoid stagnant zones reduce crevice risk. Where a crevice is unavoidable, specify a higher-alloy grade rather than relying on design alone.
Chloride Stress Corrosion Cracking (SCC) Resistance
The defining duplex advantage
Chloride stress-corrosion cracking needs three things at once: tensile stress, chlorides, and a susceptible microstructure above a threshold temperature. Remove any one and SCC stops.
Austenitic grades 304 and 316L become susceptible to chloride SCC above roughly 60°C. Duplex 2205 resists it to about 150°C, and super duplex extends that margin further. This is the single biggest operational difference between duplex and austenitic pipe in hot chloride service.
The mechanism explains why. In the two-phase structure, the ferrite arrests crack propagation. A crack that starts in the austenite is stopped or redirected at the ferrite-austenite boundaries instead of running through the section. That structural advantage is fundamental, not a coating or a treatment.
The welding caveat
The heat-affected zone is the weakest link. Unbalanced phase ratios, secondary austenite, and chromium nitrides can reduce SCC resistance right where welds create tensile stress. Controlled heat input of roughly 0.5 to 2.5 kJ/mm and over-alloyed filler such as ER2209 preserve the phase balance. Austenitic filler on duplex, chosen to “simplify” consumables, removes the corrosion advantage in the weld region.
Illustrative fabrication scenario: A fabricator welded duplex 2205 pipe with austenitic filler to simplify consumable management. The weld and heat-affected zone lost the ferrite-austenite balance, and G48 testing on the weldment showed pitting at temperatures the base metal survived easily. The issue was not the grade. Corrosion resistance is manufactured into the material, and it can be welded away. Over-alloyed filler, ER2209, plus controlled heat input restored the performance.
Other Corrosion Mechanisms
Intergranular corrosion
Low carbon content and rapid cooling after solution annealing reduce chromium-carbide sensitization at grain boundaries. This is why duplex grades are inherently less prone to intergranular attack than older high-carbon austenitic grades.
Galvanic corrosion
Duplex is noble relative to many engineering metals. In seawater service, duplex pipe in contact with less-noble materials such as carbon steel or aluminum creates a galvanic couple that accelerates attack on the less-noble member. Design reviews should account for the full assembly, not just the duplex component.
Sour service (NACE MR0175 / ISO 15156)
In H2S environments, NACE MR0175 / ISO 15156 imposes hardness and phase-balance limits for duplex materials. Super duplex generally offers higher H2S resistance than 22Cr duplex. Note that NACE compliance is a project or supplementary requirement, not an automatic grade certification. Confirm it only where the project calls for it.
What Degrades Duplex Stainless Steel Corrosion Resistance
Sigma phase
Sigma phase is the corrosion resistance killer. It precipitates in the roughly 705 to 980°C range during slow cooling or improper heat treatment. Sigma phase is rich in chromium and molybdenum, which it pulls out of the surrounding matrix, stripping the alloy of the elements that build the passive film.
Corrosion rate in chlorides rises as the sigma-phase area fraction increases. Super duplex is at higher sigma risk than 22Cr duplex because its higher alloy content drives faster precipitation.
475°C embrittlement
Sustained service in the roughly 350 to 525°C range causes ferrite spinodal decomposition, which embrittles the material and lowers toughness. Duplex is not intended for prolonged elevated-temperature service. This is one reason duplex has a practical upper service limit around 280 to 300°C depending on the grade and application.
The welding heat-affected zone
Chromium nitride precipitation, unbalanced phase ratios, and secondary austenite form in the HAZ when welding is uncontrolled. These defects concentrate corrosion attack at the weld, which is often the highest-stress location in the system.
Phase imbalance
Below roughly 25% ferrite, duplex loses its corrosion and SCC advantage and behaves more like an austenitic grade. Above roughly 70% ferrite, it loses toughness. The target window is usually 35 to 65% ferrite.
The QC connection
Proper solution annealing at roughly 1020 to 1100°C followed by rapid water quenching locks in the phase balance. Ferrite content measurement, intermetallic testing per ASTM A923, and chemistry verification are how a manufacturer protects corrosion resistance. This is the step where a buyer discovers whether the delivered duplex actually is duplex.
How Duplex Stainless Steel Corrosion Resistance Is Verified in Production
Chemistry verification
Spectrographic analysis, often performed as positive material identification or PMI, confirms chromium, molybdenum, and nitrogen, the elements behind PREN. The MTR must trace the delivered product to its heat and show the ordered chemistry.
Ferrite content
Ferrite is measured with a feritscope or by metallographic point count per ASTM E562. The typical acceptance target is 35 to 65% ferrite, with 40 to 60% common for many specifications.
Intermetallic phase testing
ASTM A923 detects detrimental intermetallic phases such as sigma and chi in duplex stainless steel. It is essential for duplex and is the test most general guides omit. If the service is critical, request ASTM A923 results as a supplementary requirement.
Illustrative receiving scenario: A buyer requested ASTM A923 testing on a duplex order plus a PMI check at receiving. The MTR’s nitrogen value sat near the lower limit for S32205, and A923 testing flagged elevated sigma-phase content from a heat-treated batch. The material was rejected before installation, preventing a chloride-service failure that would have appeared only after months in service.
Corrosion testing
ASTM G48 testing is performed when the project or service requires it. A G48 test report is direct evidence of pitting or crevice corrosion performance. Where the consequence of failure is high, request the report rather than a written claim.
Documentation to demand in an RFQ
- MTR showing nitrogen, molybdenum, and mechanical properties, traced to the heat.
- Ferrite content statement.
- ASTM A923 results where the service requires intermetallic verification.
- ASTM G48 results where pitting or crevice performance must be proven.
- Third-party inspection support, such as SGS, TUV, or BV, for witness testing.
For the product-level requirements, review the ASTM A790 duplex seamless pipe guide. At Zhongzheng, every heat is spectrographically verified, ultrasonically tested, and hydraulically proven before shipment, and third-party inspection can be arranged on request.
Choosing a Duplex Grade for Corrosive Service
Seawater temperature is a useful shorthand for grade selection because it tracks the PREN guidance used across the industry: roughly PREN 32 for ambient seawater, 35 for warm seawater, and 40 for heated seawater.
| Service condition | Recommended grade | Reasoning |
|---|---|---|
| Ambient seawater (20 to 25°C) | 2205 (PREN ~32+) | Standard choice for continuous immersion |
| Warm seawater (~35 to 40°C) | 2205 (PREN ~35) | 316L will pit at these temperatures |
| Heated seawater (40 to 60°C) | Super duplex 2507 (PREN ~40+) | Maximum corrosion margin |
| Chemical process, moderate chloride | 2205 | Balanced strength and corrosion performance |
| Subsea or severe sour service | Super duplex | PREN > 40 with a NACE/ISO 15156 envelope |
This is a starting framework, not a substitute for a corrosion assessment. Always confirm the grade against fluid chemistry, temperature, pressure, crevices, deposits, and the governing code.
Outokumpu’s duplex stainless steel overview is a useful reference for the grade family’s properties and application context. For the grade decision in detail, see the duplex vs super duplex grade comparison and the super duplex S32750 pipe specifications. Where heated service demands maximum margin, our super duplex stainless steel pipe is available with documented corrosion testing.
Frequently Asked Questions
What is PREN in duplex stainless steel?
PREN, the Pitting Resistance Equivalent Number, is a composition-based formula for comparing pitting corrosion resistance: PREN = %Cr + 3.3(%Mo) + 16(%N). Duplex 2205 calculates to roughly 35, super duplex 2507 exceeds 40, and 316L lands around 24 to 26. Higher PREN indicates a larger resistance margin, but it is a screening tool, not a service-life guarantee.
Is duplex stainless steel corrosion resistant?
Yes, within defined thresholds. Duplex resists chloride pitting, crevice corrosion, and stress-corrosion cracking far better than standard austenitic grades, but its performance depends on grade, phase balance, heat treatment, welding, and the actual service environment.
Does duplex stainless steel resist stress corrosion cracking?
Yes. The roughly 50/50 ferrite-austenite structure resists chloride stress-corrosion cracking far better than austenitic grades. Where 304 and 316L become susceptible above roughly 60°C, duplex 2205 generally resists to about 150°C. The improvement depends on phase balance, heat treatment, and weld quality.
What is the critical pitting temperature of duplex 2205?
Roughly 35 to 40°C, compared with approximately 10 to 15°C for 316L. Above its CPT, a grade is vulnerable to pitting in chloride environments. Treat the value as a comparison benchmark, not a universal service limit.
What is the maximum temperature for duplex stainless steel?
Duplex is generally not used above roughly 280 to 300°C because of sigma-phase embrittlement risk, and not below roughly -40°C because of the ductile-to-brittle transition. Confirm the qualified envelope against the current standard and the specific grade.
What is the difference between pitting and crevice corrosion?
Pitting is localized breakdown of the passive film at an exposed surface. Crevice corrosion attacks a shielded, oxygen-starved site such as under a gasket or deposit. Crevice corrosion initiates at lower temperatures than pitting, which is why crevice sites are often the first to fail.
What is ASTM G48 testing?
ASTM G48 is the standard for verifying pitting and crevice corrosion resistance. Method A covers ferric chloride pitting, Method B covers crevice corrosion, and Methods E and F measure critical pitting and critical crevice temperatures. A G48 test report is direct evidence of corrosion performance.
Which duplex grade is best for seawater?
For ambient seawater, 2205 is the standard choice. For warm seawater, 2205 with a higher PREN. For heated seawater or severe crevice risk, super duplex 2507. Confirm every selection against the actual service conditions and a corrosion assessment.
Conclusion: Duplex Corrosion Resistance Is Real, but Conditional
Duplex stainless steel corrosion resistance is not a claim to take on faith. It is the product of a controlled two-phase microstructure, a specific chemistry, correct heat treatment, and disciplined welding, and it must be verified in the delivered material.
Check the MTR chemistry against the UNS requirements. Confirm ferrite content and ASTM A923 results where the service demands it. Request the G48 test report when pitting or crevice performance is critical. Those documents are how a buyer turns a grade label into a serviceable material.
As a manufacturer, Zhongzheng controls the phase balance and documents the corrosion-critical testing on every duplex heat we ship. When you are ready to confirm a grade for a specific service, send your fluid chemistry, operating temperature, pressure, and service conditions. Our technical team will confirm the right duplex grade and document requirements within 24 hours.
