UNS S32750 Super Duplex in Seawater: Chloride and Crevice Corrosion Limits

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

UNS S32750 seawater service is one of the most common reasons buyers and engineers reach for a super duplex stainless steel. Known commercially as 2507, S32750 is widely selected for seawater cooling systems, desalination plants, offshore equipment, and aggressive chloride environments. But "seawater resistant" is not the same as "immune to seawater corrosion," and understanding where the real limits lie — especially in crevices — is the difference between a sound material choice and an expensive surprise.

This article explains why S32750 performs so well in seawater, why pitting and crevice corrosion must be considered separately, and why there is no single "chloride ppm + temperature" number that defines a safe operating limit. It is written for seawater cooling, desalination, marine, oil and gas, and chemical equipment engineers, as well as B2B buyers who need a practical, decision-oriented view.

1. Is S32750 Suitable for Seawater?

Yes. UNS S32750 is specifically suited to highly corrosive chloride-bearing environments, including seawater applications. Its advantage comes from a combination of high chromium, molybdenum, and nitrogen content, a duplex microstructure, and a high PREN value.

That said, "seawater resistant" does not mean "immune to seawater corrosion." The actual service life depends on temperature, chloride concentration, oxygen, flow velocity, crevice geometry, surface condition, and biofouling. A material with excellent seawater resistance can still corrode if it is used in a badly designed crevice, left with a contaminated surface, or operated outside the conditions for which it was selected.

2. Why 2507 Performs Well in Chloride Seawater

The performance of S32750 in chloride service comes from how its alloying elements work together:

• Chloride attacks the passive film: Chloride ions are the primary driver of localized corrosion in stainless steel, because they destabilize the thin passive oxide film that normally protects the surface.
• Chromium maintains passivity: High chromium helps form and repair the passive film, giving the material its basic corrosion resistance.
• Molybdenum and nitrogen improve localized corrosion resistance: Both elements raise resistance to pitting and crevice corrosion, which is why they appear in the PREN index.
• Duplex microstructure improves chloride SCC resistance: The ferrite in the duplex structure gives S32750 better resistance to chloride stress corrosion cracking than a fully austenitic grade.
• High strength: Super duplex grades offer very high yield strength, which is valuable in marine and pressure equipment where weight and wall thickness matter.

A common way to compare pitting resistance is the PREN (Pitting Resistance Equivalent Number), often written as:

PREN = %Cr + 3.3 × %Mo + 16 × %N

S32750 typically has a PREN of 40 or higher, placing it in the super duplex class; publicly available manufacturer data commonly shows a minimum PRE on the order of 41–42 for this grade. This is a useful comparison tool, but it is not an actual seawater service limit. PREN ranks relative pitting resistance; it does not, by itself, tell you whether a specific component will survive a specific crevice in a specific plant.

3. Pitting Corrosion in Seawater

Pitting is a localized form of corrosion that begins when the passive film breaks down at a small surface site and the attack then progresses into the metal.

Several factors drive pitting in seawater:

• Pitting initiation: Small defects, inclusions, deposits, or surface irregularities provide sites where the passive film is weaker.
• Passive film breakdown: Chloride ions destabilize the film locally, allowing attack to begin.
• Chloride concentration: Higher chloride levels generally increase pitting risk.
• Temperature: Elevated temperature makes pitting more likely.
• Oxygen: Oxygen availability influences the corrosion mechanism and the stability of the film.
• Surface condition: Rough or contaminated surfaces are more prone to pitting than clean, smooth ones.

Because of its high chromium, molybdenum, and nitrogen content, S32750 offers very high pitting resistance. One laboratory way to express this is the Critical Pitting Temperature (CPT), which measures the temperature above which pitting initiates under a specific test condition. The important caveat is that CPT comes from a defined laboratory test and cannot be directly read as the actual allowable operating temperature in natural seawater.

4. Crevice Corrosion: The Main Selection Boundary

Crevice corrosion, not pitting, is usually the controlling factor in seawater selection. A crevice is more dangerous than an open surface because the local environment inside the crevice becomes more aggressive than the bulk seawater.

• Why crevices are worse: In a tight gap, the local environment stagnates and changes.
• Oxygen depletion: Oxygen inside the crevice is consumed and cannot be replenished, breaking the balance between the inside and outside of the crevice.
• Local acidification: As corrosion initiates, the crevice solution can become more acidic, accelerating attack.
• Chloride concentration: Chloride can concentrate inside the crevice, making it more aggressive than the surrounding seawater.
• Where crevices form: Gaskets, bolted joints, flanges, supports, and under-deposit areas are all classic crevice locations.

Even though S32750 has very high pitting resistance, it should not be assumed immune to corrosion in every possible crevice geometry. Pitting corrosion and crevice corrosion are different mechanisms and must be treated separately. PREN alone does not describe crevice performance, because crevice corrosion depends heavily on geometry, temperature, and the local environment, not just alloy chemistry.

5. Temperature and Chloride: Why There Is No Single Limit

A common mistake is to look for a simple rule such as "X ppm chloride + Y°C = safe." No such single limit exists, because real selection depends on many interacting factors:

• Seawater temperature;
• Chloride concentration;
• Oxygen level;
• Flow velocity;
• Stagnant zones;
• Crevice severity;
• Deposits and fouling;
• Mechanical stress;
• Welding condition.

Laboratory values such as CPT (Critical Pitting Temperature) and CCT (Critical Crevice Temperature) are useful for ranking alloys, but they are measured under specific test conditions and are not universal design limits. For example, some manufacturer data reports that S32750 in the solution-annealed and quenched condition passes the relevant ASTM G48 pitting test at up to at least 50°C. This is a meaningful indication of high pitting resistance, but it must not be read as "50°C is the absolute safe seawater limit." The test method, the specific test environment, and the difference between a laboratory coupon and a real welded, creviced, fouled component all matter.

6. S32750 vs 2205 in Seawater

This comparison should not be read as "2507 always works where 2205 fails." The actual performance depends on product form, surface condition, welding, and the specific service environment. 2205 is an excellent material for many seawater applications; S32750 becomes the better choice when conditions are more aggressive or when a higher safety margin against localized corrosion is required.

7. S32750 in Desalination and Marine Systems

S32750 is used across a range of seawater and marine equipment where high chloride resistance and high strength are both valuable:

• Seawater cooling systems;
• Desalination plants;
• Reverse osmosis (RO) equipment;
• Seawater piping;
• Offshore equipment;
• Heat exchangers;
• Pumps, valves, and flanges;
• Marine structural components.

Public manufacturer materials list super duplex grades like 2507 for seawater cooling, desalination, and aggressive chloride environments. The reason is simple: these applications demand both high localized corrosion resistance and the high strength that allows lighter, more compact equipment.

8. Surface Condition, Welding and Crevice Design

A super duplex base metal with excellent seawater resistance does not automatically produce a component with the same performance. Fabrication can change the local behavior:

• Welding and phase balance: Welding changes the local ferrite/austenite balance, which can reduce localized corrosion resistance if not controlled.
• Heat tint and oxide scale: Weld heat tint and scale are initiation sites for pitting and should be removed.
• Poor weld condition: Undercut, lack of fusion, or poor root condition can create crevice-like sites.
• Rough surfaces: Rough or contaminated surfaces are more prone to localized attack than clean, smooth ones.
• Deposits: Deposits and fouling create the conditions for under-deposit corrosion.
• Bolted joints and gaskets: These are classic crevices and must be designed and specified carefully.

For a deeper look at weld quality in this grade, see our related article "UNS S32750 Super Duplex Welding: Filler Metal, Heat Input and Testing." The key point here is that crevice design and surface condition are as important to seawater performance as the alloy choice itself.

9. How to Specify S32750 for Seawater Service

A precise RFQ protects the project. When specifying S32750 for seawater service, include at minimum:

• UNS S32750: Stated explicitly;
• Product form: Plate, sheet, tube, pipe, or fitting;
• Applicable ASTM/EN product standard: For tube and pipe, common product standards include ASTM A789 and A790; the correct standard must match the product form;
• Dimensions: Diameter, wall thickness, and length where applicable;
• Design temperature: Minimum and maximum operating temperature;
• Seawater composition: Including chloride level where known;
• Flow conditions: Velocity and any stagnant zones;
• Crevice / gasket design: Details of bolted joints, gaskets, and supports;
• Surface finish: Required surface condition and pickling/passivation where applicable;
• Heat treatment: Requirement for solution annealing;
• Welding requirements: Applicable code and any special weld requirements;
• NDE: Non-destructive examination requirements;
• Corrosion testing: Where required by the project specification;
• MTC: Requirement for a material test certificate;
• Heat-number traceability: Material traceable to its heat number.

10. Common Purchasing Mistakes

• Treating "seawater resistant" as if the material cannot corrode;
• Looking only at PREN and ignoring everything else;
• Using CPT or CCT as a direct service temperature limit;
• Ignoring crevice geometry in the design;
• Considering only chloride concentration and ignoring temperature and oxygen;
• Ignoring welding condition and heat tint;
• Ignoring surface condition and fouling;
• Assuming 2507 can replace 2205 without limit in every case;
• Failing to confirm the product standard and heat treatment;
• Checking only the MTC chemistry without verifying the actual corrosion requirements.

FAQ

Q1: Is UNS S32750 suitable for seawater?
Yes. S32750 is specifically suited to highly corrosive chloride-bearing environments, including seawater cooling, desalination, and offshore applications, thanks to its high chromium, molybdenum, and nitrogen content and duplex microstructure.

Q2: How resistant is S32750 to chloride corrosion?
It offers very high resistance to chloride pitting and crevice corrosion, and good resistance to chloride SCC because of its duplex structure. Resistance is not the same as immunity, and real performance depends on temperature, crevice geometry, surface condition, and fabrication.

Q3: What is the PREN of UNS S32750?
S32750 typically has a PREN of 40 or higher, placing it in the super duplex class; manufacturer data commonly shows a minimum PRE on the order of 41–42. PREN is a comparison tool for relative pitting resistance, not an actual service limit.

Q4: Is S32750 resistant to pitting corrosion in seawater?
Yes, it has very high pitting resistance. However, pitting resistance in a clean laboratory coupon is not the same as performance in a real, welded, fouled component with crevices and deposits.

Q5: Is S32750 resistant to crevice corrosion?
It has high crevice corrosion resistance, but no alloy is immune in every crevice geometry. Crevices concentrate chloride and can acidify locally, so crevice design remains a key part of seawater selection.

Q6: What is the seawater temperature limit for 2507?
There is no single temperature limit. Selection depends on temperature together with chloride level, oxygen, flow, crevice severity, fouling, and surface condition. Laboratory CPT/CCT values should not be read as absolute operating limits.

Q7: Can S32750 be used in desalination plants?
Yes. S32750 is widely used in desalination and RO equipment, seawater piping, heat exchangers, and related components where high chloride resistance and high strength are required.

Q8: Is S32750 better than 2205 for seawater?
It has higher pitting and crevice corrosion resistance and higher strength than 2205, making it suitable for more aggressive conditions. 2205 remains an excellent choice for many moderate seawater applications.

Q9: Does seawater chloride concentration determine the corrosion limit of S32750?
Not by itself. Chloride concentration is one factor among several, including temperature, oxygen, flow velocity, crevice geometry, surface condition, and fabrication. No single chloride number defines the limit.

Q10: What should be specified when purchasing S32750 for seawater service?
Specify UNS S32750, product form, applicable ASTM/EN standard (such as ASTM A789/A790 for tube and pipe), dimensions, design temperature, seawater composition, flow conditions, crevice/gasket design, surface finish, heat treatment, welding requirements, NDE, corrosion testing where required, MTC, and heat-number traceability.

Related Reading

• UNS S32750 Super Duplex Welding: Filler Metal, Heat Input and Testing
• S32205 vs S32750: Duplex and Super Duplex Selection
• 2507 Desalination Applications: Material Selection Overview
• Duplex Stainless Steel Seawater Selection: A Practical Guide