How to Choose the Right Stainless Steel Grade for Seawater Applications

Seawater is one of the most aggressive natural corrosives. Not all stainless steels can handle it.

316L: Barely suitable - only for very low-salinity or short-term exposure. Pitting and crevice corrosion are serious risks.

Duplex 2205: A strong general-purpose choice for seawater structures, piping, and pump housings. PREN ≥ 35.

Super Duplex 2507 / 254 SMO / 6Mo alloys: The top tier for continuous seawater service - pipelines, heat exchangers, desalination, offshore risers. PREN ≥ 40.

One-Sentence Rule: The higher the chloride concentration, temperature, and stress - the higher the PREN value you need.

Seawater contains approximately 3.5% total dissolved salts, of which roughly 55% is sodium chloride (NaCl). The chloride ion (Cl⁻) is the primary enemy of passive stainless steel. It penetrates the thin chromium-oxide passive film that gives stainless steel its "stainless" quality, triggering two forms of localised corrosion:

Stainless Steel Grade for Seawater Applications

Corrosion Mode	What Happens	Why It Matters
Pitting Corrosion	Chloride ions locally break down the passive film, creating deep pits that perforate the metal	Can penetrate a pipe wall in months; invisible until failure
Crevice Corrosion	Oxygen-depleted zones inside gaps (under gaskets, at flanges, in threaded joints) become anodic and corrode rapidly	The most common failure mode in seawater-cooled heat exchangers and bolted flanges
Stress-Corrosion Cracking (SCC)	Tensile stress + chloride environment causes sudden brittle cracking - even in alloys with good pitting resistance	Common at elevated temperatures (>60 °C); austenitic grades most susceptible
Galvanic Corrosion	Dissimilar metals in electrical contact in seawater create an electrochemical cell; the less noble metal corrodes preferentially	Critical for mixed-metal assemblies - carbon steel, aluminium, and copper alloys paired with stainless steel

Table 1: Four Major Corrosion Modes in Seawater. Source: NACE International (AMPP), "Corrosion in the Marine Environment," TPC Publication 19, 2018; ASM Handbook Vol. 13A, "Corrosion: Fundamentals, Testing, and Protection," 2003.

Additionally, seawater temperature rises in tropical regions and inside heat exchangers, and biofouling can create anaerobic micro-environments that accelerate crevice attack. Velocity matters too: stagnant seawater is far more corrosive than fast-flowing seawater.

The Pitting Resistance Equivalent Number (PREN) is the single most important indicator for predicting a stainless steel's resistance to pitting and crevice corrosion in chloride environments. It is calculated from the alloy's chromium, molybdenum, and nitrogen content:

PREN = %Cr + 3.3 × %Mo + 16 × %N

For duplex stainless steels (which have both austenite and ferrite phases), an alternative formula is sometimes used:

PRENᵀ = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N

Where %W = weight percent tungsten (relevant for grades containing tungsten, such as Zeron 100).

A higher PREN means better resistance to pitting. The industry has established three critical threshold values that define seawater suitability:

PREN Range	Classification	Seawater Suitability	Typical Grades
<25	Insufficient for seawater	NOT recommended. Rapid pitting in seawater, even at ambient temperature	304, 304L, 430
25–35	Marginal / Limited Seawater Use	Short-term or low-salinity seawater only. Not for continuous immersion or high-chloride zones	316L (PREN ~24–26), 317L
35–40	Good - General Seawater Service	Suitable for seawater structures, piping, pump housings at ambient temperature	Duplex 2205 (PREN ~34–36), S31803
≥40	Excellent - Full Seawater Service	Continuous immersion, heat exchanger tubing, desalination, offshore. Industry-standard threshold for seawater service	2507, 254 SMO, AL-6XN, Zeron 100

Table 2: PREN Classification and Seawater Suitability. Source: NACE MR0175/ISO 15156; DNV-RP-F112 "Design of Duplex Stainless Steel Subsea Equipment"; EFC Publication No. 32, "Guidelines on Materials Requirements for Carbon and Low Alloy Steels" (adapted for stainless steels).

Source for all composition data: ASTM A240 (sheet/plate), ASTM A276 (bar), ASTM A312 (pipe), ASTM A789/A790 (duplex pipe). PREN values calculated from nominal compositions. Corrosion data from NACE International and Special Metals Corp., 2023.

Grade (UNS)	Type	%Cr	%Mo	%N	PREN	Seawater Rating	Relative Cost	Best For
304L (S30403)	Austenitic	18–20%	-	-	~18	Avoid	Lowest	Freshwater, indoor
316L (S31603)	Austenitic	16–18%	2.0–3.0%	-	~25	Marginal	Low	Light splash, brackish
317L (S31703)	Austenitic	18–20%	3.0–4.0%	-	~28	Limited	Low+	Chemical process
904L (N08904)	Austenitic	19–23%	4.0–5.0%	-	~34	Fair	Medium	Acids, sulfuric
2205 (S32205)	Duplex	22–23%	3.0–3.5%	0.14–0.20%	~35	Good	Medium+	Seawater structures
254 SMO (S31254)	6Mo Austenitic	19.5–20.5%	6.0–6.5%	0.18–0.22%	~43	Excellent	High	HX tubes, desalination
AL-6XN (N08367)	6Mo Austenitic	20.0–22.0%	6.0–7.0%	0.18–0.25%	~46	Excellent	High	Offshore, FGD
2507 (S32750)	Super Duplex	24–26%	3.0–5.0%	0.24–0.32%	~43	Excellent	High	Offshore, seawater pump
Zeron 100 (S32760)	Super Duplex	24–26%	3.0–4.0%	0.20–0.30%	~41	Excellent	High	Subsea, risers

Table 3: Stainless Steel Grade Master Comparison for Seawater. Compositions from ASTM A240/A276/A312/A790. PREN calculated as %Cr + 3.3×%Mo + 16×%N using nominal midpoint compositions. Relative cost index based on JN Alloy procurement data, H1 2026. Seawater ratings per NACE International and EFC Publication No. 32.

Not all seawater environments are equally aggressive. The following matrix maps service conditions to recommended grades, enabling precise selection decisions.

Seawater Environment Conditions

Source: NACE Technical Report 6J187, "Performance of Stainless Steels in Seawater and Related Environments," 2019; Outokumpu Stainless Steel Handbook, 13th Ed., 2021.

Service Condition	Temp. (°C)	Recommended Minimum Grade	Preferred Grade	Notes / Cautions
Atmospheric coastal (spray, mist)	Ambient	316L	2205	316L acceptable for light exposure; clean regularly to remove chloride deposits
Brackish water (low salinity, estuary)	<30	316L	2205	Chloride < 1,000 ppm; 316L borderline. Prefer 2205 for long-term reliability
Seawater piping (ambient, flowing)	<30	2205	2507 / 254 SMO	Flow velocity >1.5 m/s prevents biofouling; critical to avoid dead legs where stagnation occurs
Seawater piping (elevated temp.)	30–60	254 SMO / AL-6XN	2507	Temperature accelerates pitting; PREN ≥40 mandatory for this range
Heat exchanger tubes (seawater-cooled)	20–60	254 SMO / AL-6XN	2507 / Titanium	Most demanding seawater application; crevice attack at tube-to-tubesheet joints is primary risk
Seawater pump impellers/shafts	Ambient	2205	2507 / Duplex CD4MCu	Erosion-corrosion from entrained sand; high-strength duplex grades outperform 316L significantly
Offshore structural (splash zone)	Ambient	2205	2507 / Zeron 100	Splash zone combines chloride, oxygen, and cyclic wetting - most corrosive marine zone
Desalination (RO membranes, brine)	20–45	254 SMO / AL-6XN	2507	Concentrated brine can have 2× normal seawater chloride levels; super-alloys mandatory
Subsea (fully immersed, cathodic protection)	<20	2205 (with CP)	2507 / Zeron 100	Cathodic protection mitigates crevice risk; verify hydrogen embrittlement is not a concern for highly-stressed components
High-temp seawater (>80 °C)	>60	Titanium / Ni alloys	Hastelloy C-276	No standard SS grade is reliably safe above 80 °C in seawater; SCC risk is severe

Table 4: Seawater Environment Conditions Matrix. Source: NACE Technical Report 6J187 (2019); Outokumpu Stainless Steel Handbook, 13th Ed. (2021); DNV-RP-F112 (Duplex SS Subsea); Sandvik Corrosion Handbook (2020).

Grade 316L (UNS S31603) - Marginal for Seawater

Bottom Line: 316L is the most common industrial stainless steel, but its PREN of ~25 puts it below the recommended threshold for seawater service. Use it only for short-term coastal atmospheric exposure or very low-chloride brackish water. Budget for regular inspection and do not specify it for continuously immersed or heat exchanger service in full seawater.

Key properties: UTS ~485 MPa | Yield ~170 MPa | Elongation 40% | Good weldability | ASTM A312 TP316L for pipe.

Grade 2205 Duplex (UNS S32205 / S31803) - Recommended for General Seawater

Bottom Line: 2205 is the workhorse duplex grade and the minimum recommended for most seawater structural applications. Its PREN of ~35 puts it right at the threshold, making it suitable for piping, pump housings, heat exchanger shells, and offshore structural members at ambient temperature. Its dual austenite-ferrite microstructure gives it roughly twice the yield strength of 316L, enabling thinner-wall designs.

Key properties: UTS ~620 MPa min | Yield ~450 MPa min | Elongation ≥25% | Excellent SCC resistance (vs. austenitic) | ASTM A790 for pipe, ASTM A789 for tubing

Important: 2205 should be used with caution above 25°C seawater service and is not recommended for heat exchanger tube applications where the PREN ≥40 threshold applies.

Super Duplex 2507 (UNS S32750) - Premier Choice for Full Seawater

Bottom Line: 2507 is the most widely specified super duplex grade for demanding seawater applications. With a PREN of ~43 and exceptional strength (yield strength ≥550 MPa), it handles continuous immersion, elevated temperatures up to 60°C, high-velocity flows, and concentrated brine. It is the reference material for offshore oil and gas seawater injection/disposal lines and desalination high-pressure pump components.

Key properties: UTS ≥795 MPa | Yield ≥550 MPa | Elongation ≥15% | ASTM A790 / A789 for pipe/tube | ASTM A276 for bar

Note: Requires careful welding procedure (low heat input, correct filler, no preheating) to maintain phase balance. Use matching filler ER2594 (AWS A5.9).

254 SMO / AL-6XN (6Mo Austenitic) - Superior for Heat Exchanger Tubes

Bottom Line: The 6Mo austenitic grades (254 SMO / AL-6XN) combine PREN ≥43 with the excellent formability, weldability, and thin-wall tube-making capability of austenitic grades. This makes them the preferred choice for heat exchanger tubing in seawater service, where the duplex grades' limited ductility makes tube-to-tubesheet expansion more challenging. They are also specified for flue-gas desulfurization (FGD) scrubbers and coastal chemical plants.

Key properties (254 SMO): UTS ~650 MPa | Yield ~300 MPa | Elongation ≥35% | ASTM B673 (pipe/tube, UNS N08367) | ASTM B676 (AL-6XN seamless tube)

Source: ASM Handbook Vol. 13B, "Corrosion: Materials," 2005; NACE International immersion test data; Outokumpu research publication "Performance of Austenitic and Duplex Stainless Steels in Seawater," 2018. Test conditions: natural seawater, 25°C, 90-day immersion unless noted.

Grade	PREN	Pitting (25°C seawater)	Crevice (25°C)	Pitting Temp. (CPT, °C)*	Seawater Service Life
304L	~18	Severe pitting within weeks	Rapid attack	<10°C	Unsuitable - do not use
316L	~25	Pitting within 1–3 months at ambient	Significant crevice attack	~15–20°C	Marginal - short term only
2205	~35	No pitting at 25°C seawater (immersion)	Mild crevice risk at elevated temp.	~30°C	Good - suitable for most marine use
254 SMO	~43	No pitting up to 40°C seawater	Minimal to nil	~60°C	Excellent - industry standard
AL-6XN	~46	No pitting up to 50°C seawater	Minimal to nil	~65°C	Excellent - seawater + FGD
2507	~43	No pitting up to 40°C seawater	Minimal to nil	~50°C	Excellent - offshore premier choice

Table 5: Corrosion Performance in Seawater. *CPT (Critical Pitting Temperature) measured in 3% NaCl by ASTM G150 electrochemical method. Service life assessments based on field case studies in ASM Handbook Vol. 13B and Outokumpu Technical Report 2018. "No pitting" = corrosion rate <0.025 mm/yr in immersion test.

Q1: Can 316L stainless steel be used in seawater?

316L stainless steel (UNS S31603) has a PREN of approximately 25, which is below the PREN ≥40 threshold recommended for full seawater service. 316L is therefore only suitable for incidental or short-term seawater exposure - such as coastal atmospheric spray, low-chloride brackish water (<1,000 ppm Cl⁻), or splash zones where the surface is regularly rinsed.

For continuous immersion in seawater, heat exchangers, or any application above 20°C in seawater, engineers should specify a minimum of Duplex 2205 (PREN ~35) and preferably a super duplex or 6Mo grade (PREN ≥40).

Q2: What is the best stainless steel for seawater heat exchanger tubes?

The best stainless steels for seawater-cooled heat exchanger tubes are the 6-molybdenum austenitic grades: 254 SMO (UNS S31254, ASTM B673) and AL-6XN (UNS N08367, ASTM B676).

Both achieve PREN ≥43, providing reliable resistance to pitting and crevice corrosion in seawater up to approximately 60°C. These austenitic grades are preferred over duplex grades for tube applications because their higher ductility allows tube-to-tubesheet expansion without cracking. For temperatures above 60°C in seawater, Grade 2 Titanium (ASTM B338) is the industry standard.

Q3: What does PREN mean, and what PREN value is needed for seawater?

PREN (Pitting Resistance Equivalent Number) is a formula - PREN = %Cr + 3.3×%Mo + 16×%N - that predicts a stainless steel's resistance to pitting corrosion in chloride environments. A higher number means better resistance.

The critical threshold for seawater service is PREN ≥40: at this level, the alloy resists pitting in natural seawater at ambient temperatures. For elevated-temperature seawater (>30°C) or concentrated brine, PREN ≥43 is recommended. Common grades meeting the PREN ≥40 threshold include Super Duplex 2507 (PREN ~43), 254 SMO (PREN ~43), and AL-6XN (PREN ~46).

Q4: Is duplex stainless steel better than 316L for seawater?

Yes - for most seawater applications, Duplex 2205 and Super Duplex 2507 significantly outperform 316L stainless steel. Duplex 2205 has a PREN of ~35 (vs. ~25 for 316L), making it suitable for seawater piping, pump housings, and structural applications at ambient temperature. Super Duplex 2507 (PREN ~43) is suitable for continuous immersion, elevated-temperature service, and concentrated brine.

Additionally, duplex grades have approximately twice the yield strength of 316L (~450 MPa vs. ~170 MPa), enabling thinner-wall designs that partly offset the higher material cost. Duplex grades also have significantly better resistance to stress-corrosion cracking compared to austenitic grades like 316L.

Q5: What stainless steel is used in desalination plants?

Modern desalination plants specify different grades depending on the service zone. For piping and pressure vessels handling concentrated brine at 20–45°C, Super Duplex 2507 (UNS S32750, ASTM A790) and 254 SMO / AL-6XN are the standard choices, all with PREN ≥40.

Heat exchanger tubes use 254 SMO or AL-6XN seamless tubes (ASTM B673/B676) for intermediate-temperature brine, and Grade 2 Titanium (ASTM B338) for high-temperature stages. For the high-pressure pump components in reverse osmosis (RO) systems, Super Duplex 2507 is the preferred alloy due to its combination of high strength and corrosion resistance. 316L is not used in modern desalination plant process service due to its insufficient PREN.

Q6: What are the ASTM standards for ordering stainless steel for seawater service?

316L: ASTM A312 TP316L (seamless/welded pipe), A240 Type 316L (plate), A276 Type 316L (bar)

2205 Duplex: ASTM A790 (seamless/welded pipe), A789 (tubing), A240 UNS S32205 (plate), A276 S32205 (bar)

2507 Super Duplex: ASTM A790 UNS S32750 (pipe), A789 S32750 (tubing), A240 S32750 (plate), A276 S32750 (bar)

254 SMO: ASTM B673 UNS S31254 (pipe/tube), A240 S31254 (plate), A276 S31254 (bar)

AL-6XN: ASTM B676 UNS N08367 (seamless tube), B673 N08367 (pipe), A240 N08367 (plate)

Zeron 100: ASTM A790 UNS S32760 (pipe), A240 S32760 (plate)

Q7: How does temperature affect stainless steel selection for seawater?

Temperature is a critical multiplier of seawater corrosivity. As temperature rises, the Critical Pitting Temperature (CPT) of the alloy may be exceeded, triggering rapid pitting. The general rules:

• <20°C (ambient coastal): 2205 is generally adequate for most seawater service.

• 20–40°C: PREN ≥40 is required (2507, 254 SMO, AL-6XN).

• 40–60°C: PREN ≥43 recommended; AL-6XN with CPT ~65°C preferred.

• >60°C: No standard stainless steel is reliably safe in seawater. Specify Grade 2 Titanium or Hastelloy C-276.