S31803 Corrosion Resistance in Seawater Pitting, Crevice, and Stress Corrosion Cracking Data

Duplex 2205 (UNS S31803) performs well in seawater but is not immune: it resists pitting and crevice attack below about 35-40 deg C and 20-25 deg C respectively (in standard ferric-chloride tests), and is far more resistant to chloride stress corrosion cracking than 316L, but should be limited to roughly 60 deg C continuous service in aerated seawater with good design practices.

In one sentence: 2205 is a strong, cost-effective choice for many seawater duties below about 60 deg C, but it must be specified and fabricated carefully - especially avoiding tight crevices and stagnant zones - or it will pit and crevice-corrode like any stainless steel.

S31803 Corrosion Resistance in Seawater Pitting

Key Takeaways

PREN of S31803 is about 32-35 (S32205 grade is 34-38) - high enough for many but not all seawater applications.
Critical Pitting Temperature (CPT): about 35-40 deg C in ASTM G48 ferric-chloride testing.
Critical Crevice Temperature (CCT): about 20-25 deg C - crevices are the weak point, more sensitive than open surfaces.
Chloride Stress Corrosion Cracking (SCC): 2205 is far more resistant than 316L; safe to about 60 deg C in aerated seawater (316L fails above ~60 deg C).
Flow velocity matters: seawater moving above ~1-1.5 m/s prevents deposits and biofouling that trigger localized corrosion.
Welding must use ER2209 filler and controlled heat input; post-weld pickling restores corrosion resistance.

S31803 is the original UNS designation for Duplex 2205, a two-phase (austenite + ferrite) stainless steel; the newer, tighter-spec version is UNS S32205. Both share roughly 22% Cr, 5% Ni, 3% Mo, and nitrogen, giving a PREN of about 32-38.

The "duplex" microstructure - about half ferrite, half austenite - is the reason 2205 beats 316L in both strength and chloride corrosion. The ferritic phase blocks the path of chloride stress corrosion cracking, while chromium, molybdenum, and nitrogen raise pitting resistance.

Element	S31803	S32205 (improved)
Chromium	21-23%	22-23%
Nickel	4.5-6.5%	4.5-6.5%
Molybdenum	2.5-3.5%	3.0-3.5%
Nitrogen	0.08-0.20%	0.14-0.20%
PREN (approx)	32-35	34-38

Pitting starts where the passive film breaks down - typically under deposits, biofouling, or scratches - and chloride ions attack the exposed metal, forming small deep holes that can perforate a wall.

Seawater is aggressive because it carries about 19,000-35,000 ppm chloride plus oxygen. Chloride is the ion that breaks stainless steel passive film; oxygen (from splashing/aeration) drives the corrosion cell. Warm water accelerates the reaction. Once a pit nucleates, it is self-sustaining and grows fast in stagnant zones.

In ASTM G48 ferric-chloride tests, S31803 / S32205 shows a Critical Pitting Temperature (CPT) of about 35-40 deg C. Above this temperature the risk of pitting in aggressive chloride rises sharply.

CPT is measured per ASTM G48 Method A (6% FeCl3 at increasing temperature until pits appear). Real seawater has lower chloride than FeCl3 solution, so field CPT is higher - but the test gives a conservative ranking. Practical guidance: 2205 is reliable for open, flowing seawater below about 30-35 deg C; above that, design margins shrink and 2507 (super duplex) or higher alloys are preferred.

Alloy	PREN	CPT (ASTM G48, approx)
316L	24-26	~20-25 deg C
S31803 / 2205	32-38	~35-40 deg C
S32750 / 2507	40-43	~55-70 deg C

Crevice corrosion is worse than pitting for 2205: under bolted joints, gaskets, lap welds, or marine growth, stagnant seawater depletes oxygen and acidifies, attacking the metal inside the crevice.

How Does Crevice Corrosion Behave in Seawater

Because crevices trap water and cut off oxygen replenishment, the metal inside becomes anodic and corrodes even at temperatures where open surfaces are safe. This is the single most common failure mode for stainless steels in seawater.

The Critical Crevice Temperature (CCT) of 2205 in ASTM G48 ferric-chloride testing is about 20-25 deg C - roughly 10-15 deg C lower than its CPT, confirming crevices are the limiting factor.

Alloy	CCT (ASTM G48, approx)	Practical seawater limit
316L	~10-15 deg C	Rarely suitable
S31803 / 2205	~20-25 deg C	Good below ~20-30 deg C flow
S32750 / 2507	~40-50 deg C	Excellent to ~50 deg C

Design lesson: keep 2205 seawater service cool (below about 25-30 deg C in creviced areas), use continuous flow, and avoid tight bolted crevices without sealing or cathodic protection.

2205 is dramatically more resistant to chloride SCC than 316L thanks to its ferritic phase, which blocks crack propagation. It is generally considered safe in aerated seawater up to about 60 deg C, whereas 316L can crack above ~60 deg C.

Chloride SCC needs three things: tensile stress, chloride, and sufficient temperature. 316L (fully austenitic) fails by SCC in hot chloride surprisingly often. The ferritic half of 2205 stops cracks from running, raising the threshold temperature substantially. Industry practice (e.g., Norsok M-001) limits duplex stainless in seawater to about 60 deg C unless verified otherwise.

Alloy family	Chloride SCC threshold (seawater, approx)
Austenitic 316L	Fails above ~60 deg C (avoid)
Duplex 2205 (S31803)	Usable to ~60 deg C; conservative limit
Super Duplex 2507	Usable to ~80-90 deg C

For seawater: 316L is marginal and usually avoided for continuous immersion; 2205 is the standard economical workhorse for flowing cool seawater; 2507 is the premium choice for warm, high-velocity, or high-chloride seawater.

Metric	316L	S31803 / 2205	S32750 / 2507
PREN	24-26	32-38	40-43
CPT	~20-25 deg C	~35-40 deg C	~55-70 deg C
CCT	~10-15 deg C	~20-25 deg C	~40-50 deg C
SCC limit	~60 deg C (avoid)	~60 deg C	~80-90 deg C
Cost (per kg)	1.0x	~1.6x	~2.3x

Five factors dominate whether 2205 survives in seawater: temperature, flow velocity, oxygen, crevices/deposits, and biofouling. Control these and 2205 performs for decades.

Temperature: stay below ~30-35 deg C in creviced zones, ~60 deg C max for SCC safety.
Flow velocity: keep above ~1-1.5 m/s to prevent deposits and biofouling; avoid stagnant dead legs.
Oxygen: aerated (splashed) zones are more corrosive but also self-passivating; design for full wetting, not intermittent.
Deposits and marine growth: clean regularly; fouling creates crevices that pit.
Cathodic protection: coupling 2205 to sacrificial anodes (Zn/Al) or impressed current extends life in settled zones.

Correct welding preserves 2205 corrosion resistance; wrong heat input or skipping post-weld cleaning can ruin it. Use ER2209 filler, control interpass below 150 deg C, and always pickle/passivate after welding.

Filler: ER2209 (matching duplex) to keep PREN and phase balance.
Heat input: keep in ~0.5-2.5 kJ/mm; too hot forms brittle sigma phase, too cold leaves excess ferrite.
Phase balance: target 35-65% ferrite; verify with PMI / ferrite scope.
Post-weld: pickling + passivation restores the chromium-rich passive film; grinding alone is not enough.
Avoid carbon steel contamination (iron particles cause rust spots) - use dedicated tools.

Do not use 2205 for warm (>60 deg C) stagnant seawater, tight unvented crevices at >30 deg C, continuous immersion above its CPT, or cryogenic/LNG duty. In those cases step up to 2507, 6Mo super austenitic, or a nickel alloy.

Warm, oxygen-rich, low-flow seawater above ~35-40 deg C: choose 2507 or 6Mo.
Tight bolted flange crevices in tropical waters: seal, CP, or upgrade alloy.
Cryogenic seawater / LNG (-162 deg C): use 304L/316L, not duplex.
High H2S + chloride (sour) severe service: verify NACE MR0175 and consider 2507.

Q: Is S31803 the same as 2205?

A: S31803 is the original UNS for Duplex 2205; S32205 is the newer version with tighter, higher nitrogen/molybdenum chemistry and slightly better corrosion resistance. Most mills now supply S32205.

Q: Can 2205 be used in seawater?

A: Yes, for flowing, cool (below about 30-35 deg C crevice / 60 deg C SCC) seawater with good design. It is not for warm stagnant seawater or tight crevices without protection.

Q: What is the CPT of Duplex 2205?

A: Approximately 35-40 deg C in ASTM G48 ferric-chloride testing; field performance in real seawater is typically better but designs should keep a margin.

Q: Is 2205 better than 316L in seawater?

A: Yes - 2205 has roughly 1.3-1.5x the PREN of 316L and far better chloride SCC resistance, making it the standard choice where 316L would fail.

Q: Does 2205 need cathodic protection in seawater?

A: Not always, but in stagnant or creviced zones coupling to sacrificial anodes significantly extends life and is common practice.

Q: What temperature is safe for 2205 in seawater?

A: Keep creviced areas below about 25-30 deg C and overall service below about 60 deg C to avoid SCC; cooler and flowing is always safer.

S31803 / Duplex 2205 is a proven, cost-effective material for many seawater applications, with PREN around 32-38, CPT about 35-40 deg C, CCT about 20-25 deg C, and chloride SCC resistance far superior to 316L. Its limits are clear: crevices and warm stagnant water are its enemies, and continuous service should stay below about 60 deg C.

Rule of thumb: for cool, flowing seawater with controlled crevices, 2205 is the smart default. For warm, high-chloride, or severe crevice service, step up to Super Duplex 2507.

Need help specifying? JN Alloy stocks Duplex 2205 (S31803/S32205) and Super Duplex 2507 in pipe, plate, bar, flanges, and fittings, with ISO 9001, ASME U, and PED/CE certifications and full material test reports. Contact our engineers: Market@jnalloy.com | +86-193-3990-0211 | www.jnalloy.com.