On duplex 2205 vs 316 corrosion, the gap is a step change. Duplex 2205 is substantially more corrosion resistant than 316 stainless steel: PREN around 35 against 24 to 26, a critical pitting temperature near 35 to 40°C against 15 to 20°C, and chloride stress corrosion cracking resistance past 100°C where 316 becomes vulnerable above about 60°C. The decisive question is not which grade is better. It is the chloride level and temperature at which 316 stops working.
A chemical plant ran a 316L brine line at 45°C. The line passed every inspection at commissioning. Eighteen months later, pinhole leaks from chloride pitting shut the process down for four days.
Nothing about the grade changed. The chloride concentration and the temperature did, and the pitting limit for 316 is temperature-dependent in a way most specifications never state out loud.
Duplex 2205 vs 316 corrosion resistance is usually compressed into a single line. This article supplies what the datasheets leave out. You’ll get the chloride and temperature thresholds that decide a specification, the test methods behind the published numbers, the weld-zone chemistry that can undo a correct grade choice on site, and the environments where duplex 2205 fails too. For the full grade comparison first, see our duplex 2205 vs 316 stainless steel comparison.
Key Takeaways
- Duplex 2205 carries a PREN of about 35 against 24 to 26 for 316L, and its critical pitting temperature is roughly double: 35 to 40°C against 15 to 20°C.
- Crevice corrosion, not pitting, is the real limit on 2205. Its critical crevice temperature of 20 to 25°C sits 10 to 15°C below its pitting temperature, and crevices at gaskets and flanged joints are where 2205 projects actually fail.
- The corrosion decision is two-dimensional. At 60°C, 316L’s pitting limit falls below roughly 200 ppm chloride while 2205 tolerates 8,000 to 10,000 ppm in non-acidic service.
- NORSOK M-001 caps unprotected 316 at 60°C for chloride stress corrosion cracking and duplex at 100°C. Field cracking below both limits is documented, so treat them as minimums requiring margin.
- Welding can erase the advantage entirely. Phase balance, nitrogen loss, and sigma phase in the 600 to 1000°C band decide whether the delivered fabrication performs like duplex at all.
Duplex 2205 vs 316 Corrosion Resistance at a Glance
Duplex 2205 corrosion resistance and 316 corrosion resistance are separate on every line of the table below. Every figure is a test-derived benchmark with a named method, not a service guarantee.
| Corrosion measure | 316 / 316L | Duplex 2205 |
|---|---|---|
| PREN | 24 to 26 | 34 to 36 |
| Critical pitting temperature (ASTM G48 Method E) | 15 to 20°C | 35 to 40°C |
| Critical crevice temperature (ASTM G48 Method F) | Below 0°C | 20 to 25°C |
| Chloride SCC limit (NORSOK M-001, unprotected) | 60°C | 100°C |
| Chloride tolerance at 60°C, non-acidic | Under 200 ppm | 8,000 to 10,000 ppm |
| Seawater service (NORSOK M-001) | Not qualified | Entry-level grade |
The gap is a step change, not a marginal improvement. A ten-point PREN difference and a twenty-degree difference in pitting temperature move a component from a two-year replacement cycle to a multi-decade one.
One qualification applies to every number here. These values describe material that is correct to specification, correctly fabricated, and clean at the surface. Composition at the low end of the range, a rough weld toe, an embedded iron particle, or a stagnant crevice will move the real limit materially below the table. Our stainless steel corrosion resistance guide covers the passive-layer mechanism that duplex stainless steel corrosion resistance depends on.
Not sure which side of these thresholds your process sits on? Send us the chloride level, operating temperature, and pH, and our engineers will tell you whether 316L holds or 2205 is required. Get material selection help.
PREN: How Corrosion Resistance Is Ranked (and Where the Number Misleads)
PREN, the Pitting Resistance Equivalent Number, is the industry’s shorthand for ranking stainless steels in chloride service. Every duplex 2205 vs 316 corrosion comparison quotes it. Few explain the formula or its limits.
The PREN Formula and Worked Values
PREN = %Cr + 3.3 × %Mo + 16 × %N
Running the midpoint compositions of each grade:
- Duplex 2205: 22 + (3.3 × 3.25) + (16 × 0.17) = about 35. The older S31803 specification lands nearer 32 to 35; the tighter S32205 range lands at 34 to 38.
- 316L: 17 + (3.3 × 2.5) + (16 × 0) = about 25
- 304: 18 + (3.3 × 0) + (16 × 0.05) = about 19
- Super duplex 2507: 40 to 43, which is where the ladder goes next
The molybdenum and nitrogen terms do the work. Nitrogen is the reason 2205 overtakes far more expensive austenitic grades despite using roughly half the nickel of 316. The British Stainless Steel Association documents the formula and its coefficient variants, and IMOA maintains reference data on how molybdenum behaves in duplex grades.
Why the Number Isn’t the Whole Answer
PREN is a screening index, not a test result, and three caveats matter to anyone specifying from it.
- The coefficients are empirical. Nitrogen’s multiplier of 16 is a fitting constant, not a physical law, and some sources use 3.0 rather than 3.3 for molybdenum. A PREN difference of one point doesn’t translate to a measurable field difference, while a difference of ten does.
- PREN says nothing about geometry or temperature. It doesn’t account for crevice severity, weld microstructure, surface finish, flow, or operating temperature. Those variables decide more failures than the index does.
- The value is heat-specific. Molybdenum and nitrogen sitting at the bottom of the specification range can pull an individual heat of “2205” well below the nominal 35.
That last point isn’t academic. A European fabricator received a consignment of 2205 tube with molybdenum at 3.02% and nitrogen at 0.15%, both inside the S32205 specification.
The calculated PREN for those heats came to 32.4, not 35.
The material was fully compliant, and it was measurably less corrosion resistant than the design assumed. The buyer had specified the grade rather than the heat. Only the mill’s calculated PREN revealed the difference.
The practical response is a single line in the purchase specification: request the calculated PREN for the delivered heat on the mill certificate. Our stainless steel composition article covers how chromium, molybdenum, and nitrogen drive performance across the grade family.
The Chloride–Temperature Grid: Where 316 Stops and 2205 Starts
Corrosion resistance is not a property a grade has. It is a boundary the grade holds until the combination of chloride concentration and temperature pushes past it. That is why two plants running what looks like the same brine at different temperatures get completely different service lives from identical pipe.
The grid below is an indicative selection aid for the 50 to 70°C band at pH 2 to 6.
| Chloride level | Recommended grade | Practical note |
|---|---|---|
| Under 200 ppm | 304 / 304L | 316L is also suitable and adds margin |
| 200 to 1,000 ppm | 316L | Practical ceiling near 60°C |
| 1,000 to 5,000 ppm | Duplex 2205 or 904L | 254 SMO resists roughly 15,000 ppm at 60°C |
| Above 5,000 ppm | 2507 super duplex | Titanium where reducing acids are also present |
Three numbers explain why this table exists.
- 316L at 60°C: the pitting and crevice limit collapses to below roughly 200 ppm chloride. At lower temperatures, the same grade tolerates far more, which is why 316 works in one part of a plant and fails in another.
- 2205 at 60°C: tolerates roughly 8,000 to 10,000 ppm in non-acidic service, comparable to 904L at around 8,500 ppm.
- Seawater: contains 18,000 to 35,000 ppm chloride. That single figure is why 316L is not a seawater material, and 2205 is the entry grade rather than a luxury option.
One warning overrides the grid. Chlorides concentrate. Evaporation under insulation, splash and dry cycling, and dripping condensation can drive the local chloride at a surface to tens of percent even when the bulk fluid is dilute. A line whose bulk chemistry sits comfortably in the 316L row will still pit at a wet-dry interface, and this is the mechanism behind a large share of “unexplained” coastal plant failures.
Sourcing duplex 2205 or 316 for a chloride duty? We supply both grades in sheet, plate, coil, and stainless steel pipes, with heat-specific PREN reported on the mill certificate. Request a quote.
Pitting and Crevice Corrosion: The Two Failure Modes, Separated
Most comparisons merge 2205 vs 316 pitting corrosion and crevice corrosion into one “chloride resistance” line. They are separate mechanisms with separate thresholds, and for duplex 2205 the distinction decides the design.
Pitting: The Critical Pitting Temperature
The critical pitting temperature (CPT) is the temperature at which a stable pit can initiate and propagate in a standardized chloride test. Below it, pits tend to repassivate. Above it, they grow.
- 316L CPT: roughly 15 to 20°C by ASTM G48 Method E. Some datasets place it as low as 10 to 15°C at higher chloride concentrations, a reminder that CPT is itself chloride-dependent rather than a single fixed number.
- 2205 CPT: roughly 35 to 40°C
For a process engineer, the gap is the whole argument. A 2205 component removes a failure mode that 316L carries for most of its working temperature range.
Crevice: The Real Limit on 2205
This is the section most supplier pages omit, and it is where duplex projects fail.
- 2205 critical crevice temperature (CCT): roughly 20 to 25°C, which is 10 to 15°C below its own pitting temperature
- 316L CCT: effectively at or below ambient, with published values as low as -2°C and others in the 10 to 15°C range
Crevices are unavoidable geometry in real equipment. They form at gaskets and flanged joints, under deposits and biofouling, at weld toes and undercuts, inside threaded connections, and in tube-to-tubesheet gaps. Inside a crevice, oxygen cannot replenish and chloride concentrates, so the local chemistry is far more aggressive than the bulk fluid.
This is the honest headline for duplex 2205. Its binding constraint in seawater is not pitting at 35 to 40°C. It is crevice corrosion at 20 to 25°C.
The design response is geometric rather than metallurgical. Seal or eliminate crevices, specify full-penetration welds with smooth toes, control deposits, and keep creviced 2205 seawater service below roughly 25 to 30°C. Above that, the ladder moves to super duplex 2507.
One test-method note is worth keeping in mind. ASTM G48 tests in ferric chloride solution, which is deliberately harsher than real seawater because of its low pH and high chloride content. Field performance is generally better than the G48 ranking suggests, so treat these figures as conservative screening benchmarks rather than knife-edge service limits.
Chloride Stress Corrosion Cracking: The Failure That Ends 316
Chloride stress corrosion cracking (SCC) is the failure mode that removes 316 from warm service. It needs three things at once: chlorides, tensile stress, and temperature. Remove any one and the mechanism stops.
Fully austenitic 316 offers no microstructural barrier once a crack initiates. Duplex 2205 does. With roughly 50% ferrite by volume, a crack propagating through austenite meets ferrite and is deflected rather than driven through. That microstructural interruption, combined with the higher PREN, is the entire engineering reason duplex outperforms austenitic grades in this failure mode.
The published limits are straightforward, and their caveats are not.
- NORSOK M-001: maximum unprotected operating temperature of 60°C for 316 and 100°C for duplex stainless steels. Above those temperatures, the standard requires protective coating per NORSOK M-501.
- BS EN ISO 21457 places duplex SCC thresholds in the 80 to 100°C band.
- Operator-reported thresholds range from 70°C to 112°C. The original 1997 NORSOK limit for 22Cr duplex was 110°C, later reduced to 100°C to build in margin.
- Field cracking below the limits: documented. Duplex SCC has been reported as low as 70 to 80°C where evaporated chloride films form in humid environments, and the adequacy of the NORSOK limits is actively debated for exactly this reason.
Treat 60°C and 100°C as standards-based ceilings that require margin, not as safe operating points.
The insulated-line trap deserves its own warning, because it breaks the assumption that bulk temperature is the deciding number. A coastal chemical plant ran an insulated 316 line carrying a chloride-bearing stream at a bulk temperature well below 60°C for most of the year.
Insulation wetted by condensation and drying cycles concentrated chloride against the pipe surface. The line cracked externally, from the outside in, at a location where the metal itself had never been above 50°C. The fluid temperature was never the problem. The surface temperature under the lagging was.
Seawater Service: 2205 Is the Entry Grade, Not the Finish Line
Duplex 2205 vs 316 seawater service is where the grade decision is forced most often, and it is also where over-simplified advice does the most damage.
The Standards Position
Under NORSOK M-001, the Norwegian offshore material selection standard, 316L is not qualified for seawater service. Duplex 2205 is the entry-level seawater material, with 6Mo alloys and super duplex 2507 above it.
The reason becomes obvious when you compare the crevice data to the environment. Seawater carries 18,000 to 35,000 ppm chloride, while 2205’s critical crevice temperature is 20 to 25°C. Any creviced geometry in natural seawater is operating near or above the material’s limit on a warm day.
Industry guidance recommends a PREN of at least 40 for continuous seawater immersion, which is precisely the criterion that pushes a project past 2205 and into super duplex territory:
| Seawater condition | Recommended minimum PREN |
|---|---|
| Ambient seawater, 20 to 25°C | 32 |
| Warm seawater, 35 to 40°C | 35 |
| Heated seawater, 40 to 60°C | 40 |
Zone by Zone
Seawater is not one condition. It behaves differently by exposure zone, and the same grade can be comfortable in one and marginal in another.
- Atmospheric zone: 2205 is comfortable. Above the splash line there is no continuous electrolyte.
- Splash and tidal zone: drying cycles concentrate chloride and prevent cathodic protection from working reliably. This is the most aggressive zone for any stainless grade.
- Continuous immersion, open and flowing: 2205 performs well below roughly 30 to 35°C, provided flow prevents fouling and no crevices exist.
- Continuous immersion, creviced: the binding constraint. Keep creviced service below about 25 to 30°C with continuous flow, or seal the crevices.
- Chlorination: a residual free chlorine above about 0.5 ppm sharply reduces the safe temperature for austenitic grades and must be factored into 2205 designs as well.
Where Even 2205 Fails in Seawater
A Middle East seawater treatment plant installed 2205 filter vessels. They failed after six weeks in service. The failures were pitting at welds and crevice corrosion at flanged joints, reported in the Nickel Institute’s desalination handbook.
The instructive part is that this was not a grade failure. 2205 was the correct selection for the bulk seawater chemistry. The failures came from welding quality and joint geometry: incomplete penetration, rough weld toes, and tight bolted flanges in warm chlorinated seawater. A plant with the same grade, better welding, and welded-in-place connections in a colder seawater supply would have run for decades.
The broader service-life picture supports that reading. Published desalination experience puts 316 valves and fittings in direct seawater contact on a replacement cycle of 3 to 5 years, against 15 to 25 years for 2205 in the same duty. That is an industry figure from documented plant experience, not a guarantee, and it depends entirely on fabrication quality. Our article on 316 stainless steel for marine service covers where austenitic grades stop.
Corrosion at Welds: Why Fabrication Decides the Result
Duplex 2205 is the only material in this comparison whose corrosion resistance is a microstructure property rather than a composition property. Welding destroys that microstructure and rebuilds it, and the rebuild is rarely as good as the original. The heat-affected zone is normally the highest-risk location on a duplex fabrication.
Phase Balance
The target is roughly 35 to 65% ferrite, usually expressed as 30 to 70 ferrite number (FN). Outside that band, corrosion and mechanical performance both suffer.
- Below about 25% ferrite: the alloy loses the ferrite barrier that interrupts crack propagation and begins behaving like an austenitic grade. The SCC advantage that justified the upgrade is gone.
- Above about 70% ferrite: impact toughness drops and susceptibility to hydrogen cracking rises.
Nitrogen Is the Lever
Nitrogen is a strong austenite stabilizer, and it burns out of the weld pool during welding. Measured data from TIG-welded S32205 shows the effect directly:
| Shielding gas | Ferrite content |
|---|---|
| Pure argon | 63.3% |
| Argon with 1% N₂ | 54.2% |
| Argon with 6% N₂ | 36.5% |
This is why ER2209 filler metal is deliberately over-alloyed with 2 to 4% more nickel than the base metal. The extra nickel compensates for the nitrogen lost in the arc so the deposit can rebalance toward the target phase mix. Autogenous GTAW without nitrogen addition is discouraged for the same reason.
Sigma Phase
Sigma is a brittle, chromium- and molybdenum-rich intermetallic phase that forms between roughly 600 and 1000°C, with the fastest precipitation around 850°C. In duplex it can nucleate at ferrite-austenite boundaries within 2 to 5 minutes in the 800 to 900°C band.
The corrosion consequence is the important one here. Sigma formation strips chromium and molybdenum out of the surrounding matrix, which lowers the local PREN below the pitting threshold and creates preferential initiation sites. Roughly half of analyzed duplex failures involve sigma phase, and Charpy impact toughness can fall from above 100 J to below 20 J at the same time.
A 2205 gas flowline flange fractured 11 hours after entering service. Post-failure metallography attributed it to sigma phase in the weld and heat-affected zone. Eleven hours is not a slow degradation process. It is a component that was never duplex in the metallurgical sense, only in the compositional one.
Verifying the Weld
These tests cover the screening:
- ASTM A923 Method A: electrolytic etch with metallographic examination, checking for intermetallic phases at 400 to 500× magnification.
- ASTM A923 Method B: Charpy impact testing, with an acceptance criterion of 54 J / 40 ft·lbf minimum.
- ASTM A923 Method C: ferric chloride immersion, requiring no pitting at 20× magnification.
- Phase fraction is measured by point count per ASTM E562 or ISO 17781, and ferrite content by ferritescope per AWS A4.2, taken on root, fill, and cap passes.
The full welding procedure parameters belong to our duplex 2205 welding guide. The point that belongs here is narrower. A duplex component without weld qualification documentation has no demonstrated corrosion resistance, regardless of what the mill certificate says about the plate.
Where Duplex 2205 Fails Too: The Honest Limits
Duplex 2205 is not immune to corrosion, and a specification that treats it as a universal upgrade will produce failures. Six environments end its suitability.
Strongly reducing acids. Hydrochloric acid and other reducing environments prevent the passive film from reforming at all. PREN is irrelevant when the protective oxide cannot exist. Duplex is not the answer here; higher-nickel alloys or titanium are.
Hot concentrated brine. The chloride tolerance figures quoted earlier apply to non-acidic service at moderate temperature. Concentrated brine at elevated temperature attacks 2205 as well, and this is where the ladder steps to 2507 or a 6Mo alloy.
Tight crevices above roughly 25 to 30°C. This is the crevice limit restated as a design constraint, and it is the single most common way a correct grade choice produces an incorrect result.
Sour service. Hydrogen sulfide exposure makes the ferrite phase more susceptible to hydrogen embrittlement than an austenitic grade would be. Acceptability is governed by NACE MR0175 / ISO 15156, which sets hardness and stress limits for the service.
Over-protected cathodic systems. Cathodic protection beyond the required current generates hydrogen at the metal surface, raising the same embrittlement risk durably. Duplex in seawater needs correctly designed CP, not maximum CP.
High-temperature exposure. Sigma phase between 600 and 1000°C and 475°C embrittlement between roughly 300 and 525°C degrade both toughness and corrosion resistance. Our article on duplex 2205 temperature limits covers the exposure windows in full.
Two more mechanisms are worth naming. Microbiologically influenced corrosion affects duplex under deposits and in stagnant low-flow seawater, so flow rate and cleaning strategy matter as much as grade selection.
Galvanic coupling between 2205 and 316 then behaves in a way that surprises most designers. Classic galvanic attack is limited, because both alloys passivate at similar potentials. The real risk sits on the 316 side, which becomes the pitting and SCC concern in aggressive conditions. The coefficient of thermal expansion also differs by roughly 25% (about 8.9 against 7.2 µ·in/in·°F), adding thermal stress at dissimilar-metal joints on top of weld residual stress.
Where reducing acids or extreme chloride levels rule out duplex entirely, our article on titanium vs stainless steel corrosion covers the next step on the ladder.
Verifying Corrosion Resistance Before You Buy
Every threshold in this article assumes the delivered material is what the specification says it is. Verification is where that assumption gets tested, and it is cheap relative to a failure.
- Request the heat-specific calculated PREN on the mill certificate. Ask for the calculation, not the grade datasheet value, and check that chromium, molybdenum, and nitrogen sit where the design assumed.
- Ask for ASTM G48 and ASTM A923 test reports where the specification grades corrosion performance, on heavier sections, and on production welds for marine service.
- Confirm composition by moly spot test, XRF, or PMI on receipt, and confirm EN 10204 3.1 traceability ties the delivered material to the tested heat. This is the practical defence against a substituted heat, and the same check our field guide on how to tell 304 from 316 stainless steel describes one grade lower.
- Verify fabrication follow-through. Pickling and passivation after welding, iron-contamination control, and dedicated stainless tooling all matter. A carbon steel grinding wheel used on a duplex weld toe creates a corrosion initiation site that no certificate will capture.
Documentation quality is not administrative overhead in chloride service. It is the difference between a specification and a guess. Talk to our team about documentation scope and custom processing for your next chloride project.
The Selection Framework: 316L, 2205, or the Next Grade Up
Bring the thresholds together and the decision reduces to four questions: chloride level, temperature, crevice risk, and pH.
| Condition | Recommendation | Reason |
|---|---|---|
| Clean, cold, low-chloride, low crevice risk | 316L | 2205 premium buys nothing |
| Chloride under 200 ppm at 60°C, no crevices | 316L | Inside the pitting limit |
| 200 to 1,000 ppm, moderate temperature | 316L with margin | Watch the 60°C ceiling |
| 1,000 ppm and above, or 316L crevice risk | Duplex 2205 | Above the austenitic crevice limit |
| Seawater, ambient to warm, crevice-controlled | Duplex 2205 | Entry-level seawater grade |
| Heated seawater, or PREN 40 required | Super duplex 2507 | Above the 2205 crevice limit |
| Reducing acids, or very high chlorides | 6Mo alloy or titanium | Passive film cannot reform in 2205 |
The escalation ladder has a corrosion trigger at each step: 316L to 2205 when chlorides at temperature exceed the crevice limit, 2205 to 2507 when continuous immersion requires PREN 40 or creviced service exceeds 25 to 30°C, and past 2507 when reducing chemistry rules out stainless altogether.
At a PREN of about 35 and 1.2 to 1.5 times the price of 316L, duplex 2205 displaces super duplex, 6Mo alloys, and titanium across a large share of chloride service. Our article on duplex 2205 vs 316 cost works through the price and lifecycle consequence of each step.
The audit runs in the other direction too. If the service is clean, under 200 ppm chloride, below 60°C, free of crevices, and inside the temperature window, 316L will serve a full design life and the duplex premium buys nothing. Specifying 2205 there is a cost, not a safety margin. Our 316 stainless steel guide covers the austenitic side in full, and the 304 vs 316 chloride stress corrosion cracking comparison covers the grade decision one step below this one.
Frequently Asked Questions
Is duplex 2205 more corrosion resistant than 316?
Yes. Duplex 2205 carries a PREN of roughly 35 against 24 to 26 for 316L, with a critical pitting temperature near 35 to 40°C against 15 to 20°C. It also resists chloride stress corrosion cracking past 100°C where 316 becomes vulnerable above about 60°C.
What is the PREN of duplex 2205 vs 316?
Using PREN = %Cr + 3.3 × %Mo + 16 × %N, duplex 2205 calculates to about 35 and 316L to about 25. The value is heat-specific, so request the calculated PREN for the delivered heat rather than relying on the grade datasheet figure.
What is the critical pitting temperature of duplex 2205?
Roughly 35 to 40°C by ASTM G48 Method E in ferric chloride. The critical crevice temperature is lower, at about 20 to 25°C, and crevices are the binding constraint on 2205 in seawater. Both values are chloride-concentration dependent and are conservative relative to real seawater.
At what temperature does 316 suffer chloride stress corrosion cracking?
NORSOK M-001 caps unprotected 316 at 60°C, and field cracking has been reported below that where chlorides concentrate by evaporation under insulation. Treat 60°C as a ceiling requiring margin, not as a safe operating point, and account for wet-dry cycling at the metal surface.
Can 316 stainless steel be used in seawater?
No, not for wetted or immersed service. Under NORSOK M-001, 316L is not qualified for seawater because its critical crevice temperature is at or below ambient and it is prone to chloride pitting and SCC. Duplex 2205 is the entry-level seawater material.
Can duplex 2205 be used in seawater?
Yes, with two conditions. Keep creviced geometry below roughly 25 to 30°C with continuous flow, and control chlorination, since residual chlorine above about 0.5 ppm lowers the safe temperature. Where continuous immersion requires a PREN of 40, super duplex 2507 is the correct step up.
Why does duplex resist stress corrosion cracking better than 316?
Because of microstructure, not just chemistry. Duplex contains roughly 50% ferrite, and a crack propagating through austenite is deflected or arrested when it meets a ferrite grain. Fully austenitic 316 has no such barrier, so once a crack initiates it propagates freely.
Does welding reduce the corrosion resistance of duplex 2205?
Yes, potentially. Welding burns out nitrogen and disrupts phase balance, and slow cooling through the 600 to 1000°C band can form sigma phase, which depletes chromium and molybdenum locally. Verification by ASTM A923 Methods A, B, and C and phase measurement per ASTM E562 is how you confirm the weld performs like duplex.
Where does duplex 2205 corrode or fail?
The most common sites are crevices at gaskets, flanged joints, and threaded connections above roughly 25 to 30°C; weld heat-affected zones with poor phase balance or sigma phase; and hot concentrated brine. Strongly reducing acids such as hydrochloric acid rule 2205 out entirely.
Is duplex 2205 resistant to hydrochloric acid?
No. Hydrochloric acid is a reducing acid, and 2205 relies on a chromium oxide passive film that cannot reform in reducing conditions. PREN has no meaning when the protective film cannot exist. Higher-nickel alloys or titanium are the appropriate materials for HCl service.
Does duplex 2205 need cathodic protection in seawater?
Often yes, particularly in creviced or buried geometry, but it must be correctly designed. Over-protection generates hydrogen at the metal surface and raises the risk of hydrogen embrittlement of the ferrite phase, which is governed by NACE MR0175 / ISO 15156.
How do I verify the corrosion resistance of 2205 on a mill certificate?
Request the heat-specific calculated PREN, ASTM G48 test reports where the specification grades corrosion performance, and ASTM A923 screening on welds for marine service. Confirm composition on receipt by XRF or moly spot test, and confirm EN 10204 3.1 traceability ties the material to the tested heat.
Conclusion
Duplex 2205 wins this comparison in chloride service, and it wins by a wide margin. The PREN gap is about 35 against 25. The critical pitting temperature roughly doubles, from 15 to 20°C to 35 to 40°C. And chloride SCC resistance runs past 100°C against a 60°C ceiling for 316.
Together, that is a change in kind rather than degree.
The nuance is that the corrosion decision is two-dimensional. Chloride concentration and temperature act together, and 316L’s limit falls to under 200 ppm chloride once the temperature reaches 60°C. The second nuance is that 2205’s binding constraint is crevice corrosion at 20 to 25°C, not pitting at 35 to 40°C, which makes joint geometry and welding quality as decisive as grade selection.
The honest limits matter just as much. Hot concentrated brine, reducing acids such as HCl, sour service, over-protected cathodic systems, and high-temperature exposure all end 2205’s suitability. A duplex weld without qualification testing has no demonstrated corrosion resistance at all.
Get those conditions right and duplex 2205 vs 316 corrosion stops being a preference and becomes a specification with test evidence behind it.
Contact LIANYUNGANG DAPU METAL CO., LTD for duplex 2205 and 316/316L material, heat-specific PREN on the mill certificate, and corrosion test documentation. Send your service conditions, chloride level, and operating temperature, and we will come back with a grade recommendation, a documentation scope, and a quote.
