Duplex 2205 vs 316L for Chloride and Seawater Service

The Selection Problem

Chloride attack on stainless is localized, not uniform: the passive film survives everywhere except at weak points, and metal loss concentrates into pits and crevices that perforate walls with negligible weight loss. Three variables decide whether that happens: chloride concentration, temperature, and geometry, because crevices under gaskets, deposits, and biofilm lower the threshold dramatically. Velocity, pH, and oxidizing biocides move the same thresholds further.

Grade selection is a zoning exercise, not a single number. The same plant can justify 316L in the low-chloride make-up line, 2205 in the warm cooling return, and a superduplex or 6-moly grade in the chlorinated seawater intake; anything simpler either wastes money or leaks. The pitting resistance equivalent number, PREN, is the common currency that makes the zones comparable across grades and product forms.

What is actually being decided in a 2205 versus 316L RFQ is three things at once. First, the grade: whether the duty is inside the chloride, temperature, and crevice envelope of 316L or needs the duplex step. Second, the wall: 2205 carries roughly double the specified minimum yield, so pressure parts and spanning members can be derated in thickness, which feeds straight back into the price comparison per meter rather than per kilogram. Third, the supply chain: the fabricator must be able to weld and form the grade bought, and duplex procedures are a real capability question, not a line item.

The reference frame is the ASTM product family. Plate, sheet, and strip for 316L and 2205 are specified to ASTM A240 class chemistry and mechanicals, bar and shapes to ASTM A276 class products, welded and seamless pipe to the A312 class, and forgings such as flanges and fittings to the A182 class, with UNS designations S31603 for 316L and S32205 for the current 2205 composition (older dual-certified stock also carries S31803). European practice calls the same grades 1.4404 and 1.4462. None of this changes the corrosion logic; it changes the paperwork, and Chapter 5 returns to it in the RFQ checklist.

Finally, the money. 316L is the cheapest commonly stocked corrosion-grade stainless; 2205 commands a premium that swings with nickel and molybdenum markets but is partially self-offsetting, because the leaner nickel content (5 to 6 percent against 10 to 14) makes 2205 less hostage to nickel spikes, and the double yield buys back wall thickness. The grade decision is where corrosion engineering and procurement cost meet, and both sides deserve numbers, not adjectives.

One procurement trap deserves a name before the metallurgy starts: substitution. Mixed bills of material, where 316L flanges wander into a 2205 piping package or a length of thin-wall austenitic pipe replaces the specified duplex run, are a standard root cause of early chloride failures, because the system corrodes at its least resistant verified component. The defenses are procedural and cheap relative to the leak: positive material identification on receipt, full traceability from heat number to installation, and a change-control rule that any grade substitution re-opens the corrosion review. Projects that treat the grade line items as commodities and the welding consumables as afterthoughts are the ones that generate the failure case studies this page quotes.

Composition, PREN, and Phase Balance

The compositions are close enough to compare line by line and far enough apart to behave differently, which is exactly why the PREN arithmetic exists. The pitting resistance equivalent number ranks pitting resistance from alloy content: percent chromium plus 3.3 x percent molybdenum plus 16 x percent nitrogen. It is a comparative index, not a corrosion rate, but it tracks chloride service remarkably well and it is the number most water-industry selection charts are drawn around.

Run the arithmetic on typical heats and the separation is visible. A mid-range 316L at 17 Cr, 2.5 Mo, and 0.05 N computes to 17 + 8.25 + 0.8, about 26; lean heats sit nearer 23. A 2205 heat at 22 Cr, 3 Mo, and 0.17 N computes to 22 + 9.9 + 2.7, about 34.6, and nitrogen-rich heats reach 36. The threshold most often quoted for seawater splash and brackish service is PREN 32 and above, which 2205 clears and 316L does not approach: that single line is the most common justification for the upgrade in cooling water, desalination, and marine projects, and it should appear on the mill certificate as a computed value from the actual heat analysis, not a brochure number.

The structure matters as much as the chemistry. 2205 is about half ferrite and half austenite by volume after solution annealing. The ferrite carries the strength and near-immunity to chloride stress corrosion cracking; the austenite carries toughness and formability; the 50/50 balance delivers the roughly doubled yield strength, around 450 MPa minimum for 2205 against the 170 MPa class specification minimum for 316L, with typical annealed 316L plate actually measuring 220 to 240 MPa. In wall-thickness-driven design, pressure pipe and vessel heads in 2205 can be calculated thinner at the same allowable stress basis, which buys back part of the grade premium in every meter of stainless pipe and every ton of steel plate.

The same two phases set the boundaries covered in Chapter 3. The ferrite embrittles at sustained high temperature, through the 475 C embrittlement mechanism and sigma-type phases under long exposure, which caps standard duplex service around 300 C, and duplex toughness falls away at low temperature well before austenitic grades do. 316L has neither bookend: it remains a standard cryogenic material and serves at moderately elevated temperatures. Grade selection inside the chloride question therefore always carries a second check on the temperature band, and the phase balance is why.

One more composition line that earns its place on the RFQ: nitrogen. The deliberate 0.14 to 0.20 nitrogen in 2205 is what makes the duplex metallurgy stable, contributes heavily to PREN, and must be preserved through welding, which is why duplex practice specifies nitrogen-bearing root gases and 2209-class filler. A heat analysis that arrives at the low end of the nitrogen band with no margin is a procurement flag before a single weld is made.

Product Forms and Duty Boundaries

Both grades are full-catalog materials: steel plate and sheet to A240, bar to A276, welded and seamless steel pipe to the A312 class, forgings to the A182 class for flanges and fittings, plus cast equivalents for valve and pump bodies. Within the wider procurement families, 2205 sits in the stainless steel and nickel-bearing middle tier between the commodity austenitics and the high alloys, and it competes for the same order book as alloy steel with lining, coated carbon steel, and, at the top, nickel alloy and titanium alloy solutions.

Where 316L still wins, and wins legitimately. Cryogenic duty keeps 316L: it retains useful impact toughness at liquid-gas temperatures (about -196 C) where standard duplex has long since fallen below its toughness floor, and LNG cool-down lines, cold boxes, and cryogenic vessels are austenitic territory. Deep drawing and complex forming favor 316L: 40 percent class elongation and a work-hardening behavior that thins gracefully let press shops form heads, sinks, and convoluted shapes that a 25 percent elongation, 450 MPa yield duplex will crack or spring back from. Sustained service above roughly 300 C excludes standard duplex on embrittlement grounds, with 475 C embrittlement the classic long-exposure mechanism in ferrite, so hot headers and certain process lines stay austenitic. And where welding expertise is thin, 316L is the grade every shop can join competently with standard filler and an ordinary argon purge; duplex is not that grade, and pretending otherwise produces Chapter 6 failures.

Where 2205 wins. The chloride duties: seawater cooling circuits in the splash and spray zones, brackish and estuarine cooling water, SWRO desalination trains up to the mid and brine stages where concentration climbs, plate heat exchanger plates in chloride-bearing service, chemical and brine tanks, marine valve bodies and pump internals, coastal bridge hardware and rebar in chloride environments, and pulp bleach plant washing and filtrate circuits. In each of these, 316L pits or cracks on a schedule set by chloride level and temperature, 2205 holds, and the maintenance avoided is the business case. The doubled yield adds a second, quieter win wherever wall thickness drives cost: thinner wall pipe, longer unsupported spans, lighter structures.

Availability shapes real decisions as much as corrosion tables. Both grades are mill products with broad stocking in plate, sheet, welded pipe, and common flange and fitting sizes; 2205 has become a standard item for the major mills and distributors, which is precisely what distinguishes it from the higher tiers: superduplex and 6-moly plate and flanges are produced but stocked thinly, with longer lead times and fewer qualified forgers. Seamless stainless pipe in 2205 is stocked in common schedule sizes, while exotic diameters and heavy wall are mill-order items, so schedule pressure on a duplex line often resolves into welded pipe with 100 percent weld NDE rather than a seamless mill run. Specifiers who write grade without form and lead time in the same sentence discover the difference during procurement, when the two-week substitution debate is harder on the schedule than the original corrosion review was.

The table reads as a boundary survey. 2205 owns the warm-chloride middle of the map, 316L owns the benign edges and the temperature extremes, and neither owns true immersed seawater, which is the superduplex and 6-moly tier. When a specifier tries to stretch 2205 into the bottom rows of that table with a coating or a bit of cathodic protection, the result is usually a deferred decision that arrives as a leak, and the honest RFQ answer names the higher grade early.

Key Specification Parameters

The datasheet for either grade answers selection in a dozen lines, and most disputes trace to a line that was never read. The decode below follows ASTM A240 and A276 class values for plate and bar products; pipe and forging classes carry the same strength and PREN logic with their own tables.

Yield and tensile are the wall-thickness levers. The specification minimum of about 170 MPa for 316L against about 450 MPa for 2205, with typical annealed 316L plate measuring 220 to 240 MPa, is what allows calculated wall reductions of 35 to 45 percent in pressure parts when the design code permits strength-based design. Tensile minima around 485 MPa for 316L against 620 to 655 MPa for 2205 follow the same ratio; note that dual-certified S31803 stock carries the 620 class tensile minimum and the current S32205 composition the 655 class, so the UNS number on the certificate matters when the calculation is tight.

Elongation is the formability line. A 40 percent class elongation for 316L against 25 percent for 2205 is the numeric statement of why drawing, spinning, and aggressive bending favor the austenitic; duplex parts with heavy cold deformation may also need a re-anneal to restore phase balance, covered in Chapter 6. Hardness, typically 150 to 190 HB against 230 to 270 HB delivered, feeds machinability and galling behavior: duplex work-hardens and galls against itself more readily, so mating sliding parts in duplex assemblies deserve attention to surface finish and, where relevant, dissimilar-hardness pairing.

PREN on the certificate should be computed from the actual heat analysis by the mill formula and stated as a number. A certificate without PREN is a certificate the specifier has to recompute by hand from the heat analysis before anything is bought; Chapter 5 puts that requirement in the RFQ directly. The ferrite number line belongs to welding: sound duplex welds target 30 to 70 FN, the accepted window that keeps enough austenite for toughness and corrosion and enough ferrite for strength and SCC resistance; below the window the weld behaves austenitic and loses the duplex advantages, above it the weld turns brittle. A 316L weld is fully austenitic with only residual ferrite, and its FN line is mostly a formality.

Impact behavior sets the low-temperature boundary. 316L Charpy energy at cryogenic temperatures is why it, not duplex, is the LNG and industrial-gas standard; 2205 heats can be impact-qualified to design minimum temperatures near -50 C for specific projects, but that is a purchase condition with test evidence, not a catalog property. Sulfur and inclusion shape belong to bar machining: controlled or resulfurized calcium-treated heats break chips cleanly on high-volume machining centers at a modest cost in pitting resistance and weldability, a trade that is legitimate for fasteners and shafting in benign duty and dubious for chloride service, where the corrosion-grade sulfur level should hold.

Selection Decision Factors

Run the sequence against the actual water analysis, not the nominal one, and against the actual fabricator, not the ideal one. Each step either eliminates a grade or prices it.

1. Fix the chemistry: chloride ppm, temperature, pH, residual biocide, velocity, and crevice population from flanges, gaskets, deposits, and threads. A water analysis without temperature is not usable, because every chloride threshold moves with it.
2. Ambient dilute chlorides: 316L is generally safe only to roughly 200 ppm chloride in real flanged, crevice-bearing systems, rising to about 1,000 ppm only in clean, flowing, crevice-free conditions, and the ceiling falls as temperature rises. Inside that envelope, 316L with crevice control is the economical answer and 2205 is over-spec.
3. Warm or concentrated chloride duty: past the 316L zone, or above roughly 50 to 60 C in chlorides where austenitic stress corrosion cracking becomes a live failure mode, duplex 2205 takes over and extends service into seawater splash and higher temperatures.
4. Immersed seawater and chlorinated systems: PREN above 40; superduplex or 6-moly austenitics, with nickel alloy and titanium alloy as the further steps for the hardest duties.
5. Check the bookends: cryogenic duty keeps 316L; sustained service near or above about 300 C excludes standard duplex through 475 C class embrittlement.
6. Check the shop: duplex needs 2209-class filler, controlled heat input and cooling, argon or nitrogen-bearing backing, and procedures qualified with phase-balance verification; a fabricator without that capability is a reason to choose the grade the shop can actually weld.
7. Price the exposure: 2205 is nickel-lean (5 to 6 percent against 10 to 14), so its price tracks nickel markets less, and the wall derating from roughly double yield offsets part of the premium per kilogram in pressure parts.
8. Sour service: both grades appear in NACE MR0175 / ISO 15156 with environmental limits on temperature, chloride, and H2S partial pressure; verify the specific application against the standard's tables before committing either grade.

The two corrosion thresholds worth memorizing. First, the chloride ceiling: 200 ppm class at ambient for real 316L systems, lower as temperature rises, with the 1,000 ppm figure reserved for clean, flowing, crevice-free geometry; 2205 extends the same service into brackish water, seawater splash, and warm duty. Second, the SCC window: austenitic grades crack in chlorides above roughly 50 to 60 C under tensile stress, transgranular and fast, while duplex grades are far more resistant, which is why hot chloride lines are the classic 316L-to-2205 retrofit.

Write the RFQ so the corrosion answer, not the price per kilogram, decides the award:

• Service chloride and temperature, stated as a water analysis with pH and residual biocide, plus velocity and the crevice population of the intended system.
• PWHT prohibited: both grades are used as-welded, and duplex stress-relief treatments in the embrittlement range are not an option; state as-welded condition plainly.
• Weld procedure with phase-balance verification for 2205, targeting 30 to 70 FN in the weld and heat-affected zone, qualified to the applicable code.
• PMI on delivery: positive material identification by optical emission spectroscopy against the mill certificate, item by item, before fabrication.
• CERT with PREN: a mill certificate (EN 10204 3.1 class) carrying the full heat analysis and the computed PREN, plus the UNS number, so S31803 versus S32205 is explicit.
• Product forms and classes: A240 plate, A276 bar, A312 class pipe including seamless steel pipe where specified, A182 class flange and fittings, and the required NDE extent.

Field Practice: Fabrication, Corrosion Control, and Failure Modes

Most 2205 and 316L failures are not chemistry failures; they are fabrication and detailing failures on good chemistry. The practices below are what separate a grade choice that works from one that becomes a metallurgical post-mortem.

Solution annealing and quench. Delivery condition for both grades is solution annealed and quenched, and the quench is not negotiable for duplex. In heat treatment furnace practice, 2205 is typically soaked around 1020 to 1100 C and water quenched; 316L around 1010 to 1150 C with the same rapid cool. The fast quench locks in the 50/50 ferrite-austenite balance; slow cooling through intermediate temperatures gives intermetallic phases time to form in the ferrite and quietly consumes both toughness and corrosion resistance. Any hot forming or heavy cold work that upsets the balance needs a re-anneal on the same furnace logic, and the certified re-anneal, not the workshop's word, is the deliverable.

Pickling and passivation. Weld heat tint is a chromium-depleted skin that pits first, so welds are pickled, typically with nitric-hydrofluoric acid mixtures as immersion, spray, or paste, then rinsed and passivated to restore the passive film. On site, pickling paste discipline (contact time, rinse, neutralization) is a safety and quality item in one; unpickled 2205 welds in chloride service underperform the base metal by a wide margin, which defeats the reason the grade was bought. Mill-scale-free, passivated surfaces are the starting condition the datasheet assumes.

Gasket and flange pairing. Crevice corrosion lives at joints, so the flange and gasket pairing is a corrosion specification, not a commodity purchase. Specify gaskets that seat to the bore edge so no exposed annular crevice remains at the pipe ID, keep flange face finish within the gasket's requirements, torque to a controlled procedure, and prefer joint designs that minimize the crevice gap in chloride duty. A 2205 flange pair with a sloppy gasket line can crevice-attack in water where bare 2205 plate would have been fine, which is why the fix for flanged-system failures is often the joint detail, not the grade.

Galvanic pairing with carbon steel supports. Stainless is noble to carbon steel, so a stainless pipe on bare carbon steel supports corrodes the support, and the rust streak then stains and initiates attack on the stainless surface. Isolate contact faces with pads, sleeves, or painted interfaces, and mind the area ratio in conductive water: small anode, large cathode concentrates the damage. The same logic applies to mixed-tray storage and to drawings that pair a stainless steel item with unpainted carbon steel tooling or structure without an isolation note.

PMI and iron contamination control. Verify what arrived: positive material identification by optical emission spectroscopy against the mill certificate catches the classic mix-up, a 316L consignment in a 2205 fabrication package, before it is welded into a seawater system. Then keep it clean: dedicated grinding wheels and stainless wire brushes, no carbon steel chain slings in direct contact, wood or stainless work surfaces. Embedded iron from carbon steel tooling rust-stains within days and becomes pitting initiation sites in chloride service, and the repair is pickling, not paint.

2205 fabrication pitfalls. Excessive cold work raises the measured ferrite content and upsets the phase balance, so heavily formed areas may need re-annealing before service; hot forming belongs above roughly 1000 C followed by solution anneal, and long dwell in the 300 to 1000 C band is where sigma and 475 C class embrittlement live, so reheat cycles and weld interpass practice both need discipline. None of this is exotic; all of it is exactly what a fabricator's qualified duplex procedure exists to control, which is why Chapter 5 makes the shop check a selection step.

What the failures look like, in words an inspector can use. Pitting under deposits: tubercles and brown staining on surfaces under slime, sand, or marine growth, with pinpoint holes through the wall at the deposit edges. Crevice corrosion at the flange gasket: a grey, undercut groove revealed when the gasket is peeled, worst at the crevice mouth where oxygen was starved. Transgranular stress corrosion cracking: fine, branching cracks running straight through grains, multiple parallel cracks across a hot chloride line, usually where residual weld stress met chlorides above the 50 to 60 C window. Each pattern indicts a different root cause, deposits, joint detailing, or grade-plus-temperature, and reading them correctly is what turns a repair into a specification change.