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.
Grade selection is a zoning exercise, not a single number. The same plant can use 316L in the low-chloride make-up line, 2205 in the warm cooling return, and superduplex 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.
Composition and PREN
PREN ranks pitting resistance from alloy content: percent chromium plus 3.3 times percent molybdenum plus 16 times percent nitrogen. It is a comparative index, not a corrosion rate, but it tracks chloride service remarkably well.
| Grade | UNS | Typical Cr / Mo / N | PREN | Yield min. (A240) |
|---|---|---|---|---|
| 316L | S31603 | 16-18 / 2-3 / 0.10 | About 24 | About 170 MPa |
| Duplex 2205 | S32205 | 22 / 3 / 0.17 | 34 to 36 | About 450 MPa |
| Superduplex | S32750 class | 25 / 4 / 0.27 | Above 40 | About 550 MPa |
The structure matters as much as the chemistry. 2205 is about half ferrite and half austenite, which delivers the doubled yield strength and near-immunity to chloride stress corrosion cracking versus fully austenitic 316L. The ferrite also sets the Chapter 5 limits: a toughness floor at low temperature and embrittlement at sustained high temperature, bracketing duplex into a band where 316L has no such bookends.
Chloride Limits in Water Service
Selection charts from bodies such as CEDINOX and the BSSA draw the same picture. In ambient, near-neutral, crevice-free water, type 304 territory ends around 200 ppm chlorides, 316L around 1,000 ppm, and 2205 comfortably beyond. Raise the temperature toward 50 to 60 C and every limit falls, and the austenitic grades additionally enter the chloride SCC window above roughly 50 to 60 C under tensile stress, a failure mode duplex resists strongly.
| Water Duty | Chlorides | Working Grade |
|---|---|---|
| Fresh water, ambient | Below 200 ppm | 304 class acceptable, 316L default |
| Brackish / cooling water, ambient, crevice-light | To about 1,000 ppm | 316L with crevice control |
| Higher chlorides or warm water, 50 C and up | Past the 316L zone | Duplex 2205 |
| Immersed or chlorinated seawater | About 19,000 ppm | PREN above 40: superduplex, 6-moly |
Real hardware is not crevice-free. Flange faces, gasket lines, deposits, and threaded joints localize chemistry, so the practical limit for a flanged 316L system is well below the clean-surface number; water-industry guidance pushes 2205 for pumped duty even where bare 316L coupons look fine. Seawater is beyond both grades immersed: biofilm and crevices make 316L pitting routine and 2205 borderline.
Where 2205 Wins, and Where It Is Not Enough
Specify 2205 over 316L when the duty is warm chloride water, seawater splash or washdown, or strength-driven design. Its PREN of 34 to 36 handles cooling water, brackish service, and desalination streams where 316L pits; its SCC resistance carries hot chloride lines that would crack austenitics; and its ASTM A240 yield minimum around 450 MPa versus roughly 170 MPa for 316L halves required wall in pressure parts, often paying for the grade premium in material and welding. Its nickel content near 5% versus 10 to 14% also makes its price less hostage to nickel markets.
Do not read 2205 as an automatic seawater grade. In flowing, clean, ambient seawater without crevices it performs, but in stagnant legs, under deposits, and in chlorinated systems, the 40-PREN line asserts itself: superduplex (S32750, S32760), 6-moly super austenitics, nickel alloys, or titanium take over. A seawater spec built on 2205 with flanged joints and intermittent chlorination is a delayed leak, and the fix is the higher grade plus crevice-minimizing design.
Temperature, Welding, and Fabrication
The duplex structure imposes temperature bookends. At the top, sustained service is normally capped around 300 C by embrittlement phases in the ferrite. At the bottom, duplex toughness falls off well before 316L, which remains a standard cryogenic material to liquid-gas temperatures. Outside that band, 316L or a higher alloy may be the honest choice.
Welding is where 2205 projects succeed or fail. The weld must balance ferrite and austenite: controlled heat input, filler such as 2209, and nitrogen-bearing root purge produce sound, corrosion-resistant joints; casual procedures produce ferrite-rich welds that are brittle and pit first. Qualify welders and procedures to code, and inspect heat-affected zones in critical service. 316L is forgiving by comparison, standard filler, ordinary argon purge, and the low-carbon L grade resists sensitization, which is why small shops and field repairs still default to it.
Product forms are no barrier for either grade: both are catalog steel pipe, seamless pipe, pipe fitting, and flange items from the major mills, across the wider alloy steel and carbon steel procurement families.
Selection Decision Factors
Run the sequence against the actual water analysis, not the nominal one.
- Fix the chemistry: chloride ppm, temperature, pH, residual biocide, velocity, and crevice population. A water analysis without temperature is not usable.
- Ambient dilute chlorides, crevice-light system (below about 1,000 ppm): 316L remains the economical default; design out crevices where practical.
- Warm chloride service (roughly 50 to 60 C and up, or chloride past the 316L zone), SCC-sensitive duty, or strength-driven wall: duplex 2205.
- Immersed seawater, chlorinated systems, or severe crevice conditions: PREN above 40, superduplex or 6-moly, with titanium and nickel alloys as the further steps.
- Check the bookends: cryogenic duty keeps 316L; sustained service near or above about 300 C excludes standard duplex.
- Check the shop: duplex filler, purge, heat-input control, and qualified procedures, or choose a grade the fabricator can actually weld.
The summary rule: mild chlorides at ambient temperature, 316L with crevice control; harder chloride duty and hot water, 2205; true seawater especially chlorinated, go past both to PREN above 40. That ladder, 316L to 2205 to superduplex, is the whole decision in one line.
Specifying and RFQ Notes
Material RFQs for chloride service should state the chloride concentration, temperature and the presence of crevices or stagnant zones, because those three variables, not the alloy name, set the corrosion margin. For 2205, require weld procedure qualifications to the duplex practice with the correct heat input window, nitrogen-bearing backing gas for root passes where applicable, and post-weld inspection for phase balance where the service is critical. Confirm the product form and schedule availability early: duplex plate and fittings run on mill cycles that outlast austenitic stock. For mixed fabrication, ask the shop how it segregates duplex from austenitic scrap and weld consumables, because cross-contamination and wrong filler are the two field failures that undo the alloy upgrade.
Grade Boundaries, Filler Metals and Mixed Fabrication
Within each family the grades are not interchangeable. Around 316L sit leaner 304L, where chloride limits drop sharply, and higher-alloyed 317L and 904L on the other side; each step is a pitting-resistance band, and substitutions cross it silently. In the duplex family, lean grades like 2304 trade some corrosion margin for formability and cost, while super-duplex steps up for the harshest seawater and chemical duties; 2205 sits as the workhorse middle. Filler selection follows the parent: duplex welds use over-alloyed duplex fillers, typically 2209 for 2205, to hold the austenite-ferrite balance through fast cooling, and austenitic fillers are the exception, not the default. Fabricators running both families need color-coded consumable racks and procedure discipline, because a 316L root pass in a duplex line is a weld that passes visual and fails in service [S1][S2]. Procurement should also pin the acceptance paperwork: material test reports with the actual chemistry and mechanical values, not just the grade name, and for critical service, corrosion test data the mill can stand behind [S2].