Most readers of this journal will never buy duplex. They will buy 304 for a bench and 316 for a coastal rail, as the old custom-cut shop did. This essay exists because a few jobs outgrow 316, and because “marine grade” language sometimes pretends 316 is the top of the mountain.
Duplex stainless steels have a mixed microstructure — roughly half austenite, half ferrite. 2205 (UNS S32205 / S31803) is the grade Australian process engineers name first. Superduplex alloys such as 2507 sit higher still. They are stronger than 316, generally more resistant to chloride stress-corrosion cracking, and more expensive to buy and to weld wrongly.
ASSDA’s duplex overviews and mill data sheets (Atlas Steels among the Australian merchants who publish grade notes; Outokumpu among the mills) are the reading list. This page is the trade briefing: when to stay with 316, when to escalate, and how not to treat duplex as a thicker marine-grade sticker. We do not sell plate. We will not invent a plant that “switched to 2205 and saved 40%.”
Start with what marine-grade actually means if you are still arguing nicknames. Start with 304 versus 316 if the job is a bench or a balcony.
Why 316 hits a wall
316 is an austenitic. It is tough, formable, and familiar. In warm chlorides under tensile stress — insulation-soaked pipe, seawater, concentrated process streams — austenitics can crack. They can also pit in crevices that never dry. A balcony that gets hosed is not that duty. A seawater heat exchanger is.
Australian coastal chlorides on a washed 316 balustrade are an architectural problem. Warm immersion, residual chlorine in a process line, or evaporative concentration under insulation is a plant problem. If your only problem is tea staining on a polished post, you do not need duplex. You need coastal care, a better finish, and maybe a design that does not pond water.
316L still earns its keep in food rooms, coastal architecture, and most outdoor fabrication. Escalating the alloy because a salesperson said “premium marine” is how a café owns plate nobody on site can weld.
Chemistry: two families, not two thicknesses
316 / 316L is a chromium-nickel austenitic with about 2% molybdenum. The structure is face-centred cubic. It is generally non-magnetic annealed, highly formable, and welded with 316L filler on a known procedure.
2205 duplex balances austenite and ferrite. Chromium sits higher than 316. Molybdenum and nitrogen do real work in the pitting and strength story. Yield strength is typically in another league — shops talk about roughly twice the yield of 316, which is why engineers thin the wall. You must confirm the actual specified properties on the standard the engineer cited; this journal will not invent a design value.
Superduplex (2507 and kin) adds still more chromium, molybdenum and nitrogen for harsher seawater and process chlorides. Lean duplexes exist where the designer wants strength and a lower nickel bill without superduplex corrosion resistance. Do not substitute lean for super because both are “duplex.”
| Family | Structure | Magnet | Typical Australian job |
|---|---|---|---|
| 316 / 316L | Austenitic | Usually no (annealed) | Coastal rails, food plant, boats’ brightwork |
| 2205 duplex | Mixed austenite / ferrite | Often yes | Water, wastewater, seawater handling |
| 2507 superduplex | Mixed, higher alloy | Often yes | Severe chloride / seawater plant |
| 430 | Ferritic | Yes | Indoor appliances — not a duplex substitute |
Identifying a grade still ends at the certificate. A magnet on duplex is expected. A magnet on a “316” balcony post is a different conversation; that may be 430 or cold work, not a free upgrade.
What duplex changes on the shop floor
Yield strength is much higher. That can mean thinner wall for the same pressure, or a lighter structural member. It also means forming and springback are different. A folder set up for 316 will not guess duplex correctly.
Welding needs heat-input control so the austenite/ferrite balance survives in the weld metal. You do not pick up a 308 rod and call it a day. Consumables are matched to the duplex family. This is specialist fabrication — adjacent to, not the same as, welding 304 and 316. Intermetallic phases from a sloppy thermal cycle are a real rejection, not a polish problem.
Passivation still matters after fabrication. Heat tint and embedded iron do not become harmless because the plate is stronger.
Cutting and machining feel different. Duplex work-hardens and is tougher on tools than a soft austenitic. Shops that quote duplex like 316 sheet are learning on your job.
Australian places you will actually see it
- Water and wastewater equipment.
- Offshore and seawater handling.
- Chemical and pulp duties specified by an engineer.
- Structural members where stainless has to carry load, not just look like a rail.
It is not a café bench upgrade. It is not a substitute for A4 fasteners on a 316 handrail. If a quote suddenly says duplex on a domestic balcony, ask why the designer left 316. The marine, balustrade and outdoors topic is still 316 country for architecture.
Food plant sometimes specifies duplex in chloride-heavy process, not because a council inspector said “food grade.” Food-grade stainless remains a use case, not a third alloy. The engineer names 2205 when the stream deserves it.
Historic Melbourne custom-cut counters did not stock duplex as a walk-in sheet. That is a clue. If you can buy it off a remnant rack with no certificate, you are not buying plant metal.
Failure modes
Wrong family, right nickname. “Marine duplex” on a docket with no UNS number. Could be lean, standard, or someone’s 316.
Weld balance destroyed. High heat input, no procedure, ferrite or austenite out of range, a brittle or corrosion-prone weld. The plate was fine.
Galvanic couples. Duplex bolted to carbon steel or to aluminium in a wet plant. Isolation and coating details belong to the designer.
Balcony theatre. Duplex posts, A2 screws, ponding bases. Strength does not wash salt.
430 confusion. Magnetic plate assumed to be the cheap ferritic and painted. Or the reverse: 430 assumed to be duplex because it pulled a magnet.
Chloride stress on 316 that should have been duplex. Insulation-soaked austenitic pipe. That is the wall 316 actually hits — not a stained handrail.
How to specify it without bluffing
Name the UNS or EN number, the product form, the standard the engineer cited, and the welding specification. Keep 316 in the sentence if the rest of the job stays 316 — mixed systems create galvanic and documentation messes.
Example language: “UNS S32205 (2205), plate to the nominated product standard, mill certificate 3.1, weld procedure for duplex, not to be substituted with 316L or lean duplex without the engineer.”
For everyday alloy choice, return to 304 versus 316. For finishes on architectural leftovers of a plant job, 2B, No. 4, BA, No. 8 still applies — duplex plate often arrives 2B or pickled, not as a kitchen No. 4.
How to order
- Get the engineer’s grade, not a merchant’s nickname.
- Write the UNS / EN number on the purchase order.
- Demand the certificate and keep it with the WPS.
- Confirm the fabricator has welded that family before.
- Do not mix remnant 316 into a duplex spool.
- Leave domestic balconies on 316L unless a structural engineer wrote otherwise.