Australian merchants still sell these two alloys as if they were personalities. 304 is the everyday austenitic — benches, splashbacks, indoor fabrication, food plant that is not swimming in brine. 316 is the one the salesperson calls marine grade. Both are chromium-nickel steels. Only 316 carries useful molybdenum. That extra element is why a coastal post specified in 304 can look ten years old after one summer, and why a 316 quote makes a kitchen look greedy when the bench will never see the sea.
This is the decision essay. If you need the adjective decoded, read what marine-grade stainless steel actually means. If the metal picked up a magnet, you may not be holding either alloy — start with what 430 stainless steel is for.
What the numbers actually say
304 (UNS S30400, EN 1.4301) is often called 18/8: roughly 18% chromium, 8% nickel. 304L is the low-carbon cousin used when a weld might sensitise the heat-affected zone. 316 (UNS S31600, EN 1.4401) keeps a similar chromium-nickel base and adds about 2% molybdenum. 316L is the weldable low-carbon version you should be naming if the job is TIG’d on site.
Molybdenum improves pitting resistance in chlorides. Australian chlorides are not theoretical. They arrive as sea spray, pool chemicals, food salt, bleach, and the dust that sits in a coastal crevice and stays wet overnight. ASSDA’s public guidance on tea staining is written for this country, not for a laboratory coupon in a jar.
Neither alloy is a duplex, a super-austenitic, or a miracle. If the duty is a seawater pump shaft or a chemical tank, you may already be in duplex stainless versus 316 territory.
The Australian tests that actually matter
Coastal and outdoor
Treat 316 as the default for anything that lives outside in a coastal suburb, on a balcony, on a pier, or beside a pool fence. That is the old Alltrade catalogue instinct, and it is still sound. It is not a force field. A 316 balustrade with carbon-steel grinding dust in the polish, no fall for water, and no wash programme will stain. Read 316 stainless balustrades in Australia and coastal stainless steel care before you promise a client a mirror that stays a mirror.
Inland, 304 outdoors can be honest — a dry regional town is not Bondi. The postcode is part of the specification. So is the finish: a rough weld and a mill 2B face hold dirt. A smooth, drained, washed surface forgives more.
Food, dairy and indoor wet rooms
304 is the workhorse of Australian commercial kitchens and indoor food plant. It is what most stainless steel benchtops in Australia are made from. 316 earns the extra money in brine rooms, meat plants that hose chlorinated water, coastal fish shops, and anywhere the cleaning chemical is more aggressive than a neutral detergent. “Food grade” is not a third alloy — see food-grade stainless steel in Australia.
Budget and availability
316 costs more because nickel and molybdenum cost more. Lead times can stretch on odd thicknesses and polished sheet. If the job is an indoor cabinet in Dubbo, paying for 316 to feel safer is not engineering. If the job is a handrail at a surf club, paying for 304 to win a tender is how you buy a callback.
Magnet myths and certificates
Annealed 304 and 316 are generally non-magnetic. Cold work — a deep draw, a heavy fold, a badly beaten corner — can make either of them attract a magnet a little. That does not turn them into 430. A magnet is a screening tool, not a grade stamp. How to identify a stainless steel grade is the longer method: certificate, stamp, and, if you must, a proper test.
Ask for the heat number. Ask for 304L or 316L if you are welding. Do not accept “stainless” as a specification on a purchase order. The historic Melbourne cut-to-size counter at least named the three grades it stocked. That habit is worth keeping.
A short specification you can send
Write the alloy, the low-carbon suffix if welding, the finish, the thickness in millimetres, and the environment in one paragraph. Example: “316L, No. 4 finish, 1.5 mm, coastal balcony, drained base plates, A4 fasteners, passivate after fabrication.” That sentence does more work than a mood board. Pair it with the right sheet finish and the right A4 fasteners.
If you are still choosing between metals rather than alloys, the comparison you want is stainless versus galvanised versus aluminium.
How the old Melbourne catalogue framed the same choice
The historic alltradestainlesssteel.com.au counter did not publish a metallurgy lecture. It sold three grades and let the job pick among them. Grade 304 was “the most commonly used” for benches, food handling and indoor plant. Grade 316 was “usually regarded as the standard marine grade” with an honest rider: it is not fully resistant to seawater, and in many marine atmospheres it still shows brown staining. Grade 430 was the magnetic indoor option. That three-line split is still the most useful thing a trade counter can say.
What the old page left unsaid is the rest of this essay. 304 and 316 look the same on a truck. They take the same No. 4 brush. They fold on the same brake. The difference arrives as molybdenum and as a chloride diary — sea spray at Bondi, pool mist at a backyard fence, brine in a fish shop, bleach in a cleaner's cupboard. If you only remember one sentence from the archived shop, remember the 316 caveat. If you only remember one sentence from this journal, remember that a certificate beats a vibe.
Chemistry in a form you can actually use
| 304 / 304L | 316 / 316L | |
|---|---|---|
| Typical chromium | about 18% | about 16–18% |
| Typical nickel | about 8–10% | about 10–12% |
| Molybdenum | none of consequence | about 2–2.5% |
| Common names | 18/8, A2 in fasteners | marine grade, A4 in fasteners |
| Default Australian indoor job | café bench, splashback, cabinet | brine room, coastal indoor wet |
| Default Australian outdoor job | dry inland only, with care | coastal, pool, marina, balcony |
| Weldable low-carbon cousin | 304L | 316L |
Pitting Resistance Equivalent Number (PREN) is a mill shorthand, not a site test. 316 lands higher than 304 because molybdenum counts twice in the usual formula. That does not license a 316 boat fitting in raw seawater, and it does not condemn a 304 sink in Carlton. Use PREN to rank alloys, then write the environment in English.
Carbon is the other number people skip. Standard 304 and 316 can sensitise in the heat-affected zone of a heavy weld: chromium carbides form, the grain boundary is robbed of chromium, and the joint becomes the first place to stain. 304L and 316L keep carbon low enough that most shop TIG work stays safe. If the old Alltrade counter cut you a blank and you welded it on a driveway, the L suffix was the cheap insurance. See welding 304 and 316 stainless and passivation after fabrication.
Australian postcodes are part of the alloy
A 304 letterbox in Mildura and a 304 letterbox in Portsea are not the same specification. ASSDA’s tea-stain work is written for this country because our coasts throw salt further inland than a European textbook assumes, and our summers bake that salt onto a warm rail. Treat anything within a few kilometres of surf, a tidal river, or a lagoon as coastal until a merchant or a corrosion engineer tells you otherwise.
Pool fences sit in a special box. Hypochlorite mist is not sea salt, but it is still a chloride. 316 is the default alloy; A4 is the default fastener; drainage and a wash are still mandatory. Pool and marine stainless fittings covers the hardware. Coastal stainless steel care covers the hose.
Indoor food rooms have their own map. A Melbourne café bench in 304 No. 4 is honest. A smokehouse, a pickling line, or a coastal oyster shed is where 316 stops looking extravagant. “Hospital grade” and “food grade” are not alloys — they are sentences people use when they have not read food-grade stainless steel in Australia.
Failures that get blamed on the wrong grade
Brown stripes under a coastal handrail are often tea staining, not a mill that secretly supplied 304. Check the fastener first. An A2 (304) screw in a 316 rail is a classic couple. Carbon-steel grinding dust left in the polish is the other classic. Both look like “the 316 was fake.”
Rainbow heat tint on a weld that was never pickled will stain faster than the parent sheet. That is a fabrication issue, not an alloy swap. A magnet on a heavily folded 304 corner is cold work, not proof you were sold 430.
The reverse error is just as expensive: upgrading a dry inland laundry cabinet to 316 because a salesperson had a story. Nickel and molybdenum are not a personality. They are elements you pay for when chlorides are real.
How to order so the truck matches the drawing
- Name the alloy and the L suffix if you will weld.
- Name the finish in mill language — 2B, No. 4, BA, No. 8 — not “brushed-ish.”
- Name thickness in millimetres, not a leftover Birmingham gauge.
- Name the environment in one clause: indoor dry, indoor food, coastal outdoor, pool, brine.
- Name the fasteners in A2 / A4, not “stainless screws.”
- Ask for a mill certificate with a heat number.
- Ask what happens after fabrication: pickle, passivate, peel the film, wash at handover.
That list is the whole difference between a custom-cut blank and a callback. The historic North Melbourne and Dandenong South counters already spoke this language. This journal just writes it down.
Read how to identify a stainless steel grade if the metal is already on site, and stainless grades and alloys if you are still choosing among 304, 316, 430 and duplex.