AISI 304 or 316: how they differ and which to choose
AISI 304 or 316 — the choice is decided by one element: molybdenum. 316 has it, 304 does not.
If the medium is dry, fresh water or food production without chlorides, take 304: it is cheaper and does the job. If salt, sea water, brines, chlorine-containing chemicals or a swimming pool are nearby, you need 316 — otherwise the metal will go into pitting corrosion long before the end of its service life. Below is a table of the differences, the role of molybdenum and the boundaries where replacing one grade with the other passes without consequences.
AISI 304 or 316: how they differ
Both grades are austenitic. After annealing both are weakly magnetic, both are not afraid of the atmosphere and fresh water, and both are approved for contact with food. The difference is in the alloying and, as a consequence, in the list of media where the metal holds up.
| Parameter | AISI 304 | AISI 316 |
|---|---|---|
| Chromium | about 18% | about 17% |
| Nickel | 8–10% | 12–14% |
| Molybdenum | none | 2–3% |
| Structure | austenite | austenite |
| Magnetism after annealing | weak | weak |
| GOST equivalents | 08Х18Н10, 03Х18Н11 (304L) | 10Х17Н13М2, 03Х17Н14М2 (316L) |
| Fresh water, atmosphere | holds | holds |
| Salt, brines, sea water | pitting, corrosion in gaps | holds |
| Chlorine-containing chemicals | not suitable | a working option |
| Food media without salt and acids | suitable | suitable |
| Marinades, brine, sauces | limited | suitable |
| Welded structures | 304L or heat treatment | 316L, 316Ti |
| Typical application | kitchen, vessels, decor, water | chemicals, sea, medicine, swimming pools |
In everyday language 304 is called "18/10" — after the chromium and nickel in its composition. It is the same thing as AISI 304 in American notation and 08Х18Н10 in GOST notation. There is no separate standard "18/10", it is a trade designation.
In a specification the grade may arrive in any of the three systems. Go by the certificate of the specific batch: it confirms the composition, not the name on the invoice.
In mechanical properties the grades are almost indistinguishable. Strength, elongation and behaviour in bending are close for 304 and 316, because their structure is the same — austenite. The difference shows not in load-bearing capacity but in resistance to the medium and in the service life of the assembly. Both weld comparably, but 316 is sensitive to weld contamination and requires cleaning and passivation no less than 304.
The letter L at the end means low carbon (304L, 316L). Such grades are taken for welding when the finished structure cannot be annealed. The letters Ti mean titanium instead of low carbon: it binds carbon and prevents the weld from corroding along the grain boundaries.
Molybdenum: what it is responsible for
Chromium is what makes stainless steel stainless: a thin passive film of oxides forms on the surface and shields the metal from the medium. The film self-repairs in air, but it can be destroyed locally.
The main destroyer is the chloride ion. It settles on the film at individual points, breaks through it and starts pitting: a point ulcer that goes deep. In gaps and under deposits the same logic gives crevice corrosion.
Molybdenum works against this mechanism. It strengthens the passive film in a chloride medium and raises the threshold at which pitting begins. That is exactly why 316 goes into sea water, brines, cellulose, dyes and chlorine-containing chemicals, where 304 gives up.
The guide when choosing is not the name of the grade but the combination of chromium, molybdenum and nitrogen in the composition: it is this that sets the resistance to pitting corrosion. That is why two batches with the same "316" label may behave differently if the composition drifts within the tolerance. For critical assemblies, check not only the grade number but also the composition in the certificate.
What molybdenum does not do:
- it does not save from intergranular corrosion of the weld — that requires low carbon or titanium;
- it does not cancel stagnation: standing water, deposits and dirt will eat 316 too;
- it is not universal across acids: concentration and temperature are calculated for each medium, and for sulphuric and phosphoric acids one looks towards 904L and duplex;
- it does not protect against "flooded" rust if ordinary steel lies nearby.
The threshold at which chloride breaks through the film depends on temperature and concentration. The hotter the medium and the more salt in it, the more aggressively even a small amount of chloride behaves. That is why the same 316 holds for years in cold water, while in a hot brine under deposits it goes into pitting within a matter of months. Where the medium changes with the season or with the operating regime, the grade is calculated for the harshest combination of temperature and concentration, not for the average value.
The practical conclusion is simple: molybdenum is needed where there are chlorides. No chlorides — no point paying for it.
If the medium is at the limit of 316's capabilities, there are two paths. The first is to raise the alloying: duplex and super-austenitic grades with a higher molybdenum content hold noticeably more chlorides. The second is to change the conditions: remove gaps, ensure flow, lower the temperature, perform passivation after welding. The second path is often cheaper than switching to a special grade.
Where 304 is enough
304 is the workhorse of stainless rolled steel. It is enough in most tasks where the metal does not meet salt and chlorine.
- Indoor premises: furniture, handrails, fences, decorative panels, shop equipment.
- Fresh water: tanks, pipelines, heating and water supply systems, domestic heat exchangers.
- Food production without aggressive media: vessels, tables, sinks, utensils, equipment for milk, flour, beverages.
- Architecture and transport away from the sea: cladding, canopies, finishing parts.
- Kitchen and laundry equipment where there is no constant contact with salt and acid.
- Transport and warehouse assemblies: racks, containers, trolleys, conveyor elements in dry workshops.
The logic of the choice is simple. If there are no chlorides in the medium, and the metal does not work in hot brine or under a layer of deposits, 304 copes without any margin in grade. Overpaying for 316 in such a task gives neither service life nor strength: the mechanical properties of the grades are close, and the molybdenum alloying remains unclaimed.
A separate note on thickness. The attempt to replace the grade with a margin in wall thickness does not work. Pitting and crevice corrosion go locally and right through, so an extra millimetre only postpones the pinhole but does not remove the cause. In a chloride medium the grade decides more than the thickness.
Important caveats. 304 rusts if metal dust from ordinary steel lands on it: iron particles grow into the surface and give red streaks. The same effect is produced by stagnant water, welding without cleaning and passivation, and also by the chlorinated water of a swimming pool — 304 does not survive there.
If a part cannot be serviced and replacing it is expensive, it is cheaper to take 316 straight away than to calculate the repair.
Where only 316 will do
There are media where 304 is not up for discussion. If the object falls into at least one line of the list below, take 316 or its low-carbon variant.
- Sea water, coastal atmosphere, ships, port equipment.
- Brines, salt brine, saline solutions, fish processing, meat processing.
- Swimming pools, spas, water parks, systems with chlorine and ozonation.
- Chemical equipment: chlorine-containing media, dyes, cellulose, fertilisers.
- Pharma and medicine: sinks, chlorine disinfection, steam sterilisation.
- Heat exchangers on sea water and on media with chlorides.
- Assemblies with no access for servicing and replacement.
For welded structures from this list, 316L or 316Ti is taken. The carbon in ordinary 316 precipitates along the grain boundaries when heated in the weld zone, and the weld corrodes even in a mild medium. Low carbon or titanium removes this problem.
If even 316 does not hold — for example, hot concentrated chlorides — one moves to duplex or super-austenitic grades. This is a separate calculation by concentration, temperature and the presence of oxygen in the medium.
The summary by media looks like this.
| Medium or condition | What to take |
|---|---|
| Dry premises, decor, furniture | 201, 430 |
| Fresh water, steam, food media without salt | 304 |
| Outdoors and moisture without chlorides | 304 |
| Sea water, salt, brines | 316, 316L |
| Swimming pool, chlorinated water | 316L |
| Welded assemblies without heat treatment in an aggressive medium | 304L, 316L, 316Ti |
| Hot concentrated chlorides | duplex, super-austenite |
The table sets a direction, it does not replace a calculation. For a critical assembly, check the specific medium: temperature, chloride concentration, presence of gaps and flow rate.
The price of the question
There is no single price for stainless steel for all cases. 316 is more expensive than 304 because it has more nickel and contains molybdenum, and these elements are more expensive than iron and chromium. The higher the nickel quotations, the wider the gap between the grades.
The final amount is affected by the grade, the size, the wall or sheet thickness, the surface, the batch volume and the delivery basis. It is convenient to look at current quotations and benchmarks in the prices and quotations section.
How to calculate the economics of a grade:
- Mistake towards the cheap. The metal goes into pitting, the part is replaced together with downtime and dismantling of the structure. In chloride media, saving on molybdenum does not pay off.
- Mistake towards the expensive. You pay for alloying that does not work in a dry room. For interiors and fresh water, 316 is an overpayment with no gain.
- Switching to 304L or 316L for welding is not an overpayment but a necessity. The alternative is heat treatment of the finished structure, which is more expensive and not always possible.
- The surface changes the behaviour of the metal no less than the grade: ground and polished surfaces hold up better than areas with weld scale and burrs.
Calculate not by the price per kilogram but by the service life of the assembly in a specific medium.
When comparing offers, bring them to a single basis: grade, size, surface, unit of quantity and shipment week must match. The difference between suppliers is more often explained by the basis and the batch volume than by the metal itself. If an offer is noticeably cheaper than the rest, check the grade and the origin of the metal in the certificate.
Common mistakes when choosing between 304 and 316
- Taking 316 "just in case". In a dry workshop or on fresh water molybdenum does not work, and you pay for it across the whole batch.
- Saving on the grade in a chloride medium. Replacing 316 with 304 in brine or sea water gives pitting, not savings: repairing the assembly is more expensive than the difference in the metal.
- Welding 304 without the low-carbon variant. A weld without heat treatment corrodes along the grain boundaries. For welding one takes 304L, for critical assemblies — 304L or 321.
- Determining the grade with a magnet. Both grades are weakly magnetic after annealing, and become more magnetic after deformation. A magnet does not distinguish 304 from 316.
- Compensating for the grade with thickness. Local corrosion goes right through, so a margin in wall thickness does not replace molybdenum.
- Forgetting about gaps and stagnation. Even 316 gives up under deposits and in crevices where oxygen does not reach. The design of the assembly matters no less than the grade.
Frequently asked questions
AISI 304 or 316: what is the difference
In the molybdenum. 316 has 2–3% of it, 304 has no molybdenum. Because of this 316 holds chlorides, salt and sea water, while 304 corrodes in these media. In mechanical properties and appearance the grades are close.
Which steel is better — AISI 304 or 316
The one that matches the medium. For dry premises, fresh water and food media without salt, 304 is enough. For the sea, brines, swimming pools and chlorine-containing chemicals, 316 is needed. There is no "best in general" grade.
AISI 304 or 430: which is better
These are different classes. 430 is a ferritic grade without nickel, it is magnetic, cheaper, with lower corrosion resistance. It is taken for decor and interior finishing where there is no moisture or chemicals. For water, food media and outdoors, 304 is preferable. More details — in the analysis of AISI 430.
AISI 304 or 18/10: which is better
They are one and the same. The notation 18/10 means 18% chromium and 10% nickel, that is AISI 304 or its GOST equivalent 08Х18Н10. A separate grade "18/10" does not exist, so a choice between them is impossible.
12Х18Н10Т — an equivalent of AISI 304 or 321
- Titanium in the composition binds carbon and increases heat resistance, so 12Х18Н10Т is closer to AISI 321. The closest equivalent of 304 is 08Х18Н10 without titanium.
Is 316 magnetic
After annealing — weakly, just like 304. A magnet does not distinguish these grades. The austenitic structure is non-magnetic in the annealed state, but cold deformation and welding make the metal noticeably magnetic in individual zones.
Can 316 be replaced with 304 in a swimming pool
No. The chlorinated water of a swimming pool is a typical chloride medium in which 304 goes into pitting, especially in gaps and at welds. For swimming pools one takes 316L, and at high temperatures and concentrations one calculates duplex.
Place an order for the grade you need
When the medium is determined, all that remains is to choose the format and the grade. In the exchange catalogue there are sheet, round and profile pipe, round bar and angle made of AISI 201, 304, 316L and 430. An order fixes the grade, size, surface and shipment week, and all of this passes into the trade unchanged.
- AISI 304 and AISI 316 — catalogue positions with the product range
- How elements determine the grade — an analysis of chromium, nickel and molybdenum
- Prices and quotations — market benchmarks