If you are choosing between 304 and 316L stainless steel and only remember one thing, make it this: 316L is 304 with molybdenum added and carbon pulled down, and you pay for it only when chlorides or welding are in the picture. For a dry indoor part, 304 and 316L look identical, machine nearly identically, and 304 costs noticeably less. The moment the part sees salt air, pool water, chlorinated cleaners or a weld, the choice flips to 316L.
The two grades are close enough that a drawing will sometimes name both, and a fabricator will quietly substitute whichever is in stock. The substitution matters because the property that separates them — chloride pitting resistance — never shows up on a casual look or a magnet test.
What actually separates them
Both are austenitic (non-magnetic in the annealed state) chromium-nickel stainless steels. The difference is two alloying decisions: molybdenum, and carbon.
| Element (wt%, ASTM A240) | 304 (S30400) | 316L (S31603) |
|---|---|---|
| Carbon, max | 0.07 | 0.030 |
| Chromium | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel | 8.0 – 10.5 | 10.0 – 14.0 |
| Molybdenum | not specified | 2.00 – 3.00 |
| Manganese, max | 2.00 | 2.00 |
| Tensile, min | 515 MPa | 485 MPa |
| Yield (0.2%), min | 205 MPa | 170 MPa |
Molybdenum is what buys chloride resistance. The shorthand engineers use is the pitting resistance equivalent number, PREN = Cr + 3.3 Mo + 16 N. 304 sits around 18–19; 316L sits around 24–26. Above roughly PREN 24 the material resists the chloride levels that start pitting 304 — which is why 316L is the default answer for anything coastal, chlorinated or marine.
Carbon is the other decision. 316L's 0.030% maximum carbon is the "L" — low carbon — and it exists for welding. When a stainless weld cools slowly through roughly 450–850 °C, chromium carbides precipitate at the grain boundaries, pulling chromium out of the surrounding metal and leaving it vulnerable to intergranular attack alongside the weld. The low-carbon L grades (304L, 316L) stay below the carbide-precipitation threshold and survive welding without a post-weld anneal.
When to specify 316L
- Coastal, marine or chloride service. Facades within a few kilometres of salt water, swimming-pool halls, food plants washed with chlorinated water, laboratory bench fronts.
- Welded assemblies. Any part that will be welded — tanks, pipework, frames — should use an L grade to avoid weld decay, whether or not chlorides are present.
- Fluid and chemical handling. Process piping, pharmaceutical lines, anything that carries a medium that attacks the passive film.
- High-temperature wet service. Where a part is both hot and wet, the combined attack is enough to justify 316L's premium.
For a welded 316L assembly in chloride service, the order should state the grade, the standard (ASTM A240 for plate, ASTM A312 for pipe), the low-carbon designation explicitly ("316L", not "316"), and the certificate level — EN 10204 3.1 with the heat number. "316" on the drawing does not mean "316L": the carbon limit differs, and so does the weld behavior.
When to specify 304
- Dry interior work. Kitchen equipment, indoor architectural trim, brackets, housings — anywhere the atmosphere is dry and non-corrosive.
- Food contact in normal conditions. 304 is the standard food-grade answer for equipment washed with ordinary detergents rather than strong chlorides.
- Cost-sensitive general fabrication. Where the part is not welded, not wet and not load-critical, 304 is the correct commercial choice.
A useful rule of thumb from fabrication shops: default to 304, and only step up to 316L when you can name the chloride, the weld, or the wash cycle that forces the upgrade. If nobody on the project can say why they need 316L, they usually do not — and the money saved goes straight to the bottom line.
What gets specified wrong
The most common mistake is ordering "316" for a welded part because it sounds better than 304, then welding it and finding intergranular corrosion alongside the weld. Plain 316 (S31600) has a 0.08% carbon limit — not the 0.030% of 316L — and is not the weld-safe grade. If the part is welded, write 316L, not 316.
The second mistake is assuming 304 and 316L are interchangeable on a drawing that names one. They are not. The composition limits differ, so a mill certificate naming 304 does not satisfy a drawing calling for 316L, and the reverse substitution in a chloride environment will pit.
The third mistake is testing with a magnet and rejecting the material. Austenitic grades work-harden, so cold-formed or lightly machined 304 and 316L can show a weak magnetic pull at cut edges and bends. That is normal and does not mean the material is defective or mislabelled.
The decision in one table
| You need | Choose | Watch for |
|---|---|---|
| Dry indoor part, no weld | 304 | nothing special |
| Welded part, any environment | 304L or 316L | write the L, match filler |
| Coastal / chlorinated / marine | 316L | specify PREN if in doubt |
| Food or pharma contact | 304 or 316L | state the wash cycle |
| Hot and wet service | 316L | confirm temperature |
Bottom line
The choice is chloride resistance against cost, and weld safety against convenience. 316L is what you specify when chlorides or welding are real; 304 is what you specify when they are not. If a weld is anywhere in the picture, use an L grade — and if the part sits near salt, use 316L and stop thinking about it.
For the stainless steel sheet and plate and stainless steel tube and pipe spec ranges, see the category pages. If you are quoting a welded assembly, send the drawing and the service environment — we will tell you which grade and certificate the job actually needs.