Put a sheet of 304 next to a sheet of 316 and you cannot tell them apart. Same colour, same weight in the hand, same finish options, same behaviour under the laser and on the press brake. The difference is in the chemistry, and it comes down mostly to one element.
The chemistry, from our material database
Both are austenitic chromium–nickel steels. Taking the specification limits recorded in our material database for plate, sheet and strip to ASTM A240:
- 304 — chromium 17.5 to 19.5 %, nickel 8 to 10.5 %, carbon up to 0.07 %, and no molybdenum requirement.
- 316 — chromium 16 to 18 %, nickel 10 to 14 %, carbon up to 0.08 %, and molybdenum 2 to 3 %.
- 316L — the same as 316 with carbon held to 0.03 % at most. The L is for low carbon.
So 316 gives up a little chromium, carries a little more nickel, and adds two to three percent molybdenum. That last line is the one that matters.
What molybdenum does
The passive chromium-oxide layer on stainless steel is attacked most effectively by chloride ions: sea spray, pool water, cleaning agents, sweat. Chlorides find a weak point in the layer and dig a pit rather than spreading a general rust. Molybdenum makes the passive layer far more resistant to that pitting, and to the crevice corrosion that starts where water is trapped under a gasket or behind a bracket. The effect is large enough that metallurgists score it with one number, the pitting resistance equivalent number, in which molybdenum counts for more than three times its weight in chromium. Our PREN calculator works it from the composition; 316 scores well above 304 for that reason alone.
The extra nickel is there to keep the structure austenitic once the molybdenum is added, not for corrosion resistance of its own. It makes 316 marginally easier to deep-draw, but on a cutting bed or a press brake you will not notice.
When 316 earns its place
- Anything within reach of the sea: coastal facades, balustrades on a corniche, marina fittings. Salt in the air is the classic 304 failure.
- Pool and spa surrounds, where chlorinated water dries on the surface again and again.
- Process plant handling chlorides or acids, and food or pharmaceutical lines washed down with aggressive agents.
- Exterior architectural work in polluted or humid air, where a tea-stained facade would be noticed.
When 304 is the right answer
Indoors, in dry or normally humid air, 304 stays bright for the life of the building. Lift interiors, reception counters, kitchen benches, wall cladding, shopfronts under cover, furniture, most of the decorative sheet that goes through our finishing lines: 304. Specifying 316 there buys no visible benefit. Where 316 is chosen for interior work it is usually for a coastal building whose lobby opens to the street, and that is a judgement about the air, not about the grade's other properties.
Two things people get wrong
- 316 is not stronger. In the annealed condition the two grades' mechanical properties are close enough that an engineer treats them alike; the choice is about corrosion, not load.
- 316 does not weld or polish better. Both take a mirror, a hairline and a PVD colour identically. The L grades weld with less risk of chromium carbides forming in thick sections, which is a reason to choose 316L or 304L, not a reason to move from 304 to 316.
If you are unsure, tell us where the part will live — indoors or out, near the coast or inland, washed with what — and we will say which grade we would use and why.