01What is actually different
Two alloying elements explain almost the whole difference. Chromium forms the passive layer that makes stainless stainless; molybdenum strengthens that layer against chlorides. Nickel stabilises the austenitic structure and makes the alloy formable.
| Element | 304 (1.4301) | 316L (1.4404) | What it does |
|---|---|---|---|
| Chromium | 18–20 | 16–18 | Passive layer — the reason it does not rust |
| Nickel | 8–10.5 | 10–14 | Austenite stability, formability |
| Molybdenum | — | 2–3 | Chloride resistance |
| Carbon | ≤ 0.07 | ≤ 0.03 | The L in 316L: lower carbon, less weld sensitisation |
| Density | 7.93 g/cm³ | 7.98 g/cm³ | Weight per square metre |
| PREN | 18–20 | 24–28 | Pitting resistance, ranking only |
Note the chromium: 304 has more of it. 316L trades a little chromium for molybdenum, and the trade is worth it in chloride service — which is the only reason the alloy exists.
02PREN, worked out
Pitting resistance equivalent number is a one-line formula used to rank alloys by resistance to chloride pitting:
PREN = %Cr + 3.3 × %Mo + 16 × %N
Take the mid-point of the ranges in the table above, and put nitrogen at 0.05 % for both. 304: 19 Cr + 3.3×0 Mo + 16×0.05 N ≈ 19.8. 316L: 17 Cr + 3.3×2.5 Mo + 16×0.05 N ≈ 26.1. Molybdenum is weighted at 3.3 times chromium, which is why 2.5 % of it moves the number further than 2 % of chromium in the other direction.
Two different figures for the same alloy are therefore both correct, and that is why the table gives ranges rather than a single value. %Cr and %Mo are ranges in EN 10088-2 and ASTM A240, so PREN is a range too: a 304 heat at 18 Cr and 0.02 N lands near 18.3, the same grade at 20 Cr and 0.08 N near 21.3. Published “typical” values usually pick the bottom or the middle of the spec window without saying which. Use the number to rank the two grades against each other, never to predict a service life in years.
It predicts resistance to pitting in chloride environments. It does not describe resistance to acids, to stress corrosion cracking, or to a scratched surface in a hot chloride concentrate — those need a corrosion engineer, not a formula.
03Choosing from what touches the top
The practical rule: 304 fails where chloride sits on the surface and cannot be rinsed away. Not where salt is used — where it is left.
| Exposure | Grade | Why |
|---|---|---|
| Domestic kitchen, general | 304 | Chlorides are diluted and wiped |
| Commercial kitchen, cooked food | 304 | Standard, and lasts decades |
| Brine, pickling, seafood, dairy | 316L | Chloride concentration at the surface |
| Heavy sanitiser, bleach left wet | 316L | Hypochlorite attacks 304 at scratches |
| Laboratory | 316L | Wider chemical range, easier to validate |
| Coastal, salt air and condensation | 316L | Airborne chloride plus standing water |
| Swimming pool, spa bar | 316L | Warm chloride is the worst case |
Read the last two rows as “better of the two”, not as safe. “Bleach left wet” means hypochlorite at the dilutions used in food service — a few hundred ppm available chlorine, at room temperature, rinsed off after the contact time. 316L tolerates that better than 304; it does not tolerate a stronger mix, a warm solution, or a puddle left overnight. A pool bar or a spa is the same problem with a higher chloride load and a higher temperature, and there is no grade name that settles it: get the concentration, the temperature and the contact time from whoever runs the chemical side, and treat it as a corrosion question rather than a plate order.
Two situations where 304 is routinely specified and should not be: a coastal apartment where the top is never dried, and a bar top where citrus and sanitiser sit overnight. In both cases the failure is pitting at a scratch, and pitting is invisible until it is through.
04How a 304 top actually fails
Not with a rust stain spreading across the surface. With small dark pits, usually near a sink, a weld, or a scratch, and usually on the side that stays wet. Three conditions have to coincide: a chloride source, an oxygen-starved crevice or a breakdown of the passive layer, and time.
- Scratch plus chloride. A scouring pad removes the passive layer; salt water sits in the scratch; the layer cannot re-form because there is no oxygen under the water.
- Crevice. Under a silicone bead, under a clip, between two plates. Same mechanism, no visible warning.
- Weld without passivation. Heat burns out chromium at the weld; if the weld is not pickled and passivated afterwards it becomes the least resistant point on the part.
Full detail on the last one is in why stainless steel rusts.
05What 316L costs, and when it is wasted
316L costs more per tonne and it is not always available at the same lead time, so we quote it per order rather than pretending it is a stock switch. On a countertop the difference is a small fraction of a fitted kitchen — and it is wasted money in a dry domestic kitchen in a non-coastal city.
Spend it where the failure mode is chloride pitting: wet, salty, sanitised, or inspected. Save it where the top is dry and the client dries it. That is the whole decision.
In the shop the grade changes nothing about how the part is made. We cut, machine, fold, weld, grain and inspect the same way in either alloy, to the same ±0.05 mm edge tolerance, and we hold the mill certificate for the heat either way. What changes is the plate cost, sometimes the availability, and occasionally the packing date if the plate has to be ordered in. If you are choosing between the two on a deadline, ask us which one is on the rack before you ask which one is better — they are rarely the same answer, and we would rather sell you the one that arrives on time than the one that sounds better in a specification.
Countertop specification · Commercial countertops · Ask about 316L on your drawing