304 vs 316: what the molybdenum actually buys
The difference is two to three percent molybdenum, and it decides whether the material survives chlorides.
304 and 316 look identical, machine similarly, weld similarly and cost within about a third of each other. The entire practical difference is that 316 contains two to three percent molybdenum and 304 contains none. That single addition is what decides whether a component survives in a chloride environment.
What molybdenum does
Both grades protect themselves with a passive chromium oxide film. Chloride ions attack that film locally, and once they break through, corrosion concentrates at that point rather than spreading — which is why the failure mode is pitting rather than general thinning. Molybdenum makes the passive film substantially more resistant to that local breakdown, and helps it repair itself when it does occur.
The standard way to express this is PREN, the pitting resistance equivalent number: %Cr + 3.3 × %Mo + 16 × %N. 304 sits around 19. 316 sits around 25. That gap is the whole argument.
| 304 / 304L | 316 / 316L | |
|---|---|---|
| Chromium | 18–20% | 16–18% |
| Nickel | 8–10.5% | 10–14% |
| Molybdenum | None | 2–3% |
| Typical PREN | 19 | 25 |
| Relative cost | 1.00× | ~1.35× |
| Coastal / marine air | Will stain and pit | Suitable |
| Seawater immersion | No | Not for sustained service |
| Food and dairy | Suitable | Suitable |
| Chloride-bearing process | No | Usually suitable |
Scroll the table horizontally to see all columns.
When 304 is the right answer
Most of the time. For indoor architectural work, potable water, food and dairy processing, general process piping and structural applications away from the coast, 304 performs indefinitely and costs less. Specifying 316 in those duties is spending money for no service benefit.
When you need 316
- Anywhere chlorides are present — coastal atmosphere, de-icing salt, brine, chlorinated water
- Pharmaceutical and sanitary systems, where cleaning chemistry is aggressive
- Most chemical and petrochemical process duty
- Marine fittings and fixings above the waterline
When 316 is not enough either
This is the part most comparisons leave out. 316 is not chloride-proof. In hot or concentrated chloride solutions it still pits, and above roughly 60 °C it becomes vulnerable to chloride stress corrosion cracking — a failure mode that produces no visible warning before it produces a through-wall crack.
If the duty involves sustained seawater immersion, hot brine, or a chloride concentration that has already pitted 316 elsewhere on the plant, the conversation should move to 317L, 904L, a duplex grade or 254 SMO. Duplex is usually the most economical of those, because its higher strength allows a thinner wall.
Specifying for a live project?
Send the service conditions and we will tell you what we would specify, and what we hold.