---
title: "316 vs 304 Stainless Steel: Which to Spec"
description: "The difference is molybdenum, and it only matters in specific environments. Composition, corrosion behavior, machinability and cost, with a grade selector."
canonical: https://openspindle.com/blog/316-vs-304-stainless
author: "Tom"
datePublished: 2026-08-29T00:00:00.000Z
dateModified: 2026-08-29T00:00:00.000Z
category: "Materials"
---

# 316 vs 304 Stainless: Which to Spec

304 is the default austenitic stainless and covers most parts. 316 adds 2 to 3 percent molybdenum, which buys real resistance to chlorides: seawater, de-icing salt, pool chemistry and many process fluids. In dry indoor service the two behave almost identically, so specifying 316 there spends money on a property the part will never use. Both machine slowly next to aluminum, and 316 is marginally harder to cut than 304.

## The composition difference, and what molybdenum does

Both grades are austenitic stainless steels built on chromium and nickel. The chromium forms the passive oxide layer that makes stainless stainless. The nickel stabilizes the austenitic structure, which is what gives both grades their toughness, formability and non magnetic behavior.

The difference is 2 to 3 percent molybdenum in 316. Molybdenum makes the passive layer far more resistant to being broken down locally by chloride ions. That specific failure mode, pitting and crevice corrosion, is what kills 304 in marine and salted environments, and it is what 316 is for.

Everything else about the two grades is close enough that you would struggle to tell them apart without a test.

| Property | 304 | 316 |
| --- | --- | --- |
| Chromium | 18 to 20 percent | 16 to 18 percent |
| Nickel | 8 to 10.5 percent | 10 to 14 percent |
| Molybdenum | None | 2 to 3 percent |
| Yield strength, annealed | About 215 MPa (31 ksi) | About 205 MPa (30 ksi) |
| Tensile strength, annealed | About 505 MPa (73 ksi) | About 515 MPa (75 ksi) |
| Pitting resistance (PREN) | About 19 | About 25 |
| Max continuous service temperature | About 870 C | About 870 C |
| Relative material cost | Baseline | About 1.2 to 1.5x |

## Where 316 earns its cost

Chlorides are the whole argument. If the part will see seawater, salt spray, road salt, swimming pool water, or a process fluid containing chlorides, 304 will pit and 316 will hold up considerably longer.

Marine hardware is the obvious case. Less obvious ones catch people out: outdoor hardware within a few miles of a coast, anything washed with chlorinated sanitiser, food processing equipment cleaned with chloride based agents, and parts near de-iced roads through winter.

Pharmaceutical and food contact work often specifies 316 for a related reason. The surface is easier to keep genuinely clean through repeated aggressive washdown, and the corrosion products that would otherwise contaminate product do not form.

One caveat worth knowing: 316 is more resistant to chlorides, not immune. In hot concentrated chloride environments it pits too, and the answer moves to a duplex or super austenitic grade rather than to more 316.

## Where 304 is genuinely fine

Indoor structural brackets. Enclosures and panels. Fasteners in dry service. Kitchen and architectural parts that get washed with ordinary detergent. Anything where the corrosion exposure is humidity rather than salt.

304 is more widely stocked, slightly cheaper, and marginally easier to machine. On a part that will never meet a chloride, choosing 316 buys nothing measurable and narrows your material availability.

The honest test is to name the environment. If you can describe the exposure specifically, and chlorides are not in the description, 304 is the grade. If you find yourself specifying 316 because it sounds like the better stainless, that is the moment to check.

## Machinability and the variants that exist to fix it

Neither grade is pleasant to machine. Both work harden aggressively, which means a tool that dwells or rubs instead of cutting creates a hardened layer that destroys the next pass. Both are gummy and produce stringy chips. Expect machining times several times longer than the same part in aluminum, and expect that to dominate your price.

316 is slightly worse than 304, though the gap is smaller than the reputation suggests. The tool life difference matters more to the shop than the cycle time difference matters to you.

Three variants are worth knowing. 303 is 304 with sulfur added for machinability, and it cuts dramatically better, but the sulfur reduces corrosion resistance and it should not be used where the corrosion performance is the point. 304L and 316L are low carbon versions that resist sensitization during welding, which is why welded assemblies are usually specified in L grades. 316L in particular is the default for welded food, pharmaceutical and marine fabrication.

## Passivation, finishing and why bare is not always clean

A freshly machined stainless surface is not in its best corrosion state. Machining smears free iron from tooling across the surface, and that iron rusts, which is why a stainless part sometimes shows orange staining that looks like the grade was wrong.

Passivation removes that free iron with an acid treatment and lets the chromium oxide layer re-form uniformly. It is inexpensive, it is specified to ASTM A967 or AMS 2700, and on any part where corrosion performance matters it should be on the drawing. Electropolishing goes further, removing a thin surface layer to leave a smoother, more corrosion resistant and easier to clean finish, which is why it is common in food and medical work.

If you specify 316 for its corrosion resistance and then skip passivation, you have paid for the grade and given away part of what you bought.

## What this costs you

Material cost is the smaller half of the story. 316 typically runs 20 to 50 percent more than 304 per kilogram, but on a machined part the material is often a minority of the price because the cycle time dominates.

Availability is the more practical cost. 304 is stocked in more sizes and forms in more places, so an unusual bar diameter or plate thickness in 316 can add real lead time even when the price is acceptable. On a prototype where you need parts in a week, that can matter more than the material premium.

The worst outcome is over specifying corrosion resistance on a part that then gets a tight tolerance and a fine finish as well, because each of those independently pushes the cycle time up on a material that is already slow to cut.

| Factor | 304 | 316 |
| --- | --- | --- |
| Raw material cost | Baseline | About 1.2 to 1.5x |
| Machining time | Baseline | Slightly longer, shorter tool life |
| Stock availability | Very wide | Good, narrower in unusual sizes |
| Chloride environments | Pits and crevice corrodes | Substantially better |
| Welded assemblies | Use 304L | Use 316L |
| Best machining variant | 303, at the cost of corrosion resistance | No direct free machining equivalent |

## Grade selector by environment

Match the grade to the exposure you can actually describe, not to the grade you have seen specified elsewhere.

| Environment or application | Grade | Notes |
| --- | --- | --- |
| Indoor dry, structural or cosmetic | 304 | 316 adds nothing measurable here |
| Outdoor inland, general weather | 304 | Passivate; consider 316 for coastal proximity |
| Coastal, marine, salt spray | 316 | Specify passivation, and 316L if welded |
| Swimming pool or chlorinated water | 316 | Hot concentrated chlorides may need duplex instead |
| Food processing with chloride sanitisers | 316L | Electropolish where cleanability matters |
| Medical or pharmaceutical contact | 316L | Electropolish, and specify surface finish explicitly |
| High volume turned parts, dry service | 303 | Best machinability, reduced corrosion resistance |
| Welded fabrication of any kind | 304L or 316L | Low carbon resists sensitization at the weld |

## What to send, and what to ask

Send the drawing with the service environment stated somewhere on it or in the notes. A shop that knows the part goes on a boat will flag things a shop that only sees a geometry cannot.

Specify passivation to ASTM A967 where corrosion matters, and say whether the part is welded, because that decides between the standard and the L grade. State the surface finish requirement rather than assuming, since finish and corrosion performance are related on stainless.

Then ask three questions. Is this size stocked in 316, or does 304 have better availability for the lead time I need? Would 303 be acceptable given the environment, and what would it save? And is there a fit or feature here driving the cycle time that I could relax without affecting the corrosion performance?

## Related reading

- [Stainless steel material guide](https://openspindle.com/materials/stainless-steel.md)
- [Stainless vs aluminum](https://openspindle.com/blog/stainless-vs-aluminum.md)
- [Passivation spec](https://openspindle.com/reference/passivation-spec.md)
- [Electropolishing spec](https://openspindle.com/reference/electropolishing-spec.md)

## Frequently asked questions

### What is the main difference between 316 and 304 stainless?

316 contains 2 to 3 percent molybdenum and 304 contains none. Molybdenum makes the passive surface layer far more resistant to chloride attack, which is the pitting and crevice corrosion that destroys 304 in marine, salted and chlorinated environments. In dry indoor service the two grades behave almost identically in strength, appearance and formability.

### Is 316 stainless worth the extra cost?

Only where chlorides are present. In seawater, salt spray, road salt, pool water or chloride based cleaning chemistry, yes, decisively. In dry indoor service you are paying 20 to 50 percent more per kilogram for a property the part will never use. Describe the actual service environment; if chlorides do not appear in that description, 304 is the correct grade.

### Which is easier to machine, 304 or 316?

304, but only slightly, and neither is easy. Both work harden aggressively and produce stringy chips, and both machine several times slower than aluminum. The difference between them shows up more in tool life for the shop than in cycle time for you. If machinability is the priority and the environment allows it, 303 cuts far better than either at the cost of corrosion resistance.

### Do I need to passivate stainless parts?

On any part where corrosion resistance matters, yes. Machining smears free iron from tooling across the surface, and that iron rusts, producing orange staining that looks like a wrong grade. Passivation removes it with an acid treatment and lets the chromium oxide layer re-form. It is inexpensive, specified to ASTM A967 or AMS 2700, and skipping it gives away part of what you paid for.

### What is the difference between 316 and 316L?

316L has lower carbon. Lower carbon resists sensitization, where chromium carbides form at grain boundaries during welding and locally deplete the chromium that provides corrosion resistance. For any welded assembly, specify the L grade. For machined parts that are never welded, standard 316 is fine and slightly stronger.

### Will 316 stainless rust?

It can. 316 is more resistant to chlorides than 304, not immune to them. In hot or concentrated chloride environments it will still pit, and the answer at that point is a duplex or super austenitic grade rather than more 316. It will also stain if free iron from machining is left on the surface, which is a passivation problem rather than a grade problem.