The short answer: 304 and 316 fasteners deliver the same strength at the same property class. What the 316 premium buys is 2–3% molybdenum, which roughly doubles the critical pitting temperature in standardised chloride testing. If the joint never sees chlorides, 304 is the rational choice; near coast, de-icing salt or chloride-bearing chemicals, 316 is the safer default — and submerged seawater is beyond both.
The chemistry: one alloying element apart
304 (UNS S30400, EN 1.4301) and 316 (UNS S31600, EN 1.4401) are both austenitic chromium-nickel steels. The compositions specified in ASTM A240/A240M overlap heavily except for molybdenum:[1]
| Element | 304 (S30400) | 316 (S31600) |
|---|---|---|
| Chromium | 18.0–20.0% | 16.0–18.0% |
| Nickel | 8.0–10.5% | 10.0–14.0% |
| Molybdenum | — | 2.00–3.00% |
| Fastener designation (ISO 3506) | A2 | A4 |
| Pitting resistance equivalent (PRE) | ≈ 18 | ≈ 24 |
PRE = %Cr + 3.3 × %Mo + 16 × %N is a ranking tool computed from composition, not a service guarantee.[2]
What the corrosion data actually shows
The Outokumpu Handbook of Stainless Steel publishes typical critical pitting and crevice temperatures (CPT/CCT) measured in acidified ferric chloride under ASTM G48:[2]
| Grade | PRE | CPT, ASTM G48-E | CCT, ASTM G48-F |
|---|---|---|---|
| 304 / 304L (1.4301/1.4307) | 18 | ≈ 10 °C | < 0 °C |
| 316 / 316L (1.4401/1.4404) | 24 | ≈ 20 °C | < 0 °C |
Two numbers here deserve attention. The molybdenum doubles the pitting temperature — that is the whole case for 316. And the critical crevice temperature is below freezing for both grades, which matters because a fastener joint is a collection of crevices: under the head, along the thread engagement, beneath the washer.
This is also why simple ppm limits mislead. The same handbook notes that even chloride levels below 100 ppm can initiate pitting or crevice attack in stagnant, oxygen-starved conditions.[2] Geometry, temperature and cleaning practice decide as much as the nominal water chemistry, so any "304 is fine to X ppm" table should be treated as a starting point, not a specification.
Strength: not a reason to choose 316
ISO 3506-1 assigns the same mechanical property classes to both grades, so A2-70 and A4-70 are equivalent in strength. The minima for austenitic bolts and screws:[3]
| Property class | Condition | Min. tensile strength | Min. 0.2% proof stress |
|---|---|---|---|
| 50 | Annealed, sizes ≤ M39 | 500 MPa | 210 MPa |
| 70 | Cold worked, sizes ≤ M24 | 700 MPa | 450 MPa |
| 80 | Cold worked, sizes ≤ M24 | 800 MPa | 600 MPa |
Inch-series stainless bolts are specified under ASTM F593, where Group 1 covers 304-type and Group 2 covers 316-type alloys.[4] If more strength than class 80 is required, the answer is alloy steel with a suitable coating or a larger fastener — not a different stainless grade.
Choosing in practice
| Service environment | Sensible default |
|---|---|
| Indoor machinery, enclosures, furniture | 304 |
| Outdoor, inland, away from road salt | 304 |
| Coastal atmosphere, de-icing salt exposure | 316 — 304 will tea-stain and can pit[5] |
| Chloride-bearing process fluids, chlorinated washdown | 316, after a chemistry review |
| Continuous seawater immersion | Neither by default — review duplex or coated alloy steel |
The exterior-architecture literature draws the same line: 304 performs well inland, while coastal and de-icing environments call for a molybdenum-bearing grade.[6] Expect 316 to carry a price premium and longer lead times in less common sizes — the working rule is to default to 304 and pay for 316 only where chlorides justify it.
Three field notes
- Galling affects both grades equally. Stainless threads seize under pressure and speed; use a validated lubricant and slow the driver down. Upgrading to 316 does nothing for seized threads.[7]
- Match the nut to the bolt. A 316 bolt with a 304 nut makes the joint's corrosion resistance the weaker member. State the grade for both.
- A magnet is not a grade test. Cold working makes class-70 and class-80 fasteners slightly magnetic, 304 and 316 alike. Positive material identification or the mill certificate is the only reliable check.
What to put on the RFQ
A complete callout is size + thread + class + grade, e.g. 1/4-20 UNC-2A, ASTM F593 Group 2 or M8 × 30, A4-70. Add the exposure conditions — coastal air, cleaning chemicals, splash or immersion — and the grade choice usually makes itself.
Our stainless range covers A2-70 and A4 across hex bolts, socket cap screws and threaded rod. If the environment is borderline, send the service details and we will confirm the grade before quoting.
Need more than stainless? Read Dacromet and Zinc-Flake Coatings for High-Strength Fasteners for coated alloy-steel options, or Titanium Fasteners: Grades, Benefits and Applications when weight also matters.
References
- ASTM International — ASTM A240/A240M-24, chromium and chromium-nickel stainless steel plate, sheet and strip.
- Outokumpu — Handbook of Stainless Steel (Table 6:1, pitting and crevice corrosion data).
- British Stainless Steel Association — Stainless steel fastener grades to BS EN ISO 3506 (SSAS Information Sheet 2.31).
- ASTM International — ASTM F593-17, stainless steel bolts, hex cap screws and studs.
- ASSDA — FAQ 6: Preventing Coastal Corrosion (Tea Staining).
- International Molybdenum Association — Which Stainless Steel Should Be Specified for Exterior Applications?.
- ASSDA — FAQ 5: Galling and its Control.
From the Catalog
Products covered in this guide
View products
