Many people buy stainless bolts to “avoid rust”, then run into two frustrating issues:
- Rust-like stains (often called tea staining) even though the fastener is stainless.
- Thread seizure (galling) where the nut locks up and won’t back off.
Most of the time, the root cause is either choosing the wrong stainless grade (201/304/316) or assembly practices that increase friction and contamination.
1 201 vs 304 vs 316: what’s the real difference?
A practical, decision-friendly summary:
- Stainless 201: budget-oriented, moderate corrosion resistance. Best for indoor or dry environments.
- Stainless 304 (18-8): the general-purpose choice for most applications.
- Stainless 316: improved resistance to chlorides/salt (thanks to molybdenum). Preferred for coastal / marine and salty humidity.
If you want one rule of thumb:
- Indoor → 201 or 304
- Outdoor general → 304
- Near the sea / salty air → 316
2 Why stainless can still look “rusty” (tea staining & contamination)
Stainless steel relies on a thin passive oxide film to protect the surface. If that film is disrupted—or if iron gets smeared onto the surface—you can see brown staining.
Common real-world causes:
- Iron contamination: cutting/grinding stainless with carbon-steel tools, using a carbon-steel wire brush, or exposure to steel dust.
- Tea staining in humid + salty environments: often cosmetic surface staining, not deep corrosion.
- Crevices and trapped moisture: dirt + water in gaps promotes localized attack.
How to reduce staining:
- Use dedicated stainless tools/abrasives where possible.
- Clean after installation (remove dust, salt, oils).
- For coastal areas: strongly consider 316 and designs that avoid water traps.
3 Grade selection by environment (quick table)
| Environment | Recommended | Notes |
|---|---|---|
| Indoor, dry | 201 or 304 | 304 is the safer default |
| Mild outdoor (sheltered, low salt) | 304 | Avoid water traps |
| Fully exposed outdoor | 304 or 316 | 316 helps in persistent humidity |
| Coastal / salty air | 316 | Better against chloride-driven staining/pitting |
| Chloride contact / light chemicals | 316 (case-by-case) | Confirm with actual exposure details |
4 What is thread galling and why stainless is prone to it?
Galling is a form of cold welding: under friction and heat during tightening, stainless surfaces can smear and stick at the thread interface until the nut locks solid.
Higher-risk situations:
- Stainless-on-stainless (bolt + nut), especially same grade.
- High-speed tightening (impact tools).
- Dry threads, fine threads.
- Higher clamp loads and repeated tightening.
5 Practical ways to prevent stainless thread seizure
- Apply a thin layer of anti-seize (nickel/moly types are commonly used for stainless).
- Tighten slowly and smoothly; avoid high-speed impact tightening.
- Use washers to reduce friction under the head/nut.
- Consider matched hardware designed to reduce galling (different nut material, coated nuts, etc.).
- Prefer coarse threads when possible.
- If you torque-spec: remember lubrication changes friction and therefore clamp load—follow your supplier/spec guidance.
6 Signs you picked the wrong grade
- 201 in humid/salty areas → faster staining and spot corrosion.
- 304 near the sea → tea staining, especially in unwashed areas.
- Right grade but wrong fabrication practices → staining from iron contamination.
7 What to specify when ordering stainless bolts
Include:
- Grade: 201 / 304 / 316
- Size + thread pitch: e.g. M8×1.25×30
- Standard/type: DIN/ISO if relevant
- Environment: indoor / outdoor / coastal
Browse bolt categories here: /bu-long.
