The maintenance shed at a small marine yard stank of diesel, damp wood, and last winter's salt. Three trays sat on the bench, each holding screws pulled off one pier walkway. The 304s had lost their shine months earlier and were growing a brown beard of pitting along the threads. The 410s, on a gate hinge repair, bled rust lines. Only the 316s still looked like the day they went in. Same salt air, same tidal splash. Three alloys, three verdicts.
What the Grade Numbers Actually Promise
Every stainless steel designation is shorthand for a compromise, and the gap between 304, 316, and 410 is wider than spec sheets let on. 304 is the workhorse 18-8 austenitic: 18 percent chromium, 8 percent nickel. The chromium film self-repairs, which is why 304 shrugs off indoor humidity and clean rainwater. But the film is thin, and chlorides are patient. Hit 304 with a coastal splash zone or chlorine-dosed washdown, and pitting starts at thread roots and under heads — where you cannot see it until it lets go.
316 is 304's answer: 2 to 3 percent molybdenum lifts the Pitting Resistance Equivalent Number into the mid-twenties and blocks chloride attack at the film level. That single addition is the whole difference at the coast.
410 lives in another camp. Martensitic, 12 percent chromium, no nickel, it hardens to around HRC 38-45 — the pick when abrasion and shear, not corrosion, are the enemy. The trade-off: thin passive film, magnetic, rusts in months outdoors, not decades.
| Grade | Metallurgy & Key Alloy | PREN (approx.) | Corrosion Profile | Hardness & Magnetism | Where It Earns Its Keep |
|---|---|---|---|---|---|
| 304 | Austenitic; 18% Cr / 8% Ni | 18–19 | Self-repairing oxide film; pits under prolonged chloride + heat | HRB ~90; non-magnetic (slightly magnetic after cold heading) | Inland construction, furniture, electronics, chloride-free humidity |
| 316 | Austenitic; adds 2–3% Mo | 24–26 | Molybdenum blocks chloride attack; survives tidal splash & chemical washdown | Similar hardness; essentially non-magnetic | Coastal decks, marine fittings, chemical plants, food processing |
| 410 | Martensitic; 12% Cr, no Ni | ~13 | Thin film; rusts fast in moist outdoor air | Hardenable HRC 38–45; strongly magnetic | Dry indoor, wear-focused jobs, where hardness beats corrosion |
Read it one way and the logic snaps into place: chloride exposure is the fork between 304 and 316, while wear decides whether 410 earns a place in the cart at all.
The Questions That Keep Coming Up in Sourcing
Five questions keep circling back, in search bars and RFQ threads alike.
Does 410 stainless steel rust?
Yes, and faster than people expect. Outdoors, even under a roof, 410 shows rust within a season. Use it where hardness earns its keep and the environment stays dry.
Is 316 stainless steel magnetic?
Not really. Austenitic grades are essentially non-magnetic, though heavy cold heading can leave a 316 screw faintly magnetic. A pocket magnet settles it.
When is 304 good enough?
Inland buildings, furniture, electronics — anywhere without sustained chloride exposure. Most indoor duty never asks more of a screw than 304 can give.
Which grade for coastal decking?
316, A4 class. Tidal splash, salt wind, and pool chemicals pit 304 within a few years; 316 is built for that fight.
Is 410 stronger than 316?
Depends what "strong" means. Hardened 410 holds an edge and resists wear; 316 wins on toughness and ductility. Different conversations.
Failure Files: When the Grade Was Wrong
A seafood plant in a humid river delta learned this the expensive way: 304 screws on washdown equipment, sprayed daily with chloride-heavy cleaners, developed crevice corrosion under the heads within two years. The fix was 316 and a longer inspection interval. A coastal fence contractor picked 410 self-drillers because they were hard enough to punch through galvanized rail — the rust streaks on the posts told the rest of the story by spring. None of these were exotic failures; they were grade-selection errors at scale, and every one landed on a maintenance line nobody budgeted for. The lesson is quiet but stubborn: match metallurgy to environment first, and the fastener stops being the failure point.
In Conclusion
Choosing between 304, 316, and 410 is only half the job. The other half is trusting that the screws in the carton match the grade on the box. Fasto produces 304, 316, and 410 screws through in-house precision cold heading, with material traceability back to heat numbers and batch-consistent hardness control — so what the datasheet promises is what your line receives, shipment after shipment.