Choosing between SS304 and SS316 is one of the first decisions OEM and project buyers make when ordering stainless steel fittings from a factory. The short answer: the two grades are mechanically near-identical — 316 simply adds 2–3% molybdenum, and that one element decides how the metal behaves when chlorides arrive.
Key takeaways
- The only compositional difference that matters between SS304 and SS316 is 2–3% molybdenum in 316, which lifts its pitting-resistance number (PREn) from ~18–20 to ~24–26 — negligible in clean air and water, decisive in chlorides.
- Both grades share essentially the same strength and fabricability; SS316L caps carbon at 0.03% versus the 0.08% standard maximum to stop weld decay in the heat-affected zone of welded assemblies.
- Expect SS316 to cost roughly 30–40% more than SS304 — money well spent on marine splash, chemical dosing and chlorinated CIP, and wasted on dry indoor instrument air.
What actually differs: one element
SS304 and SS316 are both austenitic 300-series steels. Their working strength in the annealed condition is nearly identical, and both weld, bend and machine the same way. The chemistry that separates them is small and specific:
| Element | SS304 | SS316 |
|---|---|---|
| Chromium | 18.0–20.0% | 16.0–18.0% |
| Nickel | 8.0–10.5% | 10.0–14.0% |
| Molybdenum | none | 2.0–3.0% |
| Carbon (max) | 0.08% | 0.08% |
| Typical PREn | ~18–20 | ~24–26 |
Everything else in the specification — tensile properties, hardness, forming behavior — overlaps enough that drawings rarely distinguish the grades on mechanical grounds.
PREn: the number that predicts pitting
Engineers screen chloride resistance with the pitting resistance equivalent number, PREn = %Cr + 3.3 × %Mo + 16 × %N. SS304 sits around 18–20; SS316, with its molybdenum, around 24–26. That 6–8 point gap translates directly into resistance to pitting and crevice corrosion — the failure modes that hunt stainless in seawater, coastal air, de-icing salt, hypochlorite dosing and aggressive chemical cleaning.
Treat PREn as a screening tool, not a guarantee: surface finish, crevices, stagnant warm zones and weld quality all move real performance. But as a first filter it is honest — a 304 fitting in a chloride service pits eventually, and no polish will save it.
Where SS304 is the right buy
Dry indoor instrument air, plant water with low chloride content, hydraulic oil lines, most OEM panels and general utility skids — here 304 and 316 age identically, so the cheaper grade wins. SS304 is also the volume grade: factories machine, cast and stock it in the largest quantities, which means shorter lead times and less MOQ pressure on small export orders. If your line never meets chlorides, specifying 316 is insurance you will never claim on.
Where SS316 / SS316L earns its premium
Marine splash zones and coastal installations, chemical dosing skids, chlorinated cleaning regimes, pharmaceutical and high-purity water loops — anywhere chlorides concentrate — belong on 316. The molybdenum slows pitting initiation enough to matter in service, not just on datasheets.
For welded assemblies in corrosive service, specify 316L, not standard 316. Welding holds the heat-affected zone in the temperature window where carbon precipitates chromium carbides at the grain boundaries, leaving a narrow band depleted of chromium next to the weld — classic intergranular “weld decay.” The L grade caps carbon at 0.03%, starving that reaction. On weld-end fittings, welded manifolds and fabricated spools that will see chlorides, 316L is the correct line item.
Price and stock reality from the factory
Expect 316 barstock and castings to run roughly 30–40% over 304, tracking nickel and molybdenum prices — the gap widens when alloying metals spike and narrows when they soften, so treat any fixed percentage as a snapshot. Factory stock mirrors demand: 304 compression fittings and valve bodies cover the broadest size range from the shelf, while 316 and 316L common sizes are stocked but less deeply, and odd sizes in 316L may need a mill run. If your drawing calls for 316, say so plainly on the RFQ — a factory should never silently downgrade a 316 line to 304 to hit a price, and a buyer should never accept “stainless” on a quote without a grade next to it.
How to choose
- Dry indoor air, low-chloride water, cost-driven utility duty: SS304.
- Marine splash, coastal air, chemical dosing, chlorinated cleaning: SS316.
- Welded assemblies, weld-end fittings or spools in corrosive service: SS316L.
- Food, beverage or pharma with chloride-bearing CIP chemicals: SS316L, with the surface finish the process demands.
FerruleX machines and stocks stainless steel compression fittings, ball valves and needle valves in SS304 / SS316 / SS316L, with mill certificates on request. Send the grade, tube OD or thread size and quantity — we quote stocked material with a realistic ship date. A printable version of the grade comparison lives on our stainless steel grades chart sheet.
FAQ
Is SS316 stronger than SS304? No meaningful difference in the annealed condition — tensile and yield values overlap. You buy 316 for corrosion resistance, not strength. If strength is the real requirement, look at duplex or cold-worked grades instead.
Can I use SS304 fittings on a boat or dockside line? Not for anything seeing seawater splash, spray or coastal air long-term. 304 pits and crevice-corrodes in chloride exposure; 316 is the minimum sensible grade, and even 316 should be rinsed and kept free of stagnant salt deposits.
What does the L in SS316L actually do? It caps carbon at 0.03% versus 0.08% in standard 316. Low carbon prevents chromium carbide precipitation in weld heat-affected zones, so the welded joint keeps its corrosion resistance without post-weld annealing.
Will a magnet tell 304 from 316? No. Both are essentially non-magnetic in the annealed condition, and both pick up slight magnetism when cold-worked. XRF analysis separates them reliably — ask for mill certificates when the grade matters.
Is the price gap between 304 and 316 fixed? No. It tracks the nickel and molybdenum markets, so it moves monthly — a premium of roughly 30–40% is a common planning figure, but confirm current material for budget work.
