304 and 316 stainless both resist corrosion. Both austenitic. Both non-magnetic. But 316 costs 30% more — why? A 2% molybdenum addition. For chloride environments, marine service, and pharmaceutical-grade work, that 2% is the difference between a part that lasts 20 years and one that pits in 6 months.
| Element | 304 (%) | 316 (%) | Role |
|---|---|---|---|
| Chromium | 18–20 | 16–18 | Passive oxide layer, general corrosion resistance |
| Nickel | 8–10.5 | 10–14 | Austenite stabilizer, ductility |
| Molybdenum | 0.00 | 2–3 | Pitting/crevice resistance in chlorides |
| Carbon (max) | 0.08 | 0.08 | Higher C = sensitization risk at weld HAZ |
| Manganese | ≤2.0 | ≤2.0 | Sulfur neutralization |
| Iron | Balance | Balance | Matrix |
The molybdenum is the headline difference. Molybdenum atoms slot into the oxide layer on the steel surface and make that layer much more resistant to chloride attack — specifically, the kind of attack that starts as a tiny pit and grows into a through-hole over months or years.
304 in chloride environments doesn't fail by uniform rust — it pits. Tiny localized corrosion sites form, go down, and can perforate a 3 mm sheet in 2–5 years of seawater exposure. 316 with its moly addition might pit visibly in the same environment but stops at ~0.1 mm depth.
| Environment | 304 works? | 316 works? |
|---|---|---|
| Indoor dry air | ✓ Forever | ✓ Overkill |
| Outdoor clean air (inland) | ✓ 20+ years | ✓ Forever |
| Outdoor urban (pollution, some chloride) | ⚠ Slow pitting after 10+ years | ✓ 30+ years |
| Coastal air (<10 km from ocean) | ⚠ Visible pitting in 2–5 years | ✓ 15+ years |
| Splash zone / ocean spray | ✗ Through-pits in 2–5 years | ⚠ Pitting slow but present |
| Full saltwater immersion | ✗ Fails in months | ⚠ Service OK, not severe |
| Swimming pool (indoor chlorine) | ⚠ Overhead components pit fast | ✓ Standard material |
| Food contact (dairy, bakery, brewery) | ✓ USDA-accepted standard | ✓ More corrosion margin |
| Pharmaceutical (WFI, CIP chemistry) | ⚠ Limited — cleaning chemistry matters | ✓ Standard (often electropolished) |
| Marine deck hardware | ✗ Don't | ✓ Standard grade |
| Sulfuric acid <20% | ⚠ Grade dependent | ✓ Better margin |
| Nitric acid | ✓ Excellent | ✓ Excellent |
| Hydrochloric acid | ✗ No | ✗ No — need Hastelloy or titanium |
The simple rule: if chloride is anywhere in the service environment, specify 316. If not, 304 is cheaper and works.
| Property | 304 (annealed) | 316 (annealed) |
|---|---|---|
| Tensile strength (min) | 515 MPa | 515 MPa |
| Yield strength (min) | 205 MPa | 205 MPa |
| Elongation (min) | 40% | 40% |
| Hardness | 92 HRB | 95 HRB |
| Density | 8.00 g/cm³ | 8.00 g/cm³ |
| Magnetic (annealed) | No | No |
| Magnetic (cold worked) | Slightly (work-induced martensite) | Slightly less than 304 |
From a structural-analysis standpoint, 304 and 316 are interchangeable. A bracket designed in 304 will perform identically in 316. There's no weight or stiffness penalty to upgrading to 316 for corrosion margin.
Both grades are considered moderate-to-difficult to machine. Compared to free-machining steel (1215), both are slow. Compared to aluminum, both are painful.
304 machinability rating: ~45% (on the 1215 = 100% scale).
316 machinability rating: ~36% (slightly worse due to higher Ni content).
Practical implications:
303 (sulfur-added) and 316F/316 free-machining are 2× faster to machine than standard 304/316 — but sulfur destroys corrosion resistance and weldability. Use only for non-critical parts.
Standard 304 and 316 can be welded, but the heat-affected zone (HAZ) is prone to sensitization: at 500–800 °C, chromium carbides precipitate at grain boundaries, depleting the nearby chromium, creating zones that corrode preferentially.
Solutions:
Rule: if the part will be welded, specify 304L or 316L, not plain 304/316. Mechanical properties are essentially identical; cost is the same; corrosion performance near welds is dramatically better.
Base material cost (approximate, 2024 averages):
| Grade | Raw sheet $/kg | Cost multiplier vs 304 |
|---|---|---|
| 304 / 304L | $3.80 | 1.00× (baseline) |
| 316 / 316L | $5.00 | ~1.32× |
| 316Ti (titanium-stabilized) | $5.80 | ~1.53× |
| 321 (stabilized) | $5.40 | ~1.42× |
| 2205 duplex (moly + higher strength) | $7.20 | ~1.89× |
On a finished part, the cost delta is smaller than the material delta because labor and machine time are constant. A CNC-milled bracket that costs $50 in 304 typically costs $57–60 in 316 (a 15–20% part cost increase for a 32% material increase).
For procurement: don't reflexively spec 316. Over-specification is a common source of hidden cost. Use 316 when the environment genuinely includes chlorides; use 304 otherwise.
ISO 10993-compliant 316L is the default implant-adjacent material. Even for non-implant devices, 316L simplifies validation vs 304.
Florida, California, Washington, the UK, most of Japan — coastal air carries chloride. 304 pits visibly within 3–5 years of exposure.
Food-grade but not directly contacting salty brine or chloride cleaning. 304 is standard for worktables, backsplashes, equipment housings.
Indoor pool chlorine vapor attacks 304 overhead parts (ceiling hangers, light fixtures) within a year. Use 316 or upgrade to 2205 duplex.
Standard for piping, tanks, and fittings in WFI (water for injection) and CIP (clean-in-place) systems. Electropolish reduces surface area where bacteria attach.
Cost is the same as non-L. Weld zone corrosion performance is dramatically better. No reason to specify plain 304/316 if welding is in the process.
Email [email protected] with your drawing. If you aren't sure which grade suits your environment, describe the service conditions and we'll recommend — over-spec costs real money on production runs.
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