The quick answer
304 is the right call for 80% of stainless parts: indoor brackets, handrails, architectural trim, kitchen equipment, general mechanical parts, dry-environment fasteners. Step up to 316 (or 316L for welded parts) when the application sees chlorides — saltwater spray, swimming-pool chemicals, acidic food that sits for extended time, or body-fluid contact in medical devices.
The expensive mistake is specifying 316 everywhere "to be safe." A typical CNC bracket in 316 costs 15–20% more than the identical part in 304 — and if the environment doesn't contain chlorides, that money buys nothing. The other expensive mistake is specifying 304 for a part that will see salt spray, then paying to replace it 18 months later. The rest of this guide is about telling those two cases apart.
Composition and the molybdenum effect
| Element | 304 | 316 |
|---|---|---|
| Chromium (Cr) | 18.0–20.0% | 16.0–18.0% |
| Nickel (Ni) | 8.0–10.5% | 10.0–14.0% |
| Molybdenum (Mo) | — | 2.0–3.0% |
| Carbon (max) | 0.08% | 0.08% (316L: 0.03%) |
| Manganese (max) | 2.0% | 2.0% |
| Silicon (max) | 0.75% | 0.75% |
The headline difference is the 2–3% molybdenum. Molybdenum stabilizes the passive chromium-oxide layer against attack by chloride ions. In environments where chlorides penetrate the passive film — saltwater, brine, bleach, pool water, tomato paste, blood — 304's protection breaks down locally and pits form. 316's molybdenum keeps the film intact under the same conditions. That single chemistry difference drives the entire selection decision.
Mechanical properties — essentially identical
| Property | 304 | 316 |
|---|---|---|
| Ultimate tensile | 515 MPa | 515 MPa |
| Yield strength | 215 MPa | 205 MPa |
| Elongation | 40% | 40% |
| Hardness (Brinell) | ~201 HB | ~217 HB |
| Density | 7.93 g/cm³ | 7.98 g/cm³ |
For practical purposes, 304 and 316 have the same strength and the same machining behavior on our CNC floor. Cycle times are within 5%. Chip formation is similar. Surface finish after identical tooling is comparable. If you were choosing on mechanical performance alone, there's no meaningful reason to pick one over the other. Corrosion is the whole story.
When 304 is the right answer
Specify 304 when the part's operating environment is one of the following: indoor air (any humidity), freshwater contact, dry food storage, non-chloride chemical handling, architectural trim, machinery housings, or general mechanical structural parts. The chromium-nickel combination gives excellent corrosion resistance against anything that isn't actively attacking the passive layer with halides.
Real applications where we machine 304 by default: architectural balustrades and railings, indoor food-prep equipment and utensils, commercial kitchen hood frames, industrial machinery guards and fixturing, decorative trim for consumer products, pharmaceutical non-contact structural parts, brewery fermentation tank standoffs.
When 316 is required
Specify 316 (or 316L for welded assemblies) when any of these conditions apply:
- Marine atmosphere or splash zone — coastal installations, boat deck fittings, shipboard hardware above the waterline.
- Swimming pool equipment — pool ladders, fasteners, filter housings. Chlorinated water is the classic 304-failure environment.
- Acidic or salty food contact — tomato-paste tanks, pickle vats, soy sauce vessels, brewing equipment where low-pH wort contacts the surface.
- Pharmaceutical process contact — USP-grade water systems, process tanks, sanitary fittings under CIP/SIP cycles.
- Medical device housings — non-implantable surgical instruments, endoscope bodies, sterilization trays. Use 316L for weldments.
- Exterior architectural in urban / coastal environments where salt-laden air accelerates pitting.
- Chemical handling — sulfuric acid at low concentrations, phosphoric acid, mild chloride solutions.
The 316 vs 316L weldability gap
Standard 316 contains up to 0.08% carbon. When heated in the 550–850 °C range — which happens in the heat-affected zone around any weld — chromium combines with carbon to form Cr₂₃C₆ carbides at grain boundaries. This depletes the chromium in the surrounding metal below the ~12% threshold needed for passive-layer formation, and intergranular corrosion follows during service. The part passes initial inspection and fails 6–12 months later with grain-boundary cracks.
316L solves this by capping carbon at 0.03%. There isn't enough carbon present to form significant carbide precipitation during welding, so the weld heat-affected zone retains its corrosion resistance. For any 316 part that will be welded — which is most of them — specify 316L. The raw material cost premium over standard 316 is negligible, and we stock 316L as our default 316 grade. If your drawing calls out "316" without the L designation, we'll ask whether the part will be welded and upgrade to 316L if yes.
Machining behavior and surface finish
Both grades are austenitic stainless and share the classic stainless machining challenges: work-hardening, built-up edge on tool cutting edges, poor thermal conductivity that concentrates heat at the tool tip. Our CNC machining strategy for both is identical — aggressive chip loads, flood coolant, coated carbide tooling, avoid dwelling in the cut. 316 feels marginally tougher due to the higher nickel content (10–14% vs 8–10.5%), which translates to ~5% slower cycle times, but the difference is not economically significant.
For polished finishes — common on medical and food-contact parts — both grades polish to the same mirror Ra values. Post-machining electropolish (for sanitary applications) works identically on both. If you need a No. 4 brushed finish for architectural trim, 304 and 316 are visually indistinguishable.
Cost and availability
Raw 316 bar stock runs roughly 15–25% higher than 304 at current nickel and molybdenum prices, driven almost entirely by the molybdenum content. On a finished CNC part, raw material is 30–50% of cost depending on geometry, so the part-cost premium for 316 typically lands at 8–15% over the equivalent 304 part.
Availability is equivalent — both grades are stocked in round, square, hex, and flat bar across a full range of sizes. Lead time on 316L versus 304 is identical in our supply chain. The decision is cost and application-driven, never supply-driven.
Decision framework
The single question that decides 304 versus 316: will the part contact chloride ions in service? Saltwater, chlorinated pool water, bleach, acidic foods, body fluids, de-icing salt spray — any of these means 316 (or 316L if welded). Absence of chlorides means 304 is the correct and more economical answer.
Two common over-specifications we see on quote requests: (1) "316 stainless" called out on indoor machinery parts that will never see salt — upgrade to 304 saves the customer 15% at no performance cost, and (2) "304 stainless" called out on coastal installation hardware that will fail in 2 years — downgrade from 316 is costing the customer the full replacement budget later. We flag both on our quote response so you can make the real decision with full information.