304 vs 316 Stainless Steel
Corrosion resistance, machinability, cost, and the one number that tells you which grade you actually need — the Pitting Resistance Equivalent.
The One Question That Decides: Does Your Part See Chlorides?
304 and 316 are structurally nearly identical. The critical difference is molybdenum content in 316, which raises its resistance to chloride pitting by 44% (PRE 18 → 26). If your part will never contact salt water, bodily fluids, chlorine-based cleaners, or coastal outdoor air, 304 is the right choice at a lower cost. If it will, 316L is mandatory. This guide gives you the engineering data to make that call with confidence.
Composition: What Makes 316 Different
Your grade selection between 304 and 316 comes down to one element: molybdenum. Both grades are 300-series austenitic stainless steels — a family of non-magnetic, non-heat-treatable grades with a face-centered cubic crystal structure that gives them excellent ductility and formability. Chromium and nickel are the primary alloying elements; the key addition in 316 is molybdenum (Mo), which raises pitting corrosion resistance by ~40%.
Worked PRE example: PRE (Pitting Resistance Equivalent) = %Cr + 3.3 × %Mo + 16 × %N. For 304: 18 + 3.3×(0) + 16×(0.10) = 19.6. For 316: 17 + 3.3×(2.5) + 16×(0.10) = 26.9. That 7-point gap is the engineering reason 316 resists chloride pitting where 304 fails.
| Element | 304 / 304L | 316 / 316L | Why It Matters |
|---|---|---|---|
| Chromium (Cr) | 18.0–20.0% | 16.0–18.0% | Forms passive oxide layer — the primary corrosion mechanism for stainless steel |
| Nickel (Ni) | 8.0–10.5% | 10.0–14.0% | Stabilizes austenite phase, improves ductility and low-temperature toughness |
| Molybdenum (Mo) | None (0.0%) | 2.0–3.0% | Critical addition — raises PRE by 7–8 points (~40%), the single largest factor in chloride pitting resistance |
| Carbon (C) | ≤ 0.08% (304) / ≤ 0.03% (304L) | ≤ 0.08% (316) / ≤ 0.03% (316L) | "L" grade reduces carbide precipitation in the heat-affected zone (HAZ) during welding |
| Manganese (Mn) | ≤ 2.0% | ≤ 2.0% | Austenite stabilizer, deoxidizer during steelmaking |
| PRE (calculated) | ~18 | ~26 | PRE = %Cr + 3.3×%Mo + 16×%N. Higher = better chloride resistance |
Pro Tip: PRE is the Key Number
The PRE formula — %Cr + 3.3×%Mo + 16×%N — is the single best predictor of pitting corrosion resistance. 304 PRE ≈ 18; 316L PRE ≈ 26. For context, seawater immersion service typically requires PRE > 32 — meaning neither 304 nor 316 is adequate for continuous seawater submersion. Duplex 2205 (PRE ≈ 35) or super duplex 2507 (PRE ≈ 43) should be used instead.
Mechanical Properties Comparison
If your part selection is driven by strength, 304 and 316L are effectively interchangeable — the real difference is corrosion resistance, not mechanical properties. The molybdenum addition in 316 provides a slight reduction in UTS (~6%). Neither grade is heat-treatable for strength — only work hardening or cold working can increase hardness and tensile strength. If you need stainless corrosion resistance and high strength, specify 17-4 PH instead.
| Property | 304 (Annealed) | 316L (Annealed) | Notes |
|---|---|---|---|
| UTS | 515 MPa (75 ksi) | 485 MPa (70 ksi) | 316L is ~6% lower — rarely significant for structural decisions |
| Yield Strength (0.2%) | 205 MPa (30 ksi) | 170 MPa (25 ksi) | Both are low-yield austenitic grades; for high strength, use 17-4 PH H900 |
| Elongation | 40% | 40% | Excellent ductility — both grades form and bend without cracking |
| Hardness | HRB 88 max | HRB 79 max | Cannot be hardened by heat treatment — only by cold working |
| Density | 8.00 g/cm³ | 8.00 g/cm³ | 3× denser than aluminum 6061 (2.70 g/cm³) |
| Thermal conductivity | 16.2 W/m·K | 13.4 W/m·K | Low — ~10× lower than 6061-T6 aluminum (167 W/m·K). Not suitable for heat sinks. |
| CTE | 17.2 µm/m·°C | 15.9 µm/m·°C | Higher than carbon steel (12 µm/m·°C) — account for mixed-material assemblies |
| Magnetic | No (annealed) | No (annealed) | May become weakly magnetic after cold working — specify solution annealed if critical |
Corrosion Resistance: Where the Grades Diverge
Corrosion resistance is the only engineering dimension where 304 and 316L meaningfully differ — and it is the entire reason 316L exists. Use this environment matrix to make the right call for your application. If chlorides (salt, saline, coastal air, cleaning chemicals) are anywhere in your service environment, read every row carefully.
| Environment | 304 | 316L | Recommended Grade |
|---|---|---|---|
| Indoor, low-humidity air | Excellent | Excellent | 304 — use the cheaper option |
| Fresh water (municipal) | Excellent | Excellent | 304 — adequate for most water service below 60°C |
| Mild chemical (dilute acids, bases) | Good | Very good | 316L — molybdenum improves resistance to dilute H₂SO₄ and HCl |
| Salt spray / coastal outdoor | Marginal | Good | 316L — 304 may pit within months in salt-laden air |
| Chlorine-based cleaners (bleach, CIP) | Poor | Acceptable | 316L — required for food processing clean-in-place systems |
| Bodily fluids, saline solutions | Poor | Good | 316L — chloride content of biological fluids attacks 304 |
| Continuous seawater immersion | Poor | Poor | Neither — use duplex 2205 (PRE ≈ 35) or titanium |
| Autoclave sterilization (134°C steam) | Acceptable | Very good | 316L — preferred for repeated autoclave cycles |
The Crevice Corrosion Trap
Even 316L can suffer crevice corrosion in chloride environments if the design traps liquid in tight gaps (under bolt heads, in threaded connections, at mating surfaces). Stagnant chloride solution in a crevice concentrates and drops pH — overwhelming the passive layer. Design stainless parts with drainage, avoid tight lap joints, and consider sealing exposed crevices.
Passivation After Machining
Free iron from tooling contamination can compromise the passive layer on machined stainless surfaces. Electropolishing or chemical passivation (ASTM A967, nitric acid or citric acid bath) restores the chromium oxide layer. Always specify passivation on the drawing when the part will contact corrosive media or bodily fluids.
Machinability & CNC Cost Implications
Your CNC cost for 304 and 316L will be nearly identical — both are austenitic grades with similar work-hardening behavior, and both machine at roughly 45–50% the speed of free-machining steel. The real cost difference between the grades is raw material ($1–2/lb / $2–4/kg more for 316L), not machining time. Both are significantly harder to machine than aluminum or carbon steel. Here is why and what to do about it.
Work Hardening
Both 304 and 316 work-harden rapidly when the cutting tool dwells or rubs rather than cutting. This generates heat, accelerates tool wear, and can lock the tool into the cut. Solution: maintain aggressive feeds, use sharp carbide tooling, never let the cutter dwell in the material.
Tool Speed (SFM)
Run stainless at 150–300 SFM (46–91 m/min) for carbide tooling (vs. 600–900 SFM / 180–274 m/min for aluminum 6061). Higher SFM generates heat that softens the material ahead of the cutting edge — actually helping cut quality for stainless. Counter-intuitive but correct: do not machine stainless slow.
Coolant
Flood coolant is mandatory for stainless CNC machining. Insufficient cooling causes built-up edge (BUE) on the cutting tool, which worsens surface finish and accelerates tool failure. High-pressure through-spindle coolant significantly improves tool life on deep holes and difficult features.
Relative Cost vs. Aluminum
Expect stainless CNC parts to cost 2–3× more than equivalent aluminum parts. Higher raw material cost (~1.5×), lower material removal rate (~40–50% of aluminum), and faster tool wear all contribute. Design with stainless only where the corrosion resistance or biocompatibility is actually needed.
Need 304 or 316L CNC Parts with Free DFM Review?
MakerStage machines 304 and 316L stainless with 3-axis and 5-axis CNC capability. Every order includes a free DFM review — we'll flag features that are hard to machine in stainless (thin walls, deep pockets, small bores) before quoting, not after.
Get a Stainless CNC Quote with Free DFM ReviewApplication Matrix: 304 vs 316L
When in doubt, specify 316L — the $1–2/lb cost premium is small relative to the cost of replacing a corroded part in the field. Use this table to quickly identify the right grade for your use case.
| Application | Recommended | Reasoning |
|---|---|---|
| Food processing equipment (conveyor parts, bowls) | 316L | FDA 21 CFR compliance + chloride-based CIP cleaning. Low-carbon grade required for welded assemblies. |
| Laboratory instruments and fixtures | 304 or 316L | 304 for dry bench tools; 316L wherever chemical reagents, acids, or saline solutions are present. |
| Medical device components | 316L | Contact with bodily fluids (chloride-containing). ISO 10993 biocompatibility and USP Class VI compatibility. |
| Fluid system fittings (water, gas, oil) | 304 for clean water; 316L for saline, chemical, or corrosive media | Fluid chemistry determines grade. 304 adequate for potable water and most gas lines. |
| Architectural / structural (indoor) | 304 | No chloride exposure. 304 provides excellent aesthetic corrosion resistance at lower cost. |
| Outdoor structural (coastal) | 316L | Salt-laden air contains chloride ions sufficient to pit 304 over months to years. |
| High-strength fasteners and shafts | 17-4 PH H900 | 304 and 316L yield is only 30 ksi annealed — insufficient for high-load fasteners. 17-4 PH gives 170 ksi yield with stainless corrosion resistance. |
Decision Rule
Use 304 as your stainless default for indoor, low-chloride, and architectural applications. Switch to 316L the moment the word "chloride," "saline," or "marine" appears in your operating environment description. The cost delta between 304 and 316L is $1–2/lb — small relative to the cost of a field corrosion failure or product recall.
Further Reading
- Steel Grades for CNC Machining: 1018, 4140, 4340 & More — carbon and alloy steel grades when stainless is not needed.
- Aluminum vs Steel: How to Choose for Your Application — weight, stiffness, and cost decision framework.
- Material Selection Guide — broader framework covering metals, plastics, and composites.
- CNC Tolerances Guide — how stainless work hardening affects achievable tolerances.
Frequently Asked Questions
What is the main difference between 304 and 316 stainless steel?
Is 316L different from 316 stainless steel?
Can 304 stainless steel rust?
Why is 316 stainless harder to machine than 304?
What does PRE number mean for stainless steel?
Is 304 or 316 stainless steel magnetic?
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