304 vs 316 Stainless Steel: Which to Choose
Quick Answer
Use 304 (18-8, A2) for:
- General corrosion resistance
- Indoor/mild outdoor environments
- Food processing (non-marine)
- Cost-sensitive applications
- Most common applications
Use 316 (A4) for:
- Marine/saltwater environments
- Chemical exposure (chlorides, acids)
- Medical implants
- Swimming pool equipment
- Maximum corrosion resistance needed
Composition Comparison
| Element | 304 (18-8) | 316 | Effect |
|---|---|---|---|
| Chromium | 18-20% | 16-18% | Corrosion resistance |
| Nickel | 8-10.5% | 10-14% | Corrosion, workability |
| Molybdenum | 0% | 2-3% | Chloride resistance |
| Carbon | 0.08% max | 0.08% max | Strength |
| Iron | Balance | Balance | Base metal |
Key Difference: 316 contains 2-3% molybdenum, which dramatically improves resistance to chlorides and pitting corrosion.
Naming Conventions
Same Steel, Different Names
| Grade | AISI | European | Fastener Grade |
|---|---|---|---|
| 304 | 304 | 1.4301 | A2 (A2-50, A2-70, A2-80) |
| 316 | 316 | 1.4401 | A4 (A4-50, A4-70, A4-80) |
Common aliases:
- 304 = 18-8 = A2 = 1.4301
- 316 = A4 = 1.4401
Fastener Property Classes
| Class | Tensile Strength | Yield Strength | Material |
|---|---|---|---|
| A2-50 | 500 MPa | 210 MPa | 304 SS |
| A2-70 | 700 MPa | 450 MPa | 304 SS (cold worked) |
| A2-80 | 800 MPa | 600 MPa | 304 SS (heavily worked) |
| A4-50 | 500 MPa | 210 MPa | 316 SS |
| A4-70 | 700 MPa | 450 MPa | 316 SS (cold worked) |
| A4-80 | 800 MPa | 600 MPa | 316 SS (heavily worked) |
Corrosion Resistance Comparison
General Corrosion
| Environment | 304 | 316 | Winner |
|---|---|---|---|
| Indoor/dry | Excellent | Excellent | Tie |
| Mild outdoor | Very good | Excellent | 316 |
| Industrial | Good | Very good | 316 |
| Fresh water | Excellent | Excellent | Tie |
| Salt spray/marine | Fair | Excellent | 316 |
| Chlorinated water | Poor-Fair | Good | 316 |
| Acidic (mild) | Good | Very good | 316 |
| Alkaline | Excellent | Excellent | Tie |
Pitting & Crevice Corrosion
316's molybdenum content provides significantly better resistance to:
- Pitting — Localized holes from chloride attack
- Crevice corrosion — Attack in tight gaps
- Stress corrosion cracking — Under load in corrosive environment
Pitting Resistance Equivalent Number (PREN):
- 304: PREN ≈ 18
- 316: PREN ≈ 25
Higher PREN = better pitting resistance. 316 is ~40% more resistant.
Application Guide
Use 304 (A2) For:
| Application | Why 304 Works |
|---|---|
| Food equipment (non-marine) | FDA compliant, cleanable |
| Kitchen appliances | Sanitary, corrosion resistant |
| Architectural (inland) | Appearance, durability |
| Automotive trim | Good corrosion, cost effective |
| Indoor equipment | Excellent for dry environments |
| Water treatment (fresh) | Good fresh water resistance |
| General industrial | Adequate for most uses |
| Fasteners (general) | Most common SS fastener |
Use 316 (A4) For:
| Application | Why 316 Required |
|---|---|
| Marine hardware | Saltwater resistance |
| Boat/dock fasteners | Chloride exposure |
| Coastal structures | Salt air corrosion |
| Swimming pools | Chlorinated water |
| Chemical processing | Acid/chemical resistance |
| Medical devices | Biocompatibility, sterilization |
| Pharmaceutical | Purity, cleanability |
| Petrochemical | Chemical exposure |
| Pulp & paper | Chloride process chemicals |
| Desalination | Seawater exposure |
Cost Comparison
| Factor | 304 | 316 | Difference |
|---|---|---|---|
| Raw material | Baseline | +25-40% | Molybdenum cost |
| Fastener cost | 1× | 1.3-1.5× | Typical retail |
| Availability | Excellent | Good | 304 more common |
Cost consideration: For non-marine applications, 304 saves 25-40% with adequate performance.
Strength Comparison
Mechanical properties are similar:
| Property | 304 | 316 | Difference |
|---|---|---|---|
| Tensile (annealed) | 515 MPa | 515 MPa | Same |
| Yield (annealed) | 205 MPa | 205 MPa | Same |
| Hardness | 70 HRB | 79 HRB | 316 slightly harder |
| Elongation | 40% | 40% | Same |
Strength is NOT a reason to choose 316 over 304 — choose based on corrosion environment.
Magnetic Properties
| Grade | Magnetic? | Why |
|---|---|---|
| 304 | Slightly (when cold worked) | Austenite transforms |
| 316 | Less magnetic | More stable austenite |
Neither are strongly magnetic in annealed condition. Cold working can induce slight magnetism, more so in 304.
Heat Resistance
| Property | 304 | 316 |
|---|---|---|
| Max service temp | 870°C (1600°F) | 870°C (1600°F) |
| Scaling resistance | Excellent | Excellent |
| Continuous use | 925°C (1700°F) | 925°C (1700°F) |
Heat resistance is similar — not a differentiating factor.
Weldability & Galling
Welding
Both 304 and 316 are readily weldable. For critical corrosion applications, use:
- 304L or 316L (low carbon) to prevent sensitization
- Matching filler metals (308L for 304, 316L for 316)
Galling (Thread Seizure)
Both grades are prone to galling. 316 may be slightly worse due to higher nickel content.
Prevention for both:
- Use anti-seize compound
- Slow assembly speed
- Different materials (SS bolt + bronze nut)
- Proper thread fit
Chemical Resistance Comparison
| Chemical | 304 | 316 | Notes |
|---|---|---|---|
| Nitric acid | Excellent | Excellent | Both good |
| Phosphoric acid | Good | Very good | 316 better |
| Sulfuric acid | Poor | Fair | Neither great |
| Hydrochloric acid | Poor | Poor | Use Hastelloy |
| Sodium hydroxide | Excellent | Excellent | Both good |
| Chlorine solutions | Poor | Good | 316 required |
| Seawater | Fair | Good | 316 preferred |
| Ammonia | Good | Good | Similar |
| Organic acids | Good | Very good | 316 slight edge |
How to Identify 304 vs 316
Visual Identification
- Not reliably distinguishable by appearance
- Both appear similar silver color
Marking
- Fasteners should be marked: "A2" or "A4"
- Or "304" / "316"
- Check head or packaging
Testing Methods
- XRF analyzer (detects molybdenum)
- Chemical spot test for molybdenum
- Magnetic test (inconclusive)
Decision Flowchart
Common Mistakes
| Mistake | Consequence |
|---|---|
| Using 304 in marine | Pitting, failure within months |
| Using 304 with chlorine | Stress corrosion cracking |
| Assuming "stainless" = corrosion-proof | All SS can corrode |
| Mixing 304 fasteners with 316 structure | Fasteners may corrode first |
| Not specifying grade | May receive wrong material |
FAQ
Q: Is 316 worth the extra cost?
A: In marine/chloride environments, absolutely — 304 will fail. For general indoor use, 304 is fine and saves money.
Q: Can I use 304 near the ocean?
A: Coastal (salt air) applications should use 316. Inland, 304 is fine. "Near ocean" depends on how near — within 1 mile, use 316.
Q: Is 18-8 the same as 304?
A: Yes. "18-8" refers to the ~18% chromium, ~8% nickel composition of 304 stainless.
Q: Why did my stainless fastener rust?
A: Possible causes: wrong grade for environment, contamination from carbon steel, crevice corrosion, or chloride exposure on 304.
Q: Can I mix 304 and 316 in the same assembly?
A: Yes, they're galvanically compatible. Use 316 where corrosion is critical (fasteners often corrode first due to small size).
Q: Is 316L better than 316?
A: 316L is low-carbon 316, better for welded applications. For fasteners, standard 316 is typically used.
Choose 316 for marine and chemical environments, 304 for general corrosion resistance. When in doubt, 316 is the safer choice.