SUS304 vs SUS316: How to Choose Between the Two Most Common Stainless Grades

SUS304 and SUS316 are the two most specified austenitic stainless grades in Japan and globally. They look identical, machine identically, weld identically, and have the same tensile strength. The only compositional difference is 2–3% molybdenum in SUS316 — and that Mo addition raises the price 30–50% while raising the pitting resistance equivalent number (PREN) from ~20 to ~25. Whether that PREN delta justifies the cost premium is not a materials question: it is an environment question. This article gives a direct, quantified framework for making the right call.

1. What Molybdenum Actually Does

Pitting in stainless steel occurs when chloride ions (Cl⁻) break down the passive chromium oxide film at surface defects. Molybdenum raises corrosion resistance by two mechanisms:

1. Film repair: Mo enriches the passive film at incipient pit sites, slowing breakdown and promoting re-passivation
2. PREN increase: Each 1% Mo contributes 3.3 PREN units: PREN = %Cr + 3.3×%Mo + 16×%N. SUS316’s 2–3% Mo adds approximately 6.6–9.9 PREN points over SUS304

The practical result: SUS316’s critical pitting temperature (CPT) in 1M NaCl is approximately 20–25°C higher than SUS304’s. In ambient-temperature seawater (20°C), SUS316 stays passive; SUS304 pits.

2. Environment-by-Environment Decision

3. Mechanical Properties: No Useful Difference

4. Machinability and Weldability: No Practical Difference

Both grades machine at approximately 50% of AISI 1212 free-machining carbon steel. Both work-harden at similar rates under the cutting tool. SUS316’s slightly higher Ni content may cause marginally faster tool wear (~10–15% shorter tool life at identical cutting parameters) — negligible for most production operations.

Both grades weld without preheat, with the same filler metals (ER308L for 304; ER316L for 316), and with identical post-weld behavior. The sensitization risk is the same — specify the L-grade (SUS304L or SUS316L) for welded assemblies in corrosive service.

5. The Cost Decision: When Is the Premium Justified?

SUS316 costs approximately 30–50% more than SUS304 for equivalent product forms. The premium is justified when the chloride exposure is sufficient to cause pitting in SUS304 — and it is not justified when SUS304 would survive without maintenance.

6. The 500-Metre Rule and Other Field Guidelines

Several practical rules have emerged from field experience:

• Coastal distance: SUS316 is the standard specification for outdoor fixtures within ~500 m of the sea in temperate climates. In tropical coastal locations (higher temperature, higher humidity), extend this to ~1 km
• Chloride concentration: Above approximately 200 ppm Cl⁻ in solution at ambient temperature, specify SUS316 over SUS304
• Temperature effect: Corrosive damage accelerates with temperature — environments that are marginal for SUS304 at 20°C may require SUS316 at 40°C and duplex at 60°C
• Weld joints in chloride service: Always specify the L-grade (SUS316L) regardless of SUS304 vs SUS316 choice — sensitization risk is the same for both

7. Common Mistakes

8. FAQ

Q: Can I substitute SUS304 for SUS316 without changing dimensions?

Yes for mechanical purposes — same density, same strength, same elastic modulus. A direct dimensional substitution is valid. The question is whether SUS304’s lower PREN is adequate for the service environment. If it is, the substitution saves 30–50% on material cost. If it is not, the substitution will cause premature corrosion failure.

Q: Which grade should I specify as a default if I am unsure of the environment?

Specify SUS316L as the conservative default for any application involving potential moisture or chemical exposure. The cost premium is real but bounded (30–50%); the cost of specifying SUS304 in an environment that needs SUS316 includes replacement, downtime, and in regulated industries (food, pharma), potential product recalls.

Q: Is SUS316 suitable for seawater immersion?

Marginal — SUS316’s PREN ~25 is at the boundary for ambient-temperature seawater. It is adequate for splash zones and intermittent contact with regular cleaning. For continuous immersion, especially in warm water above 25°C (77°F), or for structural members under stress in seawater, specify duplex SUS329J3L (PREN 34–38).