304 vs 316 Stainless Steel
Type 304 and Type 316 are austenitic stainless steels and the two most widely used grades in the piping industry. Both offer excellent corrosion resistance, formability, and weldability. The defining difference is molybdenum: Type 316 contains 2-3% Mo, which significantly improves resistance to pitting and crevice corrosion in chloride-containing environments.
Definition and Chemistry
Type 304 (UNS S30400) is the standard 18/8 austenitic stainless steel (18% chromium, 8% nickel). It resists corrosion in most atmospheric and mildly corrosive environments and is the baseline grade for food processing, chemical, and general piping applications.
Type 316 (UNS S31600) adds molybdenum to the 304 composition (16% chromium, 10% nickel, 2% molybdenum). The molybdenum provides substantially better resistance to pitting, crevice corrosion, and chloride stress corrosion cracking.
When Each Grade Is Used
Type 304 is appropriate for freshwater systems, steam services, dilute organic acids, caustic solutions, food and beverage processing, and architectural applications where chloride exposure is minimal.
Type 316 is required for seawater and brackish water systems, offshore platforms, chemical plants handling halide-containing media, pharmaceutical and biotech facilities, and any service where chloride concentrations exceed approximately 200 ppm.
Specifications and Properties
| Property | Type 304 | Type 316 |
|---|---|---|
| UNS | S30400 | S31600 |
| Chromium | 18.0-20.0% | 16.0-18.0% |
| Nickel | 8.0-10.5% | 10.0-14.0% |
| Molybdenum | None | 2.0-3.0% |
| Carbon (max) | 0.08% | 0.08% |
| PREN | ~18-20 | ~24-28 |
| Tensile strength (min) | 515 MPa (75 ksi) | 515 MPa (75 ksi) |
| Yield strength (min) | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Pipe spec (seamless) | ASTM A312 TP304 | ASTM A312 TP316 |
| Pipe spec (welded) | ASTM A312 TP304 | ASTM A312 TP316 |
| Forging spec | ASTM A182 F304 | ASTM A182 F316 |
| Casting equivalent | CF8 | CF8M |
| Max service temp | 870°C (1600°F) intermittent | 870°C (1600°F) intermittent |
| Magnetic | No (slightly after cold work) | No (slightly after cold work) |
| Relative cost | Baseline | 15-25% premium |
Comparison: Corrosion Performance
The Pitting Resistance Equivalent Number (PREN) is the standard metric for comparing localized corrosion resistance:
PREN = %Cr + 3.3 x %Mo + 16 x %N
Type 304 achieves a PREN of approximately 19, while Type 316 reaches approximately 26. The 3.3x multiplier on molybdenum makes a decisive difference. In chloride environments above 50-60°C, Type 304 is vulnerable to pitting and crevice attack, whereas Type 316 provides reliable service.
For environments exceeding Type 316’s limits, duplex stainless steels (PREN ~35 for 2205, ~42 for super duplex 2507) are the next step up.
Low-Carbon and High-Carbon Variants
Both grades are available in low-carbon (“L”) and high-carbon (“H”) variants:
- 304L / 316L (0.03% C max): Resist sensitization during welding; standard choice for welded piping
- 304H / 316H (0.04-0.10% C): Higher creep strength for continuous service above 500°C
Most piping projects specify the “L” grades by default to ensure weldability without post-weld heat treatment requirements.
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