Stainless Steel Casting 304 vs 316: Which is Right for Your Application?
304 or 316? It's the most common material selection question in stainless steel procurement. The answer isn't simple — it depends on your environment, your fluid, and your consequences of failure. Here's the decision framework.
The Key Difference: Molybdenum
The critical addition in 316 is molybdenum (2–3%). This single element transforms corrosion resistance, especially in chloride-containing environments.
| Property | 304 (CF8) | 316 (CF8M) |
|---|---|---|
| Chromium (Cr) | 18–21% | 16–18% |
| Nickel (Ni) | 8–11% | 10–14% |
| Molybdenum (Mo) | 0% | 2–3% |
| Carbon (max) | 0.08% | 0.08% |
| PREN (pitting resistance) | ~18–22 | ~24–30 |
| Approx. cost ratio | 1.0× | 1.2–1.4× |
When 304 Is the Right Choice
- Clean water and drinking water systems
- Steam and condensate lines
- Food processing (non-chloride environments)
- Atmospheric corrosion (indoor, rural)
- nitric acid and most organic acids
- Pharmaceutical vessels (with high-purity water)
When 316 Is Essential
- Seawater: Any seawater contact — cooling, desalination, marine
- Chloride solutions: Even low concentrations (50–500 ppm) accelerate 304 pitting
- Coastal/offshor environments: Salt in air causes 304 to pit and stain
- Sulfuric acid: 316 handles up to ~10% at room temperature; 304 fails quickly
- Hot chlorides: Any chloride environment above 60°C requires 316 minimum
- Medical/pharmaceutical: Easier to electropolish to Ra <0.8 μm
- Dyed or brined food products: Chlorides in processing
PREN: The Pitting Resistance Number
PREN = %Cr + 3.3×%Mo + 16×%N
| Grade | PREN | Max Seawater Temp |
|---|---|---|
| 304 (CF8) | 18–22 | <20°C (limited) |
| 316 (CF8M) | 24–30 | <30°C acceptable |
| Duplex 2205 | 33–39 | <40°C |
| Super Duplex 2507 | 40+ | <50°C |
Temperature Limits
| Application | 304 (CF8) | 316 (CF8M) |
|---|---|---|
| Continuous service (oxidizing) | Up to 870°C | Up to 925°C |
| Continuous service (water/steam) | <800°C | <800°C |
| Carbide precipitation risk | Above 425°C (sensitization) | Above 425°C |
For parts that will be welded, specify low-carbon grades (CF3 for 304L equivalent, CF3M for 316L equivalent) to avoid sensitization — chromium carbide precipitation at grain boundaries that causes intergranular corrosion.
Frequently Asked Questions
What is the main difference between 304 and 316 stainless steel?
2–3% molybdenum in 316. This dramatically improves chloride pitting resistance. 304: PREN ~18–22. 316: PREN ~24–30. In seawater and chloride environments, 316 significantly outperforms 304.
When should I choose 316 over 304?
Choose 316 for seawater, chloride-containing environments, coastal/offshor, acidic fluids, or any application above 60°C with chloride present.
Is 316 worth the extra cost?
In corrosive environments, 316's lifespan is 5–10× longer than 304. In clean water/steam applications, 304 is sufficient and 20–30% cheaper.
What about 304L vs 316L (low carbon)?
Use low-carbon grades (CF3/CF3M) when the part will be welded after casting. Low carbon (<0.03%) avoids sensitization (grain boundary corrosion from chromium carbide precipitation).
Uncertain Which Grade to Specify?
Describe your application — fluid type, temperature, pressure, and environment. Our engineering team will recommend the right stainless steel grade and explain the tradeoffs.
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