Overview
AISI 904L (UNS N08904) is a super-austenitic stainless steel with significantly higher nickel, molybdenum, and copper content compared to standard AISI 316L (UNS S31603). This enhanced alloy composition provides superior resistance to reducing acids like sulfuric and phosphoric acid, making 904L the go-to material for demanding chemical processing environments. While 316L offers excellent general corrosion resistance at an economical price point, 904L extends the performance envelope into concentrated acid service where 316L would fail rapidly.
Chemical Composition Comparison
| Element | AISI 316L | AISI 904L |
|---|---|---|
| Carbon (C) | ≤ 0.03% | ≤ 0.02% |
| Chromium (Cr) | 16.0–18.0% | 19.0–23.0% |
| Nickel (Ni) | 10.0–14.0% | 23.0–28.0% |
| Molybdenum (Mo) | 2.0–3.0% | 4.0–5.0% |
| Copper (Cu) | — | 1.0–2.0% |
| Manganese (Mn) | ≤ 2.0% | ≤ 2.0% |
PREN and Chloride Resistance
The PREN (Pitting Resistance Equivalent Number) dramatically illustrates the gap between these grades:
- AISI 316L: PREN ≈ 22–25 (Cr~17, Mo~2.5)
- AISI 904L: PREN ≈ 34–37 (Cr~20, Mo~4.5)
With a PREN well above 32, AISI 904L offers exceptional resistance to pitting and crevice corrosion in chloride-containing media, significantly outperforming 316L in seawater, brine, and chemical process streams with halide contamination.
Sulfuric Acid Resistance
AISI 904L demonstrates markedly superior performance in sulfuric acid (H₂SO₄) across a broad concentration range:
- 0–5% H₂SO₄: Both grades perform adequately at ambient temperatures
- 5–50% H₂SO₄: 904L shows corrosion rate < 0.1 mm/year, while 316L exceeds 0.5 mm/year at elevated temperatures
- 50–80% H₂SO₄: 904L maintains acceptable rates at moderate temperatures; 316L is unsuitable
The copper addition (1–2%) in 904L is critical for improving resistance to sulfuric acid by promoting cathodic reaction polarization and enhancing passive film stability in reducing acid environments.
Phosphoric Acid & Chemical Processing
In phosphoric acid (H₃PO₄) production — including wet-process acid containing fluorides and chlorides — 904L provides reliable long-term service in heat exchangers, reactors, and storage tanks. 316L may suffer accelerated corrosion in wet-process phosphoric acid with fluoride contamination. For the chemical industry, 904L is widely specified for:
- Acid coolers and condensers
- Pickling tank heaters
- Gas scrubber internals
- Piping for mixed acid streams
International Standards
| Standard | 316L | 904L |
|---|---|---|
| UNS | S31603 | N08904 |
| EN/DIN | 1.4404 (X2CrNiMo17-12-2) | 1.4539 (X1NiCrMoCu25-20-5) |
| GB (China) | 022Cr17Ni12Mo2 | 015Cr21Ni26Mo5Cu2 |
| GOST (Russia) | 03Cr17Ni14Mo3 | 06CrNi28MDT |
Cost Analysis
AISI 904L is typically 2–3 times more expensive than 316L per kilogram, driven by its high nickel (23–28%) and molybdenum (4–5%) content plus copper addition. However, the total lifecycle cost analysis often favors 904L in acid service:
- Service life of 904L can be 5–10 times longer in aggressive acid environments
- Reduced downtime and replacement costs
- Thinner wall designs possible due to lower corrosion allowance requirements
Selection Guide
- General chemical processing, moderate chlorides? → 316L (cost-effective)
- Sulfuric acid service, any concentration? → 904L
- Wet-process phosphoric acid with fluorides? → 904L
- Seawater or high-chloride brines? → 904L (PREN > 32)
- Budget-limited, short service cycles? → Evaluate 316L with inspection program
Fabrication and Welding Comparison
Both grades are austenitic and readily welded, but 904L's higher alloy content changes the approach to heat input and filler selection.
| Aspect | AISI 316L | AISI 904L (N08904) |
|---|---|---|
| Common filler | ER316L | ER385 or a 625-type filler |
| Heat input | Standard | Keep ≤ 1.5 kJ/mm |
| Interpass | ≤ 150 °C | ≤ 100 °C |
| Preheat / PWHT | None | None; solution anneal 1100 °C if sensitised |
| Machining | Standard austenitic practice | Lower speeds, sharper tooling |
| Hot cracking risk | Low | Moderate — keep the weld pool clean |
For severely corrosive joints, over-alloying the weld metal with a 625-type filler often costs little and removes the weld as the weak link. Comparison with nickel alloys is covered in our nickel alloy versus stainless steel guide.
Frequently Asked Questions
Is 904L worth the premium over 316L?
Where the duty involves sulphuric or phosphoric acid, or warm chloride-bearing water, yes — 904L typically lasts several times longer. In mild atmospheric or fresh-water service, 316L remains the economical choice.
Can 904L be used in seawater?
For ambient-temperature, flowing seawater, yes. It is not recommended for warm, stagnant or heavily creviced seawater service, where a super-duplex or 6Mo grade performs better.
Does 904L resist stress corrosion cracking?
Better than 316L, but it is not immune. Above about 60 °C in chloride-bearing service, a nickel-base alloy or super-duplex should be specified — see our chloride SCC guide.