What is in it, and why
904L is a super austenitic stainless steel: low carbon, high nickel, high molybdenum, with copper. Each element is doing a specific job, and the combination is what separates it from the 300 series.
| Element | Range | What it is doing |
|---|---|---|
| Carbon | 0.020% max | Very low, to resist intergranular attack after welding |
| Chromium | 19.0 – 23.0% | Forms the passive film |
| Nickel | 23.0 – 28.0% | Stabilises austenite and resists chloride stress corrosion cracking |
| Molybdenum | 4.0 – 5.0% | Pitting and crevice corrosion resistance |
| Copper | 1.0 – 2.0% | Resistance to sulphuric and phosphoric acid — the defining addition |
The high nickel content is also why 904L is expensive relative to 316, and why it sits close to the nickel alloys in price rather than close to the 300 series.
Where it is genuinely the right grade
- ▸Sulphuric acid plant — storage, handling and process lines across a wide concentration range.
- ▸Phosphoric acid production, including wet-process fertiliser plant.
- ▸Flue gas desulphurisation scrubbers, where acidic condensate and chlorides occur together.
- ▸Pulp and paper bleaching equipment.
- ▸Pickling lines and acid regeneration.
- ▸Seawater and brine handling where stress corrosion cracking is a concern.
904L against the alternatives
| Consideration | SS 316L | 904L | Duplex 2205 |
|---|---|---|---|
| Sulphuric acid | Poor | Strong | Moderate |
| Chloride pitting (PREN) | ≈ 25 | ≈ 35 | ≈ 35 |
| Chloride SCC | Susceptible | Highly resistant | Highly resistant |
| Yield strength | 170 MPa min | 220 MPa min | 450 MPa min |
| Magnetic after cold work | Can become slightly | No | Yes, ferromagnetic |
| Relative cost | Baseline | High | Moderate |
The practical rule: if the problem is acid, 904L. If the problem is chlorides and you would also welcome higher strength, Duplex 2205 — it costs less and its extra yield often permits a thinner section. If the problem is neither, 316L is sufficient.
Where 904L is the wrong answer
It is not a universal acid-resistant grade. Hydrochloric acid attacks it. Hydrofluoric acid attacks it — Monel is the usual selection there. Strongly oxidising chloride media such as hot ferric chloride will pit it. And where chloride levels or temperatures are high enough, a super duplex or nickel alloy is required instead.
It is also over-specified surprisingly often. Buyers who have had a 316 failure sometimes jump to 904L when the actual failure mechanism was chloride stress corrosion cracking, for which duplex would have been both sufficient and cheaper. Diagnose the mechanism before selecting the replacement.
