AISI 316L is the standard chloride-resistant grade of stainless steel. AISI 904L falls into an altogether different class: its alloy content is so high that even its UNS number sits not in the stainless steel group (S…) but in the nickel alloy group (N08904).
Nickel, molybdenum and copper
The difference on our datasheets is striking: nickel is 10.0–14.0% in 316L and rises to 23.00–28.00% in 904L — more than double. Molybdenum rises from 2.0–3.0% to 4.00–5.00%, chromium from 16.0–18.0% to 19.00–23.00%. On top of that, 1–2% copper, which 316L does not contain at all, is added.
The high nickel content makes the material resistant to stress corrosion cracking — the point at which 316L is weakest in hot chloride environments. Molybdenum provides resistance to pitting and crevice corrosion. Copper does something particular: it provides resistance to reducing acids, especially sulphuric and phosphoric acid. That copper addition is exactly why 904L is preferred in the chemical and fertiliser industries.
Carbon is also kept very low (max 0.02%), so 904L is by its nature already an “L” grade and is protected against intergranular corrosion after welding.
Comparison in numbers
| PROPERTY | AISI 316L | AISI 904L |
|---|---|---|
| Carbon (C) | max 0.03% | max 0.02% |
| Chromium (Cr) | 16.0 – 18.0% | 19.00 – 23.00% |
| Nickel (Ni) | 10.0 – 14.0% | 23.00 – 28.00% |
| Molybdenum (Mo) | 2.0 – 3.0% | 4.00 – 5.00% |
| Copper (Cu) | — | 1 – 2% |
| Tensile strength Rm | 485 MPa | 490 MPa |
| Yield strength Rp0.2 | 170 MPa | 220 MPa |
| Elongation A50 | 40% | 36% |
| Hardness | max 217 HB | max 150 HB |
| Density | 8.00 g/cm³ | 7.90 g/cm³ |
| Thermal conductivity | 16.3 W/m·K | 11.5 W/m·K |
| UNS | S31603 | N08904 |
| W.Nr (DIN/EN) | 1.4404 | 1.4539 |
| AMS | 5653 · 5507 | — |
| ASTM | A276 · A479 · A484 · A240 | A182 · A262 · A479 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
AISI 316L is sufficient — chloride environments other than seawater, food and pharmaceutical pipework, welded tanks and pipes, general process equipment. It costs far less and is easy to source in every form.
AISI 904L is required — sulphuric and phosphoric acid lines, fertiliser production, flue gas desulphurisation systems, seawater heat exchangers, oil and gas process equipment. It comes into play where 316L begins to pit or to crack under stress.
One point to keep in mind: the strength of 904L is not much higher than that of 316L. The reason for choosing 904L is corrosion resistance, not strength. If high strength and high corrosion resistance are wanted together, the duplex steels are where to look.
Alternatives
For much higher strength in the same corrosion class there are AISI 318 (duplex 2205) and AISI F53 (super duplex). For still more aggressive environments, the move is to nickel alloys such as Hastelloy C-276 and Incoloy 825.
Technical datasheets
AISI 316L — chemical composition, mechanical values, standard equivalents
AISI 904L — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.