Last verified 19 August 2026.
The comparison
| Property | 904L | Alloy 28 | Evidence |
|---|---|---|---|
| Proof strength Rp0.2, minimum (EN) | 240 MPa | 250 MPa | E3 E3 |
| Tensile strength Rm, minimum | 530 MPa | 540 MPa | E3 E3 |
| Elongation A, minimum | 35 % | 40 % | E3 E3 |
| PREN | 34 — | 34 — | E3 E3 |
| Density | 8.0 g/cm³ | 8.0 g/cm³ | E3 E3 |
| Modulus of elasticity | 195 GPa | 195 GPa | E3 E3 |
| Thermal conductivity | 12 W/(m·K) | 14 W/(m·K) | E3 E3 |
Proof strength Rp0.2, minimum (EN) — different product forms — cold rolled coil and sheet against Quarto plate. The grades differ and so does what was tested, so the gap between these two numbers is not all material
Tensile strength Rm, minimum — different product forms — cold rolled coil and sheet against Quarto plate. The grades differ and so does what was tested, so the gap between these two numbers is not all material
Elongation A, minimum — different product forms — cold rolled coil and sheet against Quarto plate. The grades differ and so does what was tested, so the gap between these two numbers is not all material
Where each side’s numbers come from
See 904L and Alloy 28 — each page lists every source, every demotion and every gap. Nothing on this page is derived from anything not on those two, and our coverage limits are stated separately.
Where a PREN appears above, read it as a ranking number rather than a qualification: what PREN can and cannot tell you.
When to choose which
904L. Sulphuric and phosphoric acid service, and mixed acids with chlorides in them, where austenitic grades below it corrode generally rather than pit. The copper addition is specifically there for reducing acids. Pickling lines, acid plants, flue gas cleaning and some pharmaceutical process equipment are where it earns the price.
Alloy 28. Wet-process phosphoric acid plants above all — evaporators, heat exchangers, agitators — and hot sulphuric acid duty where the copper addition does the work. High chromium handles the oxidising part of the attack and copper the reducing part, which is the combination that acid process streams present.
Against 904L: Price, and the fact that a cheaper grade usually does the job. Where the problem is chlorides rather than acid, 2205 has a higher pitting ranking, twice the proof strength and a fraction of the alloy content; where the problem is severe chlorides, 254 SMO is the better answer. 904L gets specified out of habit more often than out of analysis.
Against Alloy 28: Strength, chlorides and price. Its proof strength minimum is low even for an austenitic, so a pressure part in it is thick. The pitting ranking of 34 is short of seawater territory. And it competes with nickel alloys at a price close enough that the decision usually turns on corrosion rate data, which this page does not have.
What this comparison does not cover
- No corrosion rate data, which is the only reason to choose this alloy over a duplex.
- Critical pitting and crevice temperatures are not established here.
- Elevated temperature strength, fatigue and fracture toughness.
- Composition limits, not established.
- Welding practice for high-alloy austenitics, including filler overmatching.
- No corrosion rate data, which is the only reason to choose this alloy.
- Only plate minima are established here.
- Whether the copper addition helps in your particular acid mixture. Composition alone does not answer it.
Generated from data/materials/904l.yaml and alloy28.yaml.