Material selection
Four failure patterns, four different upgrades — and a cost difference between them of up to ten times.
316L is the workhorse of corrosion-resistant steel and it covers most of the chemical industry. When it fails, the failure almost always falls into one of four patterns. Each pattern points to a different upgrade, and getting the diagnosis right is what keeps the cost difference between 1× and 10×.
| What you see | Mechanism | Where to go next |
|---|---|---|
| Isolated deep pits, surface otherwise bright | Chloride pitting | Super duplex → C-276 |
| Branching cracks, no loss of thickness | Chloride stress corrosion cracking | Duplex, titanium, 825 |
| Even thinning across the whole wetted area | General acid attack | Alloy 20 → C-276 / B-3 |
| Distortion, sagging, scaling above 550 °C | Creep and oxidation | Inconel 601 / 617, 800H |
The usual index is PREN (pitting resistance equivalent number), calculated as %Cr + 3.3 × %Mo + 16 × %N. It is a comparative figure, not a design value, but it ranks alloys in the order experience confirms.
| Grade | Typical PREN |
|---|---|
| 316L | 24 – 26 |
| 904L | 34 – 36 |
| 2205 duplex | 34 – 36 |
| 2507 / Zeron 100 super duplex | 41 – 43 |
| Hastelloy C-276 | > 65 |
Note that temperature shifts the whole picture. An alloy that holds in 3.5% chloride at ambient temperature can pit in the same solution at 80 °C. Always state the service temperature with the chloride concentration.
This is the failure mode that surprises people, because the part looks sound until it splits. Austenitic stainless steels are vulnerable to chloride SCC from roughly 60 °C upward under tensile stress. Raising the molybdenum content does not help much here; the answer is a different microstructure or a different base metal.
Two families of acid behave in opposite ways, and this is where most wrong material selections happen.
Oxidising acids (nitric, chromic) are handled by chromium. Here 304L can outperform 316L, because molybdenum is of no benefit and can be detrimental.
Reducing acids (hydrochloric, sulphuric, phosphoric, hydrofluoric) are handled by molybdenum and nickel. This is where the Hastelloy family was developed, and where stainless steel runs out quickly.
Above about 550 °C, 316L loses load-bearing capability and the question changes from corrosion to creep and oxidation. Austenitic heat-resistant steels (1.4828, 1.4841) carry you to roughly 1000 °C in oxidising atmospheres. Beyond that, or where cyclic service and sulphur are involved, the nickel alloys take over.
Rough price multipliers against 316L plate, for order-of-magnitude planning only. Actual quotations depend on form, thickness, quantity and market conditions.
| Material | Approximate multiplier |
|---|---|
| 316L | 1× |
| 2205 duplex | 1.1 – 1.4× |
| 904L | 1.8 – 2.2× |
| Super duplex (2507 / Zeron 100) | 1.8 – 2.5× |
| Titanium Grade 2 | 2.5 – 4× |
| Alloy 20 | 2.5 – 3.5× |
| Incoloy 825 | 3 – 4× |
| Inconel 625 | 5 – 7× |
| Hastelloy C-276 | 6 – 10× |
Two things worth remembering. First, duplex is often cheaper per unit of strength than 316L, because its higher yield strength lets you thin the wall — the material costs more per kilo but the vessel can weigh less. Second, titanium's density is 4.51 g/cm³ against 8.0 for stainless, so a titanium plate of the same dimensions weighs 44% less. Comparing price per kilo alone overstates the gap.
It depends on the failure mode. For pitting in a chloride environment both help and they sit at a similar PREN level, but super duplex has roughly twice the yield strength, so it allows thinner sections. For stress corrosion cracking, duplex is clearly the better choice because its ferrite phase interrupts crack propagation. For general acid attack, 904L with its higher nickel content is usually preferable.
It is technically the safest answer for most aggressive media, but at six to ten times the cost of 316L it is rarely the right commercial answer. In hot nitric acid it is also not the best performer, because C-276 is optimised for reducing rather than oxidising conditions. Diagnose the mechanism before specifying it.
Dissimilar welds are possible but need care. Filler is normally selected to the higher alloy, galvanic coupling must be considered when both are wetted, and the heat-affected zone of the lower alloy often becomes the weakest point. For a repair it is workable; for new build, a single material is usually sounder.
For corrosion service the practical limit is set by the medium, not the steel. Mechanically, allowable stresses fall away above roughly 550 °C and scaling becomes significant above about 870 °C in air. Chloride stress corrosion cracking can begin as low as 60 °C, which is well below any of those figures.
Send us the operating temperature, the medium and the pressure. We will come back with a material recommendation and the cost difference between the two options.
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