Article
316L is the go-to stainless for chloride and chemical service. But some fluids are too aggressive for it. Before jumping to expensive nickel alloys, engineers often move one step up to 317L.
What makes 317L different
317L has more chromium, nickel and especially molybdenum than 316L. Molybdenum is the key element for resisting pitting and crevice corrosion in chloride environments and improves resistance to sulphuric, acetic and other acids.
| Grade | Cr % | Ni % | Mo % | Approx. PREN |
|---|---|---|---|---|
| 316L | 16–18 | 10–14 | 2–3 | 24 |
| 317L | 18–20 | 11–15 | 3–4 | 29 |
| 904L | 19–23 | 23–28 | 4–5 | 35 |
PREN (pitting resistance equivalent number) = %Cr + 3.3 × %Mo + 16 × %N. Higher numbers indicate better resistance to pitting.
Where 317L is used
- Flue gas desulphurisation (FGD) scrubbers in power plants
- Pulp and paper bleach plants
- Chemical and petrochemical process lines
- Textile dyeing and food processing with aggressive brines
317L vs 904L
317L is a cost-effective upgrade from 316L. Where conditions are more severe, such as hot sulphuric acid or high chloride concentrations, 904L or duplex grades may be required. A corrosion engineer or the process licensor usually makes the final choice.
Fabrication
317L welds much like 316L. Use a matching or over-alloyed filler (ER317L) so the weld metal does not become the weak point, and remove heat tint by pickling.
Ordering
Pipe 4" SCH 10S, ASTM A312 TP317L, welded, pickled and passivated, EN 10204 3.1
Frequently asked questions
Is 317L better than 316L?
For pitting and many acids, yes. It has about 1% more molybdenum and higher chromium and nickel.
What is the PREN of 317L?
Roughly 29, compared with about 24 for 316L, depending on actual composition.
Where is 317L commonly used?
In FGD systems, pulp and paper, and chemical processing where 316L is marginal.
Key takeaways
- 317L adds molybdenum for better pitting and acid resistance.
- A cost-effective step up from 316L before 904L or duplex.
- Weld with matching or over-alloyed filler and pickle afterwards.
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