Article
304 and 316 are the two stainless steels most buyers meet first, and the "L" versions of each are the most common in process piping. They look the same, they are welded the same way, and they are often priced close together. The difference is in what they resist, and choosing the wrong one tends to show up as corrosion a year or two after installation rather than on day one.
The short answer
- 304 / 304L for general corrosion resistance: water, food, dairy, architectural and most mildly corrosive process duties.
- 316 / 316L where chlorides are present: seawater and coastal atmospheres, brines, many chemicals, and pharmaceutical service.
- The "L" grade whenever the material will be welded and the joint will not be solution annealed afterwards, which is almost always.
What makes them different
| Grade | UNS | Chromium % | Nickel % | Molybdenum % | Carbon, max % |
|---|---|---|---|---|---|
| 304 | S30400 | 18.0 - 20.0 | 8.0 - 11.0 | none | 0.08 |
| 304L | S30403 | 18.0 - 20.0 | 8.0 - 12.0 | none | 0.035 |
| 316 | S31600 | 16.0 - 18.0 | 10.0 - 14.0 | 2.00 - 3.00 | 0.08 |
| 316L | S31603 | 16.0 - 18.0 | 10.0 - 14.0 | 2.00 - 3.00 | 0.035 |
Limits as given for pipe in ASTM A312. Plate and forgings have slightly different limits in their own standards.
Molybdenum
The 2 to 3% molybdenum in 316 is what makes it resist pitting and crevice corrosion by chlorides. Chloride pitting is the usual way 304 fails in coastal plants and in any water that is not very clean. If chlorides are part of the service, even occasionally, 316 is the safer specification.
Low carbon
When austenitic stainless steel is heated into roughly 450 to 850 °C, as the zone next to a weld is, chromium carbides can form at the grain boundaries. That locally robs the steel of chromium and leaves it open to intergranular corrosion, known as sensitisation. Keeping carbon at 0.03 to 0.035% maximum, as the L grades do, largely prevents it. For welded piping, 304L and 316L are therefore the normal choice.
Dual-certified material
Much stainless steel is now produced to meet both the standard and the L grade at once, and certified as 304/304L or 316/316L. It has the low carbon of the L grade and the strength of the straight grade. For most buyers this is the simplest thing to order, and it is what we normally supply unless a project specifies otherwise.
Where neither is enough
In hot chloride service, strong acids or high-chloride seawater, 316L can still pit or suffer stress corrosion cracking. That is the point at which duplex grades such as 2205, or higher alloys such as 904L and 6% molybdenum grades, are considered. They cost more, but less than a failed line.
What to put on the enquiry
The grade (including whether dual certification is acceptable), the product standard (ASTM A312 for pipe, A240 for plate, A182 for forged fittings and flanges, A403 for buttweld fittings), the size, and the service if you would like us to check the choice.
See SS 316/316L seamless pipe, SS 304/304L welded pipe and SS 316 plate.
Frequently asked questions
What is the main difference between 304 and 316?
316 contains 2–3% molybdenum, which greatly improves resistance to chlorides and many chemicals.
When should the L grade be used?
Whenever the material will be welded and not solution annealed afterwards, which is almost always in piping.
Is 316L weaker than 316?
Its minimum strength is slightly lower, but dual-certified 316/316L meets both requirements.
Key takeaways
- 304/304L for general service; 316/316L for chlorides.
- Choose L grades for welded construction.
- Dual-certified material covers both grades.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.