316 vs 316L in Marine Castings
The difference is carbon, and it only shows up after welding. What to specify for salt water, where both grades quietly fail, and the test worth asking for.
10/18/20264 min read
A deck fitting came back after fourteen months with a line of rust along the weld. The drawing said 316. The mill certificate said 316. The part had not been abused, and it was not a bad casting — it was a specification that looked adequate on paper and was not.
This is the conversation we have most often about marine hardware, and it almost always starts with the same two questions: is 316L better, and will it actually stop the rust.
The difference is carbon
316 allows up to 0.08% carbon. 316L allows 0.03%. That is the entire chemical difference, and in the cast equivalents it becomes CF-8M versus CF-3M.
Everything else — chromium, nickel, molybdenum — is essentially the same, which is why the two grades behave identically until something heats them.
Why heat changes the answer
Between roughly 450 °C and 850 °C, the carbon in 316 combines with chromium at the grain boundaries to form chromium carbides. The steel next to those carbides is now short of chromium. Chromium is what makes stainless stainless, so the grain boundaries become the weakest point in the material.
This is sensitization. It happens in the heat-affected zone of a weld, and it does not announce itself. The part looks correct, passes a visual check, and starts pitting along a line three millimetres from the weld a year later.
316L's lower carbon leaves nothing to form carbides with, so it has margin. It is not immune — hold it in that range long enough and it will still sensitize — but for normal welding of marine hardware it is the difference between failing and not failing.
Cast parts behave the same way, with one addition: castings have thicker sections than sheet, so the heat-affected zone is bigger and cools more slowly. A welded cast 316 fitting is a worse case than a welded 316 sheet bracket.
What 316L costs you
Slightly less strength. Tensile strength around 485 MPa against roughly 515 MPa for 316, and a bit lower yield. For a cleat, a hinge, a deck fitting or a pump body, that difference rarely matters. For a highly stressed structural part it might — but at that point you should be looking at duplex, not at the L grade.
Tell your designer the part will be welded. That single sentence usually settles which grade to specify.
The part nobody wants to hear
Neither 316 nor 316L is a good choice for continuous immersion in seawater.
Both rely on a passive chromium oxide layer to resist corrosion, and chlorides attack that layer in crevices and under deposits, where oxygen cannot reach. Stagnant seawater in a gap between a bolt and a casting is the classic failure. The molybdenum in 316 raises the pitting resistance over 304, but not enough for immersed service.
A rough ranking by pitting resistance equivalent number (PREN, using Cr + 3.3 Mo + 16 N):
304 — PREN: ~18; Where it survives: Fresh water, atmospheric, no chlorides
316 / 316L — PREN: ~24–26; Where it survives: Splash zone, salt spray, washed surfaces
2205 duplex — PREN: ~35; Where it survives: Immersed, moving seawater
Super duplex 2507 — PREN: ~42; Where it survives: Immersed, high chloride, higher temperature
So: for a deck fitting that gets rained on and hosed down, 316L is the right answer and will last. For a fitting below the waterline or in a stagnant pocket, 316L is the wrong family of material and no amount of specification on the drawing will fix it.
What to write on the drawing
Grade and condition. "CF-3M, solution annealed" for welded cast parts. Not just "316".
Passivation after machining. Machining leaves free iron on the surface, and free iron rusts. Passivation removes it. This is cheap and it is the most commonly skipped step.
No carbon steel contact. Wire brushing with a carbon steel brush will contaminate a stainless casting permanently. It happens in every shop at least once.
A material verification step. Mill certificates are paperwork. Positive material identification (PMI) on the finished part tells you what the metal actually is. We have seen 316 stock delivered against a 316L certificate — it is not common, and it is not rare either. If the application is safety-relevant, put PMI in the purchase order.
Why this matters for castings specifically
A casting is often the part that carries the weld — a boss, a flange, a mounting pad. That puts the casting's heat-affected zone exactly where the corrosion will start.
If the part is welded after casting, specify CF-3M and say so on the drawing. If it is not welded, CF-8M is fine and slightly stronger.
The failure we opened with was a CF-8M part welded to a CF-8M part, with no passivation, in a splash zone. Three small decisions, any one of which would have prevented it.
Related reading
Why a 304 part can rust even when the grade is correct is covered in why 304 stainless steel rusts after machining — the causes are the same family, contamination and sensitization.
For food-contact rather than salt-water selection, copper vs 304 vs 316 walks through the same grades against a different set of attacks.
If you are specifying a marine casting and are unsure which grade to put down, send the drawing and describe where the part sits on the boat. Splash zone and immersed are different specifications, and the difference shows up about a year after launch.
Questions we get on this
Can I just use 316L for everything to be safe? For welded marine hardware, mostly yes. It costs slightly more and gives up a little strength, but it removes the sensitization risk. For immersed parts it does not help — that needs duplex.
Will passivation fix a part that is already rusting? If the rust is free iron contamination on the surface, yes. If it is pitting from chlorides or sensitization at a weld, no — the damage is in the metal, not on it. Passivation is prevention, not repair.Related reading
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