316L vs 316 Stainless, 304L vs 304L Stainless

We build custom sinks in domestic 316L surgical-grade stainless. The L is the part that matters once the corners are welded.

316 and 316L are the same alloy family. Both carry molybdenum, which is why they resist chlorides better than 304. The difference is carbon. Straight 316 allows up to 0.08% carbon. 316L is held to 0.03% max.

When you weld a sink corner, the heat-affected zone sits in the sensitization range, roughly 800 to 1500°F, long enough for carbon to pull chromium out of solution and form chromium carbides on the grain boundaries. The steel next to those boundaries is then short of chromium. That is weld decay: intergranular corrosion that starts at the corner, the one place a sink sees standing water, cleaner, and salt.

Straight 316 only stays sound if you solution-anneal the whole sink after welding. That means heating it to about 1900 to 2050°F (1040 to 1120°C) and quenching so the carbides dissolve and chromium goes back into solution. You will not do that on a finished corner. The bowl warps, the finish is gone, and a custom sink is not a pressure vessel you can throw back in a furnace.

316L does not form those carbides in normal weld cycles, so the corner does not need a post-weld anneal. We still use 316L filler, we still remove heat tint, and we still passivate. Low carbon is not a license to leave scale on the weld.

Strength is slightly lower. Typical annealed yield is about 25 ksi for 316L versus about 30 ksi for 316. On a 16-gauge sink that difference is irrelevant next to corrosion at the corner.

Millennia is the exception, and it is an honest one. Those sinks are 304L, not 316L. The mill that rolls our 316L had small roller defects that only showed under LED light. We flagged it years ago. Replacing the rollers was too expensive for them, so Millennia moved to 304L. It is still low-carbon, still well above commodity 304, and it fits that patented form.

Most sinks on the market are 304, sometimes called stainless with no grade at all. 304 has no molybdenum. Chlorine cleaners, coastal air, and acidic food mark it. 316 is the marine step up. 316L is the grade we weld, because the corner has to stay sound without a furnace cycle we will never run.

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