316L stainless steel is the low-carbon form of grade 316 stainless steel, an alloy of iron with chromium, nickel and molybdenum. It resists corrosion well, including from salt water. Near the coast it is generally treated as the lowest acceptable grade.
What is 316L made of?
316L is mostly iron. Its main other ingredients are chromium at 16–18%, nickel at 10–12% and molybdenum at 2–3%. It has less carbon than standard 316, and the reduced carbon helps it resist corrosion better. The molybdenum makes it more resistant than the common 304 grade. One steel supplier states that 316/316L usually holds up better than 304 in many atmospheres and corrosive substances.
Why is it used for dive watches?
Dive watches spend time in salt water, and the molybdenum in 316L helps it stand up to that. The watch brand Aquastar says 316L is the usual case metal for nearly all serious Swiss dive watches. Made to the ASTM F138/F139 standards, 316L is also biocompatible, so it is used for medical implants and body piercings.
Not to be confused with
904L is a rival watch steel. It is a super-austenitic alloy, with more chromium, nickel and molybdenum than 316L. Standard 316 has more carbon than 316L, and that matters when parts are welded. Thick welded parts made from ordinary 316 have to be annealed after welding to get the best corrosion resistance. Lower carbon spares 316L from sensitization, so heavy welded parts in 316L need no annealing.
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