What Is 316L Stainless Steel?

316L stainless steel is grade 316 with its carbon content cut to 0.03% or less; the L stands for low carbon. Everything else is essentially the same: an austenitic alloy of iron with around 16–18% chromium, 10–14% nickel and 2–3% molybdenum. The chromium forms the thin passive film that stops stainless steel rusting, the nickel keeps the structure austenitic, tough and formable, and the molybdenum makes that film far harder for chlorides to break down.

Low carbon matters when the steel is heated. Between roughly 450 and 850 °C, the range a weld’s heat-affected zone passes through, carbon in ordinary 316 can combine with chromium to form chromium carbides along the grain boundaries. The metal next to those carbides loses chromium and with it some corrosion resistance, a problem called sensitisation that can lead to intergranular corrosion. With carbon held at 0.03%, there is too little to form many carbides, so welded 316L joints keep their corrosion resistance without heat treatment afterwards.

In the UNS system 316L is S31603. The European equivalent is EN 1.4404, also written X2CrNiMo17-12-2, and 1.4435 is a related grade with slightly more nickel and molybdenum, often specified for pharmaceutical equipment.

The two grades share their chromium, nickel and molybdenum ranges, so the differences are carbon, strength and how they behave when welded. The figures below follow ASTM A240 for plate, sheet and strip.

Away from welds, the two resist corrosion almost identically. Much of the material sold today is dual-certified 316/316L, meaning its carbon is low enough for 316L while its strength still meets the 316 minimums, so one stock covers both specifications.

316L handles chlorides much better than 304, which is why it is chosen where salt, brine or chlorinated sanitizers are present. It is not immune, though. Warm, stagnant chloride solutions can still cause pitting and crevice corrosion, and above roughly 60 °C chlorides can cause stress corrosion cracking in stressed parts. Our explainer on food grade stainless steel covers what the grade alone does and does not guarantee.

Finish counts as much as grade. Fabrication can leave iron particles and heat tint on the surface, so parts are usually passivated in nitric or citric acid to restore a clean chromium-rich film. Pharmaceutical and food equipment is often electropolished as well, which smooths the surface, makes it easier to clean and leaves fewer places for deposits to start corrosion.

316L is one of the easiest stainless grades to weld, which is a large part of its popularity. Welders normally use a matching low-carbon filler, such as ER316L, so the weld metal keeps the same carbon limit as the parent plate. Pipework is purged inside with argon while it is welded, because oxygen on the inner surface leaves a rough, oxidised layer that corrodes and traps product. Pharmaceutical and dairy lines are often joined by orbital welding, which gives consistent, smooth beads that can be inspected with a borescope.

After welding, the heat tint is removed by pickling, and the part is passivated. Workshops also keep stainless separate from carbon steel tools and grinding discs, because embedded iron particles rust on the surface and look like the stainless itself is corroding.

Its weldability and chloride resistance make 316L the default stainless grade in several industries.

Many stainless steel computer housings are 304, which suits dry and lightly cleaned areas. 316L is the step up where chlorinated sanitizers are used every shift, or where salt air reaches the equipment, and its weldability suits the fully welded, crevice-free housings that hygienic design calls for. Our guide to hygienic design for washdown computers explains how grade, finish and sealing work together, and our comparison of 304 vs 316 stainless steel covers the wider choice between the two families. Several of our stainless steel panel PCs offer SUS316L as an option to the standard SUS304 housing.

We supply stainless steel panel PCs and monitors as part of a complete system, with the housing grade, sealing and mounting matched to the cleaning regime as standard.