AISI 304 vs AISI 316L: which stainless to choose
Chemical composition, corrosion resistance and price — a practical comparison of the two most common stainless steel grades for food equipment.
AISI 304 and AISI 316L are the two most common stainless steel grades in food machinery. They are visually indistinguishable, but they behave differently in aggressive environments. Let’s break down where saving on the grade is justified and where it ends in corrosion.
How the chemical composition differs
Both steels are austenitic chromium-nickel. The austenitic structure makes them non-magnetic, ductile and readily weldable — which is exactly why these grades dominate in food machinery. The key difference is molybdenum. AISI 316L has 2–3% molybdenum added, which sharply increases resistance to pitting corrosion in chloride-containing environments. The letter “L” means low carbon content — this reduces the risk of intergranular corrosion after welding.
| Parameter | AISI 304 | AISI 316L |
|---|---|---|
| Chromium | 18–20% | 16–18% |
| Nickel | 8–10.5% | 10–14% |
| Molybdenum | — | 2–3% |
| Carbon | up to 0.08% | up to 0.03% |
| Chloride resistance | moderate | high |
| Relative price | base | +25…40% |
Corrosion resistance in practice
AISI 304 holds up excellently in water, in air, in contact with most food products. But chlorides — salt, brines, chlorine-based disinfectants — gradually break through its passive film, forming pitting ulcers. Thanks to molybdenum, AISI 316L withstands such environments much longer.
The corrosion resistance of both steels rests on a passive film — a thin layer of chromium oxide that self-repairs in air. As long as the film is intact, the metal does not rust. Chloride ions are its main enemy: they locally destroy the film, and pitting starts at that point. The molybdenum in 316L slows this process several times over. So the difference between the grades shows not immediately but after months of operation: on 304 in a salty environment, fine brown spots appear first, then through-pits, especially on welds and in stagnation zones.
Why welds are the most vulnerable spot
A weld is a zone where the metal was heated and cooled, so its corrosion resistance is always lower than that of the base metal. When welding steel with a normal carbon content, near the weld the carbon binds chromium into carbides, and the area loses its protection — this is intergranular corrosion. That is exactly why food machinery uses grades with the letter “L” (low carbon): the reduced carbon prevents carbide formation. After welding, the welds are always pickled and passivated — a chemical treatment that restores the passive film. A skipped passivation is the most common reason that expensive 316L equipment rusts along the welds within a year.
Temperature should also be considered separately. Both grades work well both at the sub-zero temperatures of freezing chambers and when heated to several hundred degrees — unlike polymers, stainless steel does not stiffen in the cold or melt in an oven. This makes it a universal material for equipment that passes both a freezing zone and a heat-treatment zone on the same line.
Where AISI 304 is enough
For most food tasks AISI 304 is the optimal balance of price and quality:
- conveyors and transporters in dry and wet workshops;
- technological tables, racks, carts;
- bunkers for grain, flour, sugar;
- vegetable and fruit washing lines with fresh water.
Where AISI 316L is needed
316L is justified where there is constant contact with chlorides or acids:
- fish and seafood processing, contact with salt water;
- salting lines, marinades, brines;
- lactic acid and washing with aggressive disinfectants;
- seaweed and products with high salt content.
Engineer’s tip. Don’t overpay for 316L “just in case”. If the environment is chloride-free — AISI 304 will last just as long. But in fish production, saving on the grade ends in through-corrosion of welds within 2–3 years.
Care and surface cleanliness
No stainless grade is “eternal” on its own — its resistance depends on correct operation. The most common mistake is contact with carbon steel: sparks from an angle grinder, swarf, an ordinary tool leave iron particles on the surface that rust and “infect” the stainless steel with contact corrosion. So tools for working with stainless steel are kept separate. Chlorine-based disinfectants are not left on the surface for long — after treatment they are rinsed off. Scratches and matte spots are best avoided: on a smooth polished surface the passive film recovers faster. With proper care, even budget 304 lasts for decades.
Conclusion
AISI 304 covers 80% of food production tasks; AISI 316L is needed where chlorides and acids are present. The right grade choice is a balance of budget and the real operating environment. On our projects we always clarify the environment composition before choosing steel for equipment — more on materials in the articles tagged materials. Get in touch, we’ll advise the optimal grade.
