316, 304 and 201 stainless steel cups: the differences and why they can rust

These are the three materials you’ll hear about most when ordering stainless steel cups. They’re all stainless steel, but they’re made of different things, so they resist rust differently and cost different amounts. And none of them is completely rust-proof.

What they’re made of

304: 18–20% chromium and 8–11% nickel. It’s also called 18/8 stainless steel.

316: the same as 304 with 2–3% molybdenum added, and more nickel, 10–14%.

201: only 3.5–5.5% nickel. Manganese and nitrogen replace part of the nickel, which is why it’s cheaper.

Why stainless steel resists rust, and why it still can rust

“Stainless” means the steel doesn’t rust easily. It doesn’t mean it never rusts.

Once steel contains 10.5% chromium or more, its surface reacts with the oxygen in the air and forms a layer of chromium oxide only 1–3 nanometres thick. That layer keeps water and oxygen away from the iron underneath. If it gets scratched, it grows back on its own as long as there’s oxygen. Stainless steel rusts when this layer is damaged faster than it can grow back, or when it can’t form at all.

Corrosion resistance

316 > 304 > 201.

The molybdenum makes 316 more resistant to salt and acid. 304 is fine for everyday water, tea, coffee and juice. 201 has the lowest corrosion resistance. It rusts more easily if it holds salty or acidic drinks for a long time, or if it’s kept somewhere damp or near the sea.

Resistance to pitting is measured with a number called PREN: % chromium + 3.3 × % molybdenum + 16 × % nitrogen. The higher the number, the better the steel stands up to salt and chlorine.

Low-nickel 201 (J-series): about 15–17

304: about 18–20

316: about 24–26

Price

316 > 304 > 201.

How they’re used in cups

The inner liner touches the drink, so it’s usually 304 or 316. 201 is usually only used for the outer shell.

Common combinations are 316 inside with 304 outside, 304 inside and out, and 304 inside with 201 outside.

Five common reasons a cup rusts

1. Chloride. Salt, soy sauce, seawater, chlorine bleach, or soup left in the cup for a long time eats small pits into the surface. This is called pitting. 304 is vulnerable to it. 316 holds up much better.

2. Welds. During welding, the band of metal that reaches 450–850°C loses chromium, because the chromium bonds with carbon. That leaves both sides of the weld short of chromium. If the weld isn’t pickled and passivated afterwards, the yellow or dark areas along the seam are the first to rust.

3. Iron particles from outside. If the polishing or grinding tools have touched ordinary carbon steel, or the cup has been scrubbed with steel wool, iron particles get stuck in the surface. What rusts is the iron, not the cup itself, and it wipes off.

4. Gaps with no oxygen. Under a silicone seal, inside a thread, or in a joint, oxygen can’t get in, so the layer can’t grow back. Rust starts in the gap.

5. The wrong material. The cup is sold as 304 but is really 201 or something worse. The next section shows how to check.

How to tell them apart

A magnet test isn’t reliable. 304 and 316 aren’t magnetic on their own, but they pick up a little magnetism once the steel is drawn into a cup shape, and so does 201. Whether a magnet sticks or not doesn’t tell you much.

The reliable ways are to check the material test report, or to test the steel with a nickel testing solution. A drop on the steel shows whether there’s enough nickel for 304.

Which one to choose

304 is enough for most orders.

If you want a selling point, or the cup will hold acidic or salty drinks, use 316 for the inner liner.

For the lowest budget, use 304 inside and 201 outside. The outer shell doesn’t touch the drink.

If you’re not sure which one to use, tell me what the cups are for and your target price, and I’ll help you pick the material.

materialrust