Why does stainless steel resist rust?

1. Two spoons are left wet by the sink. After a few weeks the ordinary steel spoon is brown with rust, but the stainless steel spoon is as shiny as ever. What is its secret?
2. Steel is iron with a tiny bit of carbon. To make it stainless, steelmakers melt in another metal, chromium: at least about one part in every ten.
3. Chromium atoms at the surface grab oxygen from the air. Together they make a very thin, tight layer that covers the whole spoon. Water and air cannot get through it to the iron.
4. What if the shield gets scratched? Fresh chromium is uncovered, it meets oxygen, and the shield grows back in a moment. It mends itself, again and again.
5. Ordinary steel has no shield. Water and oxygen reach the iron and turn it into rust. Rust is crumbly and full of holes, so it flakes away and lets the water in deeper.
6. That is why spoons, sinks, pots and doctors' tools are made of stainless steel. It needs oxygen to keep its shield, and very salty water can still be hard on it.

The magnifier shows the very top of the spoon, so close that you can see atoms. Tap each part.

• Iron atoms. Most of the atoms are iron. Iron makes the steel strong, but on its own it joins with oxygen and water to make rust.
• Chromium atoms. Chromium atoms are mixed in among the iron. At least about one atom in ten must be chromium for the shield to cover everything.
• Invisible shield. The shield is made of chromium joined with oxygen. It is far thinner than a soap bubble, so the metal still looks shiny, but it is tight enough to keep water out.
• Oxygen from the air. Oxygen from the air is what the chromium joins with. The shield needs oxygen to form and to mend itself.
• Water drop. Water sits on the shield and cannot get through to the iron underneath, so no rust can start.
• Scratch, healing. A scratch tears the shield. The chromium underneath meets oxygen and the shield grows back before rust can begin.

Keep both spoons wet and let the days pass. Scratch them. Then change how much chromium is in the right-hand spoon.

Experiment: You scratch the stainless steel spoon and leave it wet for two weeks. What happens at the scratch?

1. Keep both spoons wet and move the days forward. Watch the close-ups.
2. Scratch both spoons, then let more days pass.
3. Now lower the chromium in the right-hand spoon a little at a time. When does rust appear?

What we found: The ordinary spoon rusts all over, and fastest when wet. The stainless spoon stays bright, and its scratch heals because fresh chromium meets oxygen. With too little chromium the shield has gaps, and rust starts in them.

Turn on JavaScript to try it yourself.

Chromium costs a lot. Make a spoon that stays shiny for 30 wet days using as little chromium as you can.

Steel is mostly iron, and iron has a weakness. When it is wet, it joins with oxygen from the air and slowly turns into rust. Rust is crumbly and full of tiny holes. It flakes away, and the water gets to the fresh iron below, so the rusting carries on.

Stainless steel has another metal mixed in, called chromium: at least about ten parts in every hundred. Chromium joins with oxygen even more eagerly than iron does. But what it makes is very different from rust. It is a thin, tight, see-through layer that sticks firmly to the metal.

That layer works like a shield. It is thousands of times thinner than a hair, so you cannot see it and the spoon still shines. Water and air cannot pass through it to the iron. If you scratch it, the chromium underneath meets oxygen and a new shield grows in a moment.

It is called stainless, but it is not stain-proof. The shield needs oxygen, so stainless steel can rust where dirt keeps the air away, and very salty water can break through. Our scraped-knee picture fits the healing, but skin takes days to grow back. This shield takes less than a second.

Key words

Alloy

A metal made by melting two or more elements together. Steel and stainless steel are both alloys.

Chromium

A hard, silvery metal. Mixed into steel, it builds the protective layer that makes the steel stainless.

Oxide

What you get when a material joins with oxygen. Rust is iron oxide. The shield is chromium oxide.

Passive film

The invisible, protective layer on stainless steel.

Corrosion

The slow eating away of a metal as it reacts with its surroundings. Rusting is the corrosion of iron.

Pitting

Corrosion that makes small, deep holes where something, usually salt, has broken through the protective film.

Check your thinking

1. What protects stainless steel from rust?

   Show the answer

   A thin, invisible layer made of chromium and oxygen. Chromium at the surface joins with oxygen to make a tight shield that water cannot pass.

2. A stainless steel sink gets scratched. Why does it not rust at the scratch?

   Show the answer

   Chromium under the scratch meets oxygen and the shield grows back. The shield is made by the metal itself, so fresh metal simply makes a fresh shield.

3. Why does rust not protect ordinary steel the way the shield protects stainless steel?

   Show the answer

   Rust is crumbly and full of holes, so it flakes off and lets water in. A shield only works if it is tight and stays on. Rust is neither.

4. A steel has only a little chromium, about 5 parts in 100. What would you expect?

   Show the answer

   The shield has gaps, so rust starts in them. There are too few chromium atoms at the surface to join into one complete layer.

5. Why do railings by the sea use a grade of stainless steel with more chromium and added molybdenum?

   Show the answer

   Chloride ions from salt can break the passive film locally and cause pitting. Chlorides attack weak points in the film and start pits. Higher chromium and molybdenum make the film harder to break and quicker to heal.