A Moment of Science

Why Stainless Steel Does Not Rust But Other Metals Do

Have you ever wondered why stainless steel does not rust?

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Photo: d00d (flickr)

Stainless Steel is commonly used for kitchen sinks because it will not rust.

Our modern world, with its skyscrapers, suspension bridges, and automobiles, relies heavily on steel. If you own an aging car however, you know firsthand that steel has one annoying problem: it rusts. All steel, that is, except stainless steel. What keeps this remarkable material, used commonly in kitchen sinks and cookware, from rusting?

Rust-Proof

Steel’s main ingredient is iron. When steel rusts, its iron combines with oxygen and reverts to iron ore, the raw state from which it came. As this happens, the steel turns brown and begins to crumble.

There are many ways to protect steel from corrosion. Steel is sometimes painted or greased, or coated with a metal that is less likely to rust.

Sacrificial Metal

Sometimes a metal that is MORE likely to rust is attached. Known as a sacrificial metal, this works by drawing the corrosion process away from the steel. For example, bars of zinc are attached to the hulls of some ships. The zinc rusts heavily, but the steel hull stays relatively safe.

Stainless Steel Avoids Rust

The best way to avoid rust is to use stainless steel. Like all steel, stainless steel is mostly iron, but it also contains nickel and chromium.

These are not just a protective coating, but are melted into the steel itself. The mixture must contain at least ten percent chromium, because it’s the chromium that protects stainless steel from corrosion.

What Happens?

What happens is this: Like a sacrificial metal, the chromium rusts first. Unlike iron however, rusting chromium doesn’t crumble apart. Instead, it forms an invisibly thin layer that protects the iron underneath. The nickel in stainless steel helps hold this protective layer of chromium rust in place.

Remember that chromium and nickel are present throughout stainless steel, not just on the surface. Because of this, the microscopic layer will form itself anew, even when the steel is cut or scratched.

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