Why Doesn’t Stainless Steel Rust?

Leaving a garden tool out in the rain seems like a simple oversight until you need to use it and find how quickly ordinary steel can turn orange and flaky. Meanwhile, the stainless steel spoon in your kitchen drawer, the sink in your bathroom, and even the metal on city skyscrapers can stay shiny for years despite constant exposure to water and air.

So what gives? If both types of steel are made mostly of iron, why does one rust while the other seems immune from the phenomenon? The answer is that the iron atoms in stainless steel are protected by another metal.

This metal not only prevents rust, but does so by teaming up with the very thing that causes it in the first place.

Iron Needs Its Own Armor

Rust is a type of corrosion that occurs when iron reacts with oxygen and water. That’s why it often occurs when something metal is left outside or gets wet. The reddish-brown material that forms — technically known as iron oxide — is weak and flaky. 

Worse, it doesn’t stay put. As it flakes away, it exposes fresh iron underneath, allowing the corrosion to continue in a vicious, damaging cycle. Stainless steel contains iron too. Not mass-produced until the 19th century, steel is an alloy that adds carbon to iron to make it a stronger, harder metal.

There are different types of steel alloys — which include different proportions of various metals like nickel and manganese — for different applications. Stainless steel is at least 10.5% chromium (and no more than 1.2% carbon), which is its secret weapon against rust. That small addition makes a big difference.

You can think of the included chromium as a microscopic suit of armor. When it comes into contact with oxygen in the air, it forms an extremely thin layer of chromium oxide on the surface of the metal.

Unlike rust, this layer is stable, tightly bonded, and remarkably durable. It acts as a barrier that prevents oxygen and water from reaching the iron below. The layer is microscopic — you can’t see this invisible shield with the naked eye.

The chromium oxide forms so quickly and effectively that it prevents the deeper, destructive corrosion we recognize as rust. The protective coating is only a few atoms thick, yet that’s enough. Better still, this coating can repair itself. If the surface of the metal is scratched, fresh chromium is exposed, and a new protective layer forms almost immediately as long as oxygen is present. 

A self-healing suit of armor? It doesn’t get much cooler than that. That doesn’t mean stainless steel is indestructible, though.

In extremely harsh conditions — such as prolonged exposure to saltwater or certain chemicals — can eat away at the chromium layer. A lack of oxygen also prevents chromium oxide from replenishing itself. Then, the iron in the steel is unprotected from the elements that cause it to rust.

That’s why marine-grade stainless steel used on ships often contains additional elements such as molybdenum, which provide additional defense.

Still, for everyday uses, stainless steel’s self-renewing shield makes it one of the most durable materials around. It might not be nearly as valuable per pound as gold or platinum, but when it comes to keeping the sink shiny and the knives rust-free, it’s an MVP.