If left unprotected, something made of iron will dissolve into dust. That’s because of rust.

Some metals, like gold and platinum, are naturally stable and keep their shiny appearance.

Other metals like aluminum, copper and silver develop patina or tarnish that protects the raw metal beneath.

Iron is different. It’s the most abundant element on Earth, followed by oxygen—together they make up over 60% of all elements. And they like to get together:

Almost all iron on the surface of Earth appears as an iron oxide—a mixture of iron and oxygen.

We smelt iron oxides to get pure iron, which we use as a metal or blend into alloys like steel. But once it’s been separated from oxygen, it wants to bond again.

When water gets involved, rust forms.

It’s not well defined as a chemical process, but initially the iron oxidizes, then water turns that molecule into a new compound we call rust, which replaces the metal.

And because the rust molecule is larger, it doesn’t fit in the same place and flakes off as rust. This exposes new metal, which will rust and flake off—until all the iron is gone.

With so much of our buildings, bridges, pipelines and other infrastructure made of iron or steel, it’s important to keep them oiled, painted, coated or alloyed to be protected.

Because as Neil Young said, rust never sleeps.