Why Does Metal Rust?

Rust is one of those everyday phenomena that we all recognise but rarely stop to think about. You’ve seen it on old cars, neglected tools, or that bicycle you left out in the rain too long.

That reddish-brown, flaky coating isn’t just an eyesore—it’s a sign of a chemical process at work. But why does metal rust, and what’s really happening when it does? Let’s break it down.

At its core, rusting is an oxidation reaction. It happens when iron (or an iron alloy like steel) reacts with oxygen in the presence of water or moisture. The result is iron oxide, commonly known as rust. This process is a type of corrosion, a natural degradation that metals undergo when exposed to certain environmental conditions.

While rust is most associated with iron, other metals corrode too—think of the green patina on copper statues—but rust specifically refers to iron oxide.

The chemistry behind rusting is straightforward yet fascinating. When iron meets water and oxygen, a redox reaction kicks off. Iron loses electrons (oxidation) and oxygen gains them (reduction).

The iron atoms transform into iron ions, which then combine with oxygen and water to form hydrated iron oxide—Fe₂O₃·nH₂O, if you want the technical formula. This compound is brittle and flaky, unlike the strong, cohesive structure of the original metal. Over time, rust eats away at the iron, weakening it and eventually causing structural failure if left unchecked.

Moisture is the key player here. Dry iron exposed to oxygen won’t rust—think of a steel beam in a desert. But add water, even just humidity in the air, and the process begins. That’s because water acts as an electrolyte, facilitating the movement of electrons between iron and oxygen. Salt accelerates this even further by increasing the water’s conductivity, which is why coastal areas or salted winter roads are rust’s best friends.

Not all metals rust the same way, though. Pure iron rusts readily, but alloys like stainless steel resist it thanks to chromium. When chromium is added to steel, it reacts with oxygen to form a thin, protective layer of chromium oxide. This “passive layer” shields the iron beneath from further oxidation, making stainless steel a go-to for everything from kitchen knives to skyscraper cladding. Aluminum, similarly, forms its own protective oxide layer, which is why it doesn’t rust like iron despite being highly reactive.

Left alone, iron will always revert to a more stable, oxidized state, a process driven by thermodynamics. It’s why ancient shipwrecks turn into coral-encrusted relics and why we’ve engineered paints, coatings, and galvanization (coating iron with zinc) to fight back.

Yet, rust also has its charm. Artists use it for aesthetics, and in nature, iron oxide colors cliffs and canyons with stunning reds and oranges.

So, the next time you spot rust, don’t just see decay. See a slow dance between metal, oxygen, and water—a natural process as inevitable as it is transformative. It’s a small glimpse into how the world works, one flaky patch at a time.