Definition
Corrosion is a chemical reaction between a metal and the oxygen or moisture around it. The metal is slowly eaten away and loses its original strength and properties. It is also the process of metal, which people worked hard to refine, returning to the most stable form it has in nature: ore.
Why does a bicycle left in the rain turn red with rust?
You've probably seen it. A bike left outside on a rainy day, and a few days later the chain is stiff and shedding red powder. That everyday red rust is the most familiar example of corrosion.
Pure metal is high in energy and unstable in nature. When iron meets oxygen and moisture in the air, it tends to lose electrons and become oxidized. Iron, oxygen, and water combine into iron oxide, a new, red, crumbly substance.
Just as wood rots and returns to soil, hard metal that people smelted with fire keeps trying to return to its original mineral state. It isn't a curse. It's a big law of nature: metal looking for a more comfortable, stable chemical state.
To be more precise: a reaction that steals electrons
In chemistry, corrosion is a classic redox reaction, in which a substance loses electrons. When a metal atom has its electrons taken, it becomes a positive ion and breaks away from the metal. The firm framework of metallic bonds falls apart.
Even one tiny water drop on a metal surface works like a precise little battery. At the center of the drop, iron loses electrons and dissolves. At the edge of the drop, oxygen from the air accepts those electrons. Salt in seawater and the de-icing salt (calcium chloride) spread on roads in winter are full of ions, so they carry electricity well. That can speed up corrosion several times over.
To be more precise, corrosion isn't just dirt on the surface. The metal itself is disappearing chemically. Once its atoms lose electrons and the structure is destroyed, the metal can no longer bear the load it was built to carry.
Different shields that protect precious iron
When big bridges, skyscrapers, and ships are weakened by corrosion, the result can be a terrible collapse. So people have developed careful shields that keep metal from touching oxygen and water directly.
The most common is a coating: paint applied carefully, or a layer of oil, to cut off contact with air. But it has a weakness. One small scratch, and corrosion can race in through the gap.
To make up for this, a more reactive metal such as zinc is coated onto the surface (plating). Even if the surface is scratched, the zinc gives up its electrons first, in place of the iron, and is oxidized. This is called sacrificial anode protection. Alloys that include chromium, such as stainless steel, form a thin, dense, transparent protective film on their own surface. It keeps the metal inside from rusting.
Common misconceptions
Stainless steel is a perfect metal that never corrodes at all.
Stainless steel actually has a very thin corroded (oxidized) layer on its surface. But the oxide film made by chromium is so dense that oxygen and moisture can't get deeper inside.
Where you meet it
The underside of a car parked near the sea rusts red much faster than one parked inland.
After rain, a monkey bar on an outdoor playground or the chain of a forgotten bike gets stiff and leaves red powder on your hands.
In one sentence
Corrosion is a redox reaction in which metal meets oxygen and water, loses electrons, and falls apart as it returns to a stable oxide.