Corrosion is the gradual destruction of metal through chemical reactions, usually with moisture and oxygen in the air. When an old bicycle is left outside in the rain and its chain turns orange and flaky, that's corrosion. When a car's undercarriage develops rust holes after years of driving through salty winter roads, corrosion caused the damage.

The most familiar type of corrosion is rust, which happens specifically to iron and steel. But other metals corrode too: copper turns green (like the Statue of Liberty), silver tarnishes black, and aluminum develops a whitish coating. Each metal reacts differently with its environment.

Corrosion costs billions of dollars every year in repairs and replacements. Engineers work hard to prevent it by painting metal surfaces, using special coatings, or choosing metals that resist corrosion better. Ships, bridges, pipelines, and buildings all need protection from corrosive forces, especially in harsh environments like the ocean or areas with lots of pollution.

The word can also describe gradual wearing away in a broader sense. Someone might say that constant criticism causes a corrosion of confidence, meaning it slowly eats away at how you feel about yourself, similar to how rust eats away at metal.