What we call “rust” scientists call “oxidation.” It’s what happens when metal are exposed to oxygen for long periods of time. Eventually, the oxygen molecules “bond” with the metal molecules, creating a new compound called oxide. The metal becomes weaker. Rust-encrusted iron or steel is called iron oxide; rust-encrusted aluminum is called aluminum oxide.
What causes rust? Dihydrogen oxide—otherwise known as good old-fashioned water. The water seeps through the iron or steel, into minute pits and cracks that are sometimes invisible to the naked eye. The hydrogen in the water, bonded with other elements, creates acids that eat at the metal and expose it to more damage. Sodium (also known as salt) speeds up the process. This is why seawater is particularly corrosive, and metals used in making boats and bridges must be especially protected against rust.
Thankfully, technological development has led to an arsenal of rust-resistant tools. Some iron and steel are manufactured in such a way that even if rust forms on the surface, it doesn’t spread to the core. There are also paints and other surface barriers such as oil that protect the metals from water.
However, thin metals are particularly prone to rust, and no amount of precaution can stop the process. For example, steel wool pads will begin to rust almost immediately after it is dampened and then left in the air. The acid created when the metal, water and oxygen combine will trigger such a reaction that the heat released actually warms the air around the steel wool by several degrees. The only solution would be to just eventually buy a new steel wool pad.