Rust begins with an electron handoff
When bare iron is exposed to oxygen and moisture, atoms at its surface can lose electrons. That loss is oxidation: the iron becomes positively charged iron ions. Oxygen takes up electrons in reactions that are helped by a thin film of water, which lets charged particles move across the surface.
The iron ions then combine with oxygen and water to form rust, a mixture of iron compounds that commonly includes hydrated iron oxides. Rust’s color and texture can vary, but its loose, flaky structure is the important part: it does not cling to the metal as a tight, continuous barrier.
Why the damage keeps moving inward
A protective coating works by separating fresh metal from the substances that react with it. Rust often fails at that job. Its pores and cracks let oxygen and moisture reach iron below the surface, where more atoms can lose electrons and join the corrosion process. As rust builds and flakes away, it can expose still more metal.
That is why rust is unlike the thin oxide layer that protects some metals, such as aluminum. Iron’s corrosion product leaves routes back to the material beneath it. A small patch of rust can therefore mark an ongoing process, not merely a stain sitting harmlessly on top.
