Electrons depart from iron atoms

In a humid environment, iron atoms are more likely to lose electrons and become iron ions in solution. This process is called oxidation; at the same time, oxygen gets electrons at another place and participates in subsequent reactions.

A drop of water is a tiny battery

Tiny anodes and cathodes form at different locations on the rusted surface. The water film transports ions and electrons move along the interior of the metal. Therefore, the rust spots visible to the naked eye are actually the result of a large number of micro-batteries working simultaneously.

Anti-rust is cutting off the reaction chain

Painting, galvanizing, adding corrosion inhibitors, and using weather-resistant alloys all block water and oxygen, or allow materials that are more susceptible to sacrifice to bear the oxidation process first.