* They have undergone a chemical reaction with their environment. This reaction typically involves the oxidation of the metal, where the metal atoms lose electrons.

* They have a change in their physical properties. This change can include:

* Color change: The metal may become discolored, often with a dull or rusty appearance.

* Surface roughness: The surface may become pitted, rough, or flaky.

* Loss of strength: The metal may become weaker and more brittle.

* Loss of conductivity: The metal may become less conductive to electricity or heat.

* They are less resistant to further corrosion. Once a metal begins to corrode, it becomes more susceptible to further damage.

The specific chemical reactions and the resulting appearance of corrosion can vary greatly depending on the type of metal, the environment, and the presence of other factors. However, the fundamental principle of chemical reaction and the resulting change in properties remain the same.