Corrosion

Corrosion is the deterioration of a material due to a chemical reaction with its environment.

• Rusting of iron is a corrosion process (oxidation of iron).
• Tarnishing of silver is a type of corrosion (formation of silver sulfide).
• Development of a green coating (patina) on copper and bronze is a form of corrosion (formation of copper carbonate and other compounds).
• Production of hydrogen by electrolysis of water is not a corrosion process. It's an electrochemical process where water is decomposed into hydrogen and oxygen.

Therefore, the process which is not an example of corrosion is the production of hydrogen by electrolysis of water.