Chemical corrosion is a process of gradual destruction of a metal's surface. It is characterized by the oxidation of the metal due to its reaction with its environment. This process is often accelerated by acids and leads to the formation of metal oxides. A common example is the corrosion of iron, or in other word the formation of rust. Iron is the reducing agent in this electrochemical reaction and releases electrons:
Fe -> Fe₂₊ + 2 e-

These electrons react with water and oxygen in the air to form hydroxide ions (OH-):
O₂ + 4 e- + 2 H₂O -> 4 OH-

Subsequently, the hydroxide ions react with the iron ions to form iron hydroxide (Fe(OH)₂):
Fe₂+ + 2 OH- -> Fe(OH)₂

The iron hydroxide dehydrates (releases water) and forms iron oxide (FeO):
Fe(OH)₂ -> FeO + H₂O

FeO is one of the products of the corrosion of iron. Similar reactions often simultaneously lead to the formation of Fe₂O₃ and Fe₃O₄ as well. The availability of water increases the speed of these reactions. Therefore, iron corrodes faster when exposed to moist air or when being submerged in water.

There are different solutions to prevent corrosion. A simple solutions is the application of a protection layer to metals that prevents oxygen and water reaching the metal itself. This can often be achieved by painting metal pieces. Another solutions is so called cathodic protection. This means that a different metal piece with a lower electronegativity will be connected to the metal that should be protected from corrosion. The free electrons from the first above mentioned reaction will flow to the less electronegative metal, for example a zinc piece connected to an iron piece. If the free electrons are localized in the piece of zinc, the formation of hydroxide ions will happen at the zinc piece and therefore the zinc will corrode and not the iron piece. This type of protection is commonly used in cars, when the steel body parts of a car are coated with a layer of zinc (before applying paint) to protect them from corrosion and increase their lifetime.