How do oxides form protective layers on metals?

Oxides form protective layers on metals through a process called oxidation, which prevents further corrosion.

Oxidation is a chemical reaction that occurs when a substance, in this case, a metal, reacts with oxygen. This reaction is often facilitated by water, heat, or exposure to the atmosphere. When a metal oxidises, it forms a metal oxide. This metal oxide forms a layer on the surface of the metal, which acts as a barrier, preventing further oxidation and thus protecting the metal from further corrosion.

For example, when aluminium is exposed to air, it reacts with the oxygen to form aluminium oxide. This aluminium oxide forms a thin, hard layer that adheres tightly to the surface of the aluminium. This layer is resistant to further corrosion, effectively protecting the underlying aluminium from further damage. This is why aluminium is often used in outdoor applications, such as in the construction of aircraft and buildings, where it is exposed to the elements.

Similarly, stainless steel, which contains chromium, forms a protective layer of chromium oxide when exposed to air. This layer is invisible, but it is extremely effective at preventing further corrosion. If the layer is scratched or damaged, it will self-heal, as the exposed chromium will react with the oxygen in the air to form more chromium oxide, restoring the protective layer.

However, not all metal oxides form protective layers. For example, iron oxide, or rust, does not adhere well to the surface of the iron and flakes off, exposing more iron to the air and allowing the corrosion process to continue. This is why iron and steel, which is primarily made of iron, need to be painted or otherwise protected to prevent rusting.

In summary, the formation of a protective oxide layer is a crucial process that helps to preserve metals and prevent their degradation. This process is a key concept in the study of chemistry and materials science.