Why do some metals resist corrosion?

Some metals resist corrosion due to their ability to form a protective layer of oxide on their surface.

Corrosion is a natural process that occurs when metals react with their environment, particularly with oxygen and moisture. This reaction often leads to the formation of oxides, which can degrade the metal's properties. However, some metals, such as aluminium and stainless steel, have a unique ability to resist this degradation. They do this by forming a thin, protective layer of oxide on their surface, which prevents further corrosion.

Aluminium, for instance, reacts with oxygen in the air to form aluminium oxide. This oxide layer is extremely hard and adheres tightly to the surface of the metal, preventing further oxygen from reaching the underlying aluminium and causing more corrosion. The oxide layer is also self-repairing, meaning that if it is scratched or damaged, the exposed aluminium will react with the oxygen to repair the layer.

Stainless steel, on the other hand, resists corrosion due to the presence of chromium in its composition. When exposed to oxygen, the chromium forms a thin, invisible layer of chromium oxide on the surface of the steel. This layer is also self-repairing and prevents further corrosion by blocking oxygen and moisture from reaching the underlying steel.

In contrast, metals like iron do not form a protective oxide layer. When iron reacts with oxygen and moisture, it forms iron oxide, or rust, which flakes off and exposes more iron to the environment, leading to further corrosion. This is why iron and its alloys need to be protected by paint or other coatings to prevent corrosion.

In summary, the ability of some metals to resist corrosion is due to their ability to form a protective, self-repairing oxide layer on their surface. This layer acts as a barrier, preventing further reaction with the environment and thus preserving the metal's properties.