Passivation

Passivation is a chemical process by which a material, typically a metal, becomes less reactive at its surface due to the formation of a stable, protective film. The film is usually an oxide or other compound that adheres strongly to the surface and is self-healing if damaged, thereby slowing dissolution and corrosion. Passivity is especially important for metals such as stainless steel, aluminum, and titanium, which form protective surface layers in air or water.

In stainless steel, a thin chromium oxide layer forms on the surface and provides corrosion resistance; the presence of chromium in sufficient quantity enables a continuous, self-healing film. Aluminum forms a native aluminum oxide layer that quickly grows to a protective thickness, while titanium forms a stable titanium dioxide layer. Copper can develop oxide films that are less protective and more prone to breakdown, and iron often forms oxides that may slow further attack but can be porous or mechanically weak in some environments.

Passivation can be induced unintentionally or deliberately. Chemical passivation uses oxidizing acids, such as nitric or citric acid, to remove surface contaminants and promote a uniform oxide film. Electrochemical passivation, or anodic treatment, forms the protective layer by controlled oxidation at a set potential. The resulting film is typically thin, adherent, and chemically stable, but its effectiveness depends on surface finish, environment, and the presence of aggressive species like chlorides.

Limitations of passivation include damage to the film from mechanical abrasion, pitting in chloride-rich environments, and dependence on temperature and pH. When protective films are compromised, corrosion resistance can degrade, and alternative coatings or inhibitors may be required. Passivation is also used in semiconductor and biomaterial contexts to stabilize surfaces and reduce reactivity.