passivfilmskiktet

Passivfilmskiktet, also called the passive film layer, is a thin, discontinuous oxide coating that naturally forms on the surface of certain metals and alloys, notably stainless steels, titanium, and aluminum. The layer is typically only a few nanometres thick and is composed mainly of metal oxides and hydroxides. Its primary function is to act as a barrier to corrosive agents, preventing further oxidation of the underlying metal. The passive film is self‑regenerating; when it is damaged or disrupted by mechanical action or chemical attack, it reforms instantaneously as long as the metal is exposed to an oxidizing environment.

The formation of the passive film is governed by thermodynamic and kinetic factors. The free energy of formation of the metal oxide must be sufficiently negative, and the kinetics must allow rapid oxidation under the prevailing conditions. Electrochemical potential of the metal, pH of the surrounding medium, temperature, and the presence of sensitising elements such as sulfur or chlorine strongly influence the film’s stability. In stainless steel, for example, chromium oxides dominate the film composition, and the presence of about 10‑20 % chromium is essential for the formation of a robust passive layer.

In industrial practice, the passive film is exploited to achieve long‑term corrosion resistance without the need for additional protective coatings. It is critical in the construction of pressure vessels, piping, and structural components exposed to corrosive environments. However, the film can be compromised by stress corrosion cracking, pitting, and galvanic coupling, particularly in chloride‑rich media. Re‑passivation procedures, such as anodisation or the application of passivating solutions, are commonly employed to restore the integrity of the film. Understanding the behaviour of passivfilmskiktet is therefore essential for material selection, design, and maintenance in many engineering fields.