Passivointi

Passivation, known in Finnish as passivointi, is a surface treatment aimed at reducing the chemical reactivity of metals by forming a protective, adherent oxide or other compound on the surface. The resulting film acts as a barrier to corrosion and lowers the rate at which the metal reacts with its environment.

The protective layer is typically thin, self-forming, and chemically stable within certain environmental conditions. It can be damaged mechanically or chemically, but the surface may re-passivate if the environment permits oxide formation. The film is most effective when it passivates metals that have a sufficient content of alloying elements, such as chromium, aluminum, or titanium, which contribute to stable oxide formation.

Common methods include chemical passivation, where acids such as nitric acid or citric acid remove free iron and promote oxide growth on stainless steels, and electrochemical passivation, which uses controlled anodic potentials to enhance film formation. In practice, passivation is often used as a pretreatment before subsequent coatings or in-service applications where corrosion resistance is critical.

Materials commonly subjected to passivation include stainless steels, aluminum and its alloys, titanium, and certain nickel-based alloys. The process is widely used in industries such as healthcare, food processing, chemical processing, aerospace, and construction to improve durability and longevity of components exposed to aggressive environments.

Limitations exist: passivation does not render a surface invulnerable to corrosion, and films can be compromised by abrasion, high concentrations of chlorides, extreme pH, or aggressive cleaning regimes. Regular inspection and appropriate maintenance are necessary to ensure the long-term effectiveness of a passivated surface.