Pasivaci

Pasivaci, often referred to as passivation in English, is the process by which a metal or alloy forms a thin, adherent, protective oxide or other compound on its surface that reduces its chemical reactivity and slows corrosion. The resulting passive film is usually invisible, self-healing after minor damage, and enhances the material’s long-term durability in corrosive environments.

The protective film is typically a few nanometers to tens of nanometers thick and is composed of metal oxide or hydroxide. Its effectiveness arises from low solubility, tight adherence to the substrate, and the ability to reform after disruption in the presence of oxygen or oxidizing conditions. Metals such as chromium, aluminum, and titanium readily form stable passive films, which is a key reason for their widespread use in corrosion-resistant applications. In stainless steels, a chromium oxide layer is fundamental to passivity.

Passivation can occur spontaneously when a clean metal surface is exposed to air or water, but it is often induced deliberately through chemical or electrochemical treatments. Chemical passivation commonly uses oxidizing acids (for example, nitric or citric acid) to remove active film-free sites and promote uniform oxide growth. Electrochemical passivation enhances the protective film by controlled anodic treatment. Electropolishing may accompany passivation to create a smoother surface and improve film uniformity.

Limitations exist: aggressive media, especially chloride-rich environments, can break down the passive film and cause pitting or crevice corrosion. Not all metals form protective passivation layers, and alloy composition (notably chromium, molybdenum content) strongly influences film stability. Passivation is distinct from coatings; it is a self-formed, dynamic surface layer that complements other corrosion protection strategies.

See also: corrosion, passive film, oxide layer, electrochemistry.