passivate

Passivation is a chemical or electrochemical process that renders a material less susceptible to corrosion or other forms of attack by forming a thin, adherent surface layer, typically an oxide, on the material. The passive film acts as a barrier to diffusion of aggressive species and is often self-healing if damaged, maintaining protection under most operating conditions.

In metals and alloys, passivation commonly produces a protective oxide film on the surface. Stainless steels rely on a chromium-rich oxide, such as Cr2O3, to create a continuous, tenacious barrier. The protective film is usually only a few nanometers thick but markedly reduces corrosion rates. The film can form naturally in air or be enhanced by deliberate treatments. The film’s effectiveness depends on alloy composition (notably chromium content), environmental conditions, and mechanical state of the surface.

Methods of passivation include chemical passivation, which uses acids to remove free iron and other contaminants and promote oxide formation, and electrochemical passivation (anodic treatments) that grow or restore the passive layer through controlled oxidation. Common chemical agents include nitric and citric acids, among others. Proper rinsing and drying are important to maintain a stable, adherent film.

In semiconductors and electronics, surface passivation reduces dangling bonds and surface states that cause charge trapping or recombination. Silicon, for example, is commonly passivated with silicon dioxide or silicon nitride layers deposited by thermal oxidation or chemical vapor deposition, improving device performance and stability.

Limitations include the potential for film breakdown in aggressive environments (such as chlorides), mechanical damage that exposes fresh metal, and the need for sufficient alloying elements to sustain a continuous passive film. Passivation is not a thick coating but a self-protective surface film.