passivated

Passivated refers to a surface that has formed a thin, protective film that reduces chemical reactivity and corrosion. In metals, this typically means a self-generated or induced oxide layer that is dense, adherent, and capable of slowing or stopping further anodic dissolution. The most familiar example is the chromium oxide film that forms on stainless steel, but passivation also occurs with aluminum, titanium, nickel, and other metals when they form stable surface oxides.

Formation and mechanism: Passivation can occur naturally when a metal is exposed to air or water, a process known as self-passivation. It can also be achieved through chemical treatment, where the surface is cleaned of contaminants and exposed to oxidizing solutions (for example, nitric or citric acid) to promote and stabilize a protective oxide film. The resulting film is typically only a few nanometers thick but significantly lowers the metal’s reactivity by impeding active sites for corrosion.

Materials and applications: Stainless steel relies on a chromium oxide layer that forms when chromium content exceeds about 10.5%. Aluminum forms a protective aluminum oxide, while titanium develops a stable titanium oxide film. Passivation improves corrosion resistance in harsh environments, and is common in chemical processing equipment, biomedical implants, and aerospace components. In electronics and photovoltaics, passivation also refers to thin dielectric layers (such as silicon oxide or silicon nitride) applied to reduce surface states and moisture ingress.

Limitations: Passive films can be damaged by abrasion, high temperatures, or aggressive species such as chlorides, which can lead to localized corrosion like pitting. Therefore, maintenance and periodic inspection are important for critical applications.