In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some electrochemical applications such as electrocoagulation for wastewater treatment, amperometric chemical sensing, and electrochemical synthesis.

When exposed to air, many metals naturally form a hard, relatively inert surface layer, usually an oxide (termed the "native oxide layer") or a nitride, that serves as a passivation layer - i.e. these metals are "self-protecting". In the case of silver, the dark tarnish is a passivation layer of silver sulfide formed from reaction with environmental hydrogen sulfide. Aluminium similarly forms a stable protective oxide layer which is why it does not "rust". (In contrast, some base metals, notably iron, oxidize readily to form a rough, porous coating of rust that adheres loosely, is of higher volume than the original displaced metal, and sloughs off readily; all of which permit & promote further oxidation.) The passivation layer of oxide markedly slows further oxidation and corrosion in room-temperature air for aluminium, beryllium, chromium, zinc, titanium, and silicon (a metalloid). The inert surface layer formed by reaction with air has a thickness of about 1.5 nm for silicon, 1–10 nm for beryllium, and 1 nm initially for titanium, growing to 25 nm after several years. Similarly, for aluminium, it grows to about 5 nm after several years.

In the context of the semiconductor device fabrication, such as silicon MOSFET transistors and solar cells, surface passivation refers not only to reducing the chemical reactivity of the surface but also to eliminating the dangling bonds and other defects that form electronic surface states, which impair performance of the devices. Surface passivation of silicon usually consists of high-temperature thermal oxidation.

There has been much interest in determining the mechanisms that govern the increase of thickness of the oxide layer over time. Some of the important factors are the volume of oxide relative to the volume of the parent metal, the mechanism of oxygen diffusion through the metal oxide to the parent metal, and the relative chemical potential of the oxide. Boundaries between micro grains, if the oxide layer is crystalline, form an important pathway for oxygen to reach the unoxidized metal below. For this reason, vitreous oxide coatings – which lack grain boundaries – can retard oxidation. The conditions necessary, but not sufficient, for passivation are recorded in Pourbaix diagrams. Some corrosion inhibitors help the formation of a passivation layer on the surface of the metals to which they are applied. Some compounds, dissolved in solutions (chromates, molybdates) form non-reactive and low solubility films on metal surfaces.

It has been shown using electrochemical scanning tunneling microscopy that during iron passivation, an n-type semiconductor Fe(III) oxide grows at the interface with the metal that leads to the buildup of an electronic barrier opposing electron flow and an electronic depletion region that prevents further oxidation reactions. These results indicate a mechanism of "electronic passivation". The electronic properties of this semiconducting oxide film also provide a mechanistic explanation of corrosion mediated by chloride, which creates surface states at the oxide surface that lead to electronic breakthrough, restoration of anodic currents, and disruption of the electronic passivation mechanism ("transpassivation").

The fact that iron doesn't react with concentrated nitric acid was discovered by Mikhail Lomonosov in 1738 and rediscovered by James Keir in 1790, who also noted that such pre-immersed Fe doesn't reduce silver from nitrate anymore. In the 1830s, Michael Faraday and Christian Friedrich Schönbein studied that issue systematically and demonstrated that when a piece of iron is placed in dilute nitric acid, it will dissolve and produce hydrogen, but if the iron is placed in concentrated nitric acid and then returned to the dilute nitric acid, little or no reaction will take place. In 1836, Schönbein named the first state the active condition and the second the passive condition while Faraday proposed the modern explanation of the oxide film described above (Schönbein disagreed with it), which was experimentally proven by Ulick Richardson Evans only in 1927. Between 1955 and 1957, Carl Frosch and Lincoln Derrick discovered surface passivation of silicon wafers by silicon dioxide, using passivation to build the first silicon dioxide field effect transistors.

Aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in many environments. Some aluminium alloys, however, do not form the oxide layer well, and thus are not protected against corrosion. There are methods to enhance the formation of the oxide layer for certain alloys. For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and peroxide alternating with deionized water. The nitric acid and peroxide mixture oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.

Generally, there are two main ways to passivate aluminium alloys (not counting plating, painting, and other barrier coatings): chromate conversion coating and anodizing. Alclading, which metallurgically bonds thin layers of pure aluminium or alloy to different base aluminium alloy, is not strictly passivation of the base alloy. However, the aluminium layer clad on is designed to spontaneously develop the oxide layer and thus protect the base alloy.