Passivation (chemistry)

In <a href="/facts/Physical_chemistry/WqwnJjRM">physical chemistry</a> and engineering, passivation is <a href="/facts/Coating/fGt3R3VC">coating</a> a material so that it becomes "passive", that is, less readily affected or <a href="/facts/Corrode/lIC7H4we">corroded</a> 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 <a href="/facts/Oxidation/Pyd5RVcj">oxidation</a> in the air. As a technique, passivation is the use of a light coat of a protective material, such as <a href="/facts/Metal_oxide_adhesion/e8Wl96jl">metal oxide</a>, to create a shield against <a href="/facts/Corrosion/lIC7H4we">corrosion</a>. Passivation of <a href="/facts/Silicon/cgWx0hdr">silicon</a> is used during fabrication of <a href="/facts/Microelectronic/HzCusb86">microelectronic</a> devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some <a href="/facts/Electrochemical_engineering/QcHfW5RE">electrochemical applications</a> such as <a href="/facts/Electrocoagulation/aaI6CBHR">electrocoagulation</a> for wastewater treatment, <a href="/facts/Amperometry/5sS1zNRG">amperometric chemical sensing</a>, and <a href="/facts/Electrosynthesis/hzU3XYxC">electrochemical synthesis</a>.
When exposed to air, many metals naturally form a hard, relatively <a href="/facts/Chemically_inert/oglbxzgk">inert</a> surface layer, usually an <a href="/facts/Oxide/DXtoVe1C">oxide</a> (termed the "native oxide layer") or a <a href="/facts/Nitride/9vkrCbJ4">nitride</a>, that serves as a passivation layer - i.e. these metals are "self-protecting". In the case of <a href="/facts/Silver/xXe41BGg">silver</a>, the dark <a href="/facts/Tarnish/GCskvFrN">tarnish</a> is a passivation layer of <a href="/facts/Silver_sulfide/muzsPR6v">silver sulfide</a> formed from reaction with environmental <a href="/facts/Hydrogen_sulfide/UbCqjMMk">hydrogen sulfide</a>. Aluminium similarly forms a stable protective oxide layer which is why it does not "rust". (In contrast, some base metals, notably <a href="/facts/Iron/QK1JYy8E">iron</a>, oxidize readily to form a rough, <a href="/facts/Porosity/UoyAJU8X">porous</a> coating of <a href="/facts/Rust/sajUmaz8">rust</a> 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 <a href="/facts/Aluminium/7wmR8jYE">aluminium</a>, <a href="/facts/Beryllium/dv295voP">beryllium</a>, <a href="/facts/Chromium/6sVk6Uu7">chromium</a>, <a href="/facts/Zinc/80b1qgk3">zinc</a>, <a href="/facts/Titanium/53ArnDoy">titanium</a>, and silicon (a <a href="/facts/Metalloid/uMFVXuTq">metalloid</a>). The inert surface layer formed by reaction with air has a thickness of about 1.5 nm for silicon, 1–10 nm for <a href="/facts/Beryllium/dv295voP">beryllium</a>, and 1 nm initially for <a href="/facts/Titanium/53ArnDoy">titanium</a>, growing to 25 nm after several years. Similarly, for aluminium, it grows to about 5 nm after several years.
In the context of the <a href="/facts/Semiconductor_device_fabrication/Jd23Mu1z">semiconductor device fabrication</a>, such as silicon <a href="/facts/MOSFET/eTDJT2Km">MOSFET transistors</a> and <a href="/facts/Solar_cell/3L814UCC">solar cells</a>, surface passivation refers not only to reducing the chemical reactivity of the surface but also to eliminating the <a href="/facts/Dangling_bond/HmG4tu3x">dangling bonds</a> and other defects that form electronic <a href="/facts/Surface_state/XQA64So2">surface states</a>, which impair performance of the devices. Surface passivation of silicon usually consists of high-temperature <a href="/facts/Thermal_oxidation/FNPWW95h">thermal oxidation</a>.

Passivation (chemistry) open-in-new

Passivation (chemistry)