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Passivation (chemistry)
Passivation, in physical chemistry and engineering, refers to a material becoming "passive," that is, being less affected by environmental factors such as air and water. Passivation involves a shielding outer-layer of base material, which can be applied as a microcoating, or oxidation which occurs spontaneously in nature. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shell against corrosion. Passivation can occur only in certain conditions, and is used in microelectronics to enhance silicon.〔IUPAC (Goldbook )〕 The technique of passivation is used to strengthen and preserve the appearance of metallics.
When exposed to air, many metals naturally form a hard, relatively inert surface, as in the tarnishing of silver, in contrast to metals such as iron, where uniform corrosion produces a somewhat rough surface by removing a substantial amount of metal, which either dissolves in the environment or reacts with it to produce a loosely adherent, porous coating of corrosion products. The amount by which a corrosion coating reduces the rate of corrosion varies, depending on the kind of metal and its environment, and is notably slower in room-temperature air for aluminium, chromium, zinc, titanium, and silicon (a metalloid); the shell inhibits deeper corrosion, and so is the key factor of passivation. The inert surface layer, termed the ‘’native oxide layer‘’, is usually an oxide or a nitride, with a thickness of a monolayer (1-3 Å) for a noble metal such as platinum, about 15 Å for silicon, and nearer to 50 Å for aluminium after several years.〔http://www.semi1source.com/glossary/default.asp?searchterm=native+oxide〕〔Fehlner, Francis P, Low-Temperature Oxidation:The Role of Vitreous Oxides, A Wiley-Interscience Publication, John Wiley & Sons, New York , 1986 ISBN 0471-87448-5〕
==Mechanisms==
There has been much interest in determining the mechanisms which describe how the thickness of the oxide layer on a material increases with time. Some of the important issues include: the relative volume of the oxide compared to the parent metal, the mechanism by which oxygen diffuses through the metal oxide to the metal - oxide interface and the relative chemical potential for the oxide to form. 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.〔Fehlner, Francis P, ref.3.〕
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, dissolving in solutions (chromates, molybdates) form non-reactive and low solubility films on metal surfaces.
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