Passivation, in
physical chemistry and engineering, refers to a material becoming "passive," that is, less affected or corroded by the environment of future use. 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 from 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 shell against
corrosion.
[1] Passivation can occur only in certain conditions, and is used in
microelectronics to enhance
silicon.
[2] The technique of passivation strengthens and preserves the appearance of
metallics. In electrochemical treatment of water, passivation reduces the effectiveness of the treatment by increasing the circuit resistance, and active measures are typically used to overcome this effect, the most common being polarity reversal, which results in limited rejection of the fouling layer. Other proprietary systems to avoid electrode passivation, several discussed below, are the subject of ongoing research and development.
When exposed to air, many metals naturally form a hard, relatively
inert surface, as in the tarnish of silver. In the case of other metals, such as iron, a somewhat rough porous coating is formed from loosely adherent corrosion products. In this case, a substantial amount of metal is removed, which is either deposited or dissolved in the environment. Corrosion coating reduces the rate of corrosion by varying degrees, depending on the kind of base metal and its environment, and is notably slower in room-temperature air for
aluminium,
chromium,
zinc,
titanium, and silicon (a
metalloid). The shell of corrosion product inhibits deeper corrosion, and operates as one form of passivation. The inert surface layer, termed the "native oxide layer", is usually an
oxide or a
nitride, with a thickness of a monolayer of 0.1-0.3
nm (1-3 Å) for a noble metal such as platinum, about 1.5 nm (15
Å) for silicon, and nearer to 5 nm (50 Å) for aluminium after several years.
[3][4][5]
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