TecKnow of the Day
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. Passivation can occur only in certain conditions, and is used in microelectronics to enhance silicon. 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.
When exposed to air, many metals naturally form a hard, relatively inert surface, as in the tarnish of silver; others, like iron, corrosion to a somewhat rough surface by removal 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. 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 inhibits deeper corrosion, and operates as one form 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.