What is "passivation"

n. 1 (context chemistry materials science English) The process of making a material passive (non-reactive) in relation to another material prior to using the materials together. 2 (context chemistry English) The spontaneous formation of a hard non-reactive surface film (usually an oxide or nitride) that inhibits further corrosion. 3 (cx control theory English) Conversion of a non-passive dynamical system into a passive one.

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. 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; 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.

The passivation of a spacecraft is the removal of any internal energy contained in the vehicle at the end of its mission or useful life. Spent upper stages are generally passivated after their use as launch vehicles is complete, as are satellites when they can no longer be utilized for their initial design purpose.

For satellites in geosynchronous orbit, both the International Telecommunications Union (ITU) and United Nations (UN) recommends that GEO-satellites be designed such that they can, at end of life, remove themselves to a disposal orbit some above the GEO belt, and then passivate themselves by removing any internally stored energy. "There is no [international] enforcement of these recommendations" but the majority of these GEO spacecraft are being boosted up into the recommended graveyard orbits.

The largest component of internally stored energy is generally unused propellant, although batteries are also important sources of stored energy that also require passivation.

Empirical history has shown that stored energy that is not dissipated can often become an energy source for initiating an explosion or fragmentation, resulting in unwanted manmade space debris.

Passivation may refer to:

• Passivation (chemistry)
• Passivation (spacecraft)
• Feedback passivation — a concept in nonlinear control