Passivation (chemistry)

passivationpassivatingpassivation layerpassivatedpassivatepassivepassivantpassivatespassive layerpassivity
Passivation, in physical chemistry and engineering, refers to a material becoming "passive," that is, less affected or corroded by the environment of future use.wikipedia
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Aluminium

aluminumAlall-metal
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 of passivation. Pure aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in many environments.
Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation.

Corrosion

corrosion resistancecorrodecorrosive
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.
As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance.

Oxide

oxidesmetal oxideO
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.
For example, aluminium foil develops a thin skin of Al 2 O 3 (called a passivation layer) that protects the foil from further corrosion.

Chromium

Crchromechromium(III)
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 of passivation.
Chromium metal left standing in air is passivated by oxidation, forming a thin, protective, surface layer.

Titanium

Tititanium oretitanian
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 of passivation.
It is, however, slow to react with water and air at ambient temperatures because it forms a passive oxide coating that protects the bulk metal from further oxidation.

Zinc

ZnZn 2+ zinc alloy
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 of passivation. Chromate conversion is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys.
The surface of the pure metal tarnishes quickly, eventually forming a protective passivating layer of the basic zinc carbonate, by reaction with atmospheric carbon dioxide.

Corrosion inhibitor

corrosion inhibitorsrust inhibitorinhibitor
Some corrosion inhibitors help the formation of a passivation layer on the surface of the metals to which they are applied.
A common mechanism for inhibiting corrosion involves formation of a coating, often a passivation layer, which prevents access of the corrosive substance to the metal.

Iron

FeFe 2+ Fe(III)
In the mid 1800s, Christian Friedrich Schönbein discovered that when a piece of iron is placed in dilute nitric acid, it will dissolve and produce hydrogen, but if the iron is placed in concentrated nitric acid and then returned to the dilute nitric acid, little or no reaction will take place.
Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.

Aluminium oxide

aluminaaluminum oxideAl 2 O 3
Pure aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in many environments.
Metallic aluminium is very reactive with atmospheric oxygen, and a thin passivation layer of aluminium oxide (4 nm thickness) forms on any exposed aluminium surface.

Anodizing

anodizedanodized aluminumanodised
Generally, there are two main ways to passivate aluminum alloys (not counting plating, painting, and other barrier coatings): chromate conversion coating and anodizing.
Anodizing (also spelled anodising in British English) is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.

Chromate conversion coating

zinc yellowchromate conversionchromating
Generally, there are two main ways to passivate aluminum alloys (not counting plating, painting, and other barrier coatings): chromate conversion coating and anodizing.
Chromate conversion coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, magnesium, and tin alloys.

Pourbaix diagram

(Pourbaix)Eh-pH diagramspotential-pH
The conditions necessary (but not sufficient) for passivation are recorded in Pourbaix diagrams.
Passivation occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of aluminium because of the alumina layer formed on its surface when exposed to air.

Nitric acid

nitricHNO 3 aqua fortis
In the mid 1800s, Christian Friedrich Schönbein discovered that when a piece of iron is placed in dilute nitric acid, it will dissolve and produce hydrogen, but if the iron is placed in concentrated nitric acid and then returned to the dilute nitric acid, little or no reaction will take place.
The formation of this protective layer is called passivation.

Magnesium

MgMg 2+ magnesian
Chromate conversion is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys.
The free element (metal) can be produced artificially, and is highly reactive (though in the atmosphere, it is soon coated in a thin layer of oxide that partly inhibits reactivity – see passivation).

Copper

CuCu 2+ cupric
Chromate conversion is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys.
Copper does not react with water, but it does slowly react with atmospheric oxygen to form a layer of brown-black copper oxide which, unlike the rust that forms on iron in moist air, protects the underlying metal from further corrosion (passivation).

Bluing (steel)

bluedbluingblued steel
Older, less-effective but chemically-similar electrochemical conversion coatings included black oxidizing, historically known as bluing or browning.
Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish.

Tin

SnGray tintinfoil
Chromate conversion is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys.
A protective oxide (passivation) layer prevents further oxidation, the same that forms on pewter and other tin alloys.

Parkerizing

parkerizedphosphatephosphating
As the uncoated surface is water-soluble, a preferred method is to form manganese or zinc compounds by a process commonly known as parkerizing or phosphate conversion.
Passivation can be used for protecting other metals.

Stainless steel

stainless-steelstainlessstainless steels
Stainless steels are corrosion-resistant, but they are not completely impervious to rusting.
In comparison, stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in air and even the small amount of dissolved oxygen in water.

Plating

platedsilver plateplate
Generally, there are two main ways to passivate aluminum alloys (not counting plating, painting, and other barrier coatings): chromate conversion coating and anodizing.
Corrosion resistance relies on what is called the passivation layer, which is determined by the chemical composition and processing, and is damaged by cracks and pores.

Steel

steel industrysteelworkersteels
Ordinary steel forms a passivating layer in alkali environments, as reinforcing bar does in concrete.
To inhibit corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel.

Oxygen

OO 2 molecular oxygen
Pure aluminium naturally forms a thin surface layer of aluminium oxide on contact with oxygen in the atmosphere through a process called oxidation, which creates a physical barrier to corrosion or further oxidation in many environments.
The surface of most metals, such as aluminium and titanium, are oxidized in the presence of air and become coated with a thin film of oxide that passivates the metal and slows further corrosion.

Nickel

NiNi 2+ Nickel (Ni)
Nickel can be used for handling elemental fluorine, owing to the formation of a passivation layer of nickel fluoride.
Pure nickel, powdered to maximize the reactive surface area, shows a significant chemical activity, but larger pieces are slow to react with air under standard conditions because an oxide layer forms on the surface and prevents further corrosion (passivation).

Fluorine

Ffluorofluorinated
Nickel can be used for handling elemental fluorine, owing to the formation of a passivation layer of nickel fluoride.
Alkali metals cause explosions and alkaline earth metals display vigorous activity in bulk; to prevent passivation from the formation of metal fluoride layers, most other metals such as aluminium and iron must be powdered, and noble metals require pure fluorine gas at 300–450 °C (575–850 °F).

Rust

rustingferruginouscorrosion
The passivation process removes exogenous iron, creates/restores a passive oxide layer that prevents further oxidation (rust), and cleans the parts of dirt, scale, or other welding-generated compounds (e.g. oxides).
As with other metals, like aluminium, a tightly adhering oxide coating, a passivation layer, protects the bulk iron from further oxidation.