Passivation, in physical chemistry and engineering, is a material becoming "passive" in relation to being less affected by environmental factors such as air or water. It means a shielding outer layer of corrosion which can be demonstrated with a micro-coating or found occurring spontaneously in nature. Passivation is useful in strengthening, and preserving the appearance of, metallics. As a technique, passivating is using a light coat of material such as metal oxide to create a shell against corrosion. Passivation can only occur in certain conditions, and is used in microelectronics to enhance silicon.[1]
In air, almost all metals form a hard inert surface naturally. The reduction of the corrosive rate will vary individually in various shells, but is most notably pronounced in aluminium, zinc, titanium, and silicon (a metalloid). The shell inhibits deeper corrosion, and that is the key factor. The layer is usually an oxide or nitride with a thickness of nanometers.
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The conditions necessary 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.
In the area of microelectronics, the formation of a strongly adhering passivating oxide is important to the performance of silicon.
In the area of photovoltaics, a passivating surface layer such as silicon nitride, silicon dioxide or titanium dioxide can reduce surface recombination - a significant loss mechanism in solar cells.
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 most environments. Aluminium alloys, however, offer little protection against corrosion. There are three main ways to passivate these alloys: alclading, chromate conversion coating and anodizing. Alclading is the process of metallurgically bonding a thin layer of pure aluminium to the aluminium alloy. Chromate conversion coating is a common way of passivating not only aluminum, but also zinc, cadmium, copper, silver, magnesium, and tin alloys. Anodizing forms a thick oxide coating. This finish is more robust than the other processes and also provides good electrical insulation, which the other two processes do not.
For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and peroxide alternating with deionized water. The nitric acid and peroxide oxidizes and dissolves any impurities on the inner surface of the container, and the deionized water rinses away the acid and oxidized impurities.
Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using phosphoric acid and further protected by surface coating. 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. Older, less-effective but chemically-similar electrochemical conversion coatings included black oxiding, historically known as bluing or browning. Ordinary steel forms a passivating layer in alkali environments, as reinforcing bar does in concrete.
Stainless steels are corrosion-resistant by nature, which might suggest that passivating them would be unnecessary. However, stainless steels are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because grain boundaries or embedded bits of foreign matter (such as grinding swarf) allow water molecules to oxidize some of the iron in those spots despite the alloying chromium. This is called rouging. Some grades of stainless steel are especially resistant to rouging; parts made from them may therefore forgo any passivation step, depending on engineering decisions.
A typical passivation process of cleaning stainless steel tanks involves cleaning with sodium hydroxide and citric acid followed by nitric acid (up to 20% at 120 °F) and a complete water rinse. This process will restore the film, remove metal particles, dirt, and welding-generated compounds (e.g. oxides).[3]
One aircraft manufacturer's corporate standard for passivation of stainless steel parts involves coating or submerging them in nitric acid solution for 40 to 60 minutes, then wiping them with water-soaked cloth.
Nickel can be used for handling elemental fluorine, owing to the formation of a passivation layer of nickel fluoride. This fact is useful in water treatment and sewage treatment applications.