What Is Metal Oxide Passivation?
Upon exposure to oxygen, many metals will oxidize. For some metals, this oxide layer "passivates" the underlying metal in the sense that it forms a protective layer that prevents further oxidation. For other metals, the oxidation continues until the entire piece of metal has been oxidized.
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Metals
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About 80 percent of the known elements in the periodic table are metals. They are characterized by their good conduction of heat and electricity, their shininess and their malleability (their ability to be hammered or machined into different shapes).
Metal Oxides
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Oxidation involves a transfer of electrons from the metal to the oxygen molecules. As a result, the metal forms an oxide layer at its surface. Some metals, such as gold and platinum, are highly resistant to oxidation, whereas others, such as iron, oxidize readily in air, particularly if moisture is present.
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Passivation
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Some metals, upon oxidation, form a passivation layer of metal oxide at their surface. This is essentially a layer of metal oxide at the surface of the metal that is only a few atoms deep. How tightly that oxide layer binds to the underlying (unoxidized) metal determines whether the metal corrodes (such as iron, which rusts) or passivates (as with aluminum).
The metal oxides of elements like iron do not bind well to the metal surface. The metal oxide layer tends to separate from the boundary between the metal oxide and metal. Eventually, the oxide layer will flake off, exposing a new layer of metal to oxidation. This effect is technically called corrosion. Metals like iron tend to "pit" from corrosion, and the oxide (rust) layer will eventually permeate the entire piece of metal. This is why iron sheet metal (such as in a car body panel) tends to form rust holes. In general, corrosion compromises the structural integrity of the metal.
The oxide layers of metals such as aluminum, copper, magnesium, titanium and zinc, however, bind tightly to the metal's surface. As a result, the oxide layer rarely penetrates more than a few atoms deep. The oxide layers on such metals are therefore said to "passivate" the metal against further oxidation. In some cases, the oxide layer is deliberately formed by an electrochemical process so that its thickness and uniformity can be carefully controlled.
Uses
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Metals that form passivation layers tend to be used where the structural integrity of the metal is paramount, such as in an aircraft. Passivated metals are also used in equipment that operates in severe (highly corrosive) environments, such as on boats and oil rigs.
Fun Fact
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According to the American Galvanizers Association, the annual direct costs of corrosion in the United States exceeds $275 billion.
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References
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- Photo Credit rust image by Stanislav Halcin from Fotolia.com
