Why is this tractor rusting away?
The tractor above was once a very useful piece of farm equipment. But it got old, probably broke down a lot, and you eventually couldn’t get parts for it. You bought a new, more efficient tractor with all the latest gadgets. There is not much of a market for old tractors, so it sits parked out of the way, exposed to the weather, and quietly rusting away.
Rust is a combination of several different oxides of iron. The equations below show the steps involved in one of the many processes of rust formation.
Iron is first oxidized to iron(II) ions by oxygen. In the second step, the iron(II) ions are further oxidized and combine with water and oxygen gas to produce a hydrated form of iron(III) oxide known as rust. Rusting is one of many example of corrosion. Corrosion is the deterioration of metals by redox processes. Corrosion causes enormous amounts of damage to buildings, bridges, ships, cars, and other objects. It has been estimated that corrosion costs the U.S. economy over 100 billion dollars each year. A great amount of time and effort is spent to try to limit or prevent corrosion.
Some metals, such as gold and platinum, do not corrode easily because they are very resistant to oxidation by common substances. Some other metals begin to be oxidized, but are further protected from additional corrosion by a coating formed on the surface. Aluminum reacts with oxygen to form aluminum oxide, which remains tightly packed on the surface. The aluminum oxide prevents the interior of the aluminum from corroding. Not all corrosion is a result of reaction with oxygen. Copper corrodes by reaction with carbon dioxide to form copper(II) carbonate. This distinctive green compound is also called patina and prevents the copper underneath from further corrosion (see Statue of Liberty in Figure below).
Statue of Liberty.
Conversely, the iron oxides that form during the corrosion of iron form a surface that flakes off easily rather than remaining tight to the surface. This allows the iron to corrode completely through until the iron object is destroyed.
One way to prevent corrosion is to protect the surface of the metal. Covering the surface of the metal object with paint or oil will prevent corrosion by not allowing oxygen to contact it. Unfortunately, scratches in the paint or wearing off of the oil will allow the corrosion to begin. Corrosion-sensitive metals can also be coated with another metal that is resistant to corrosion. A “tin can” is actually made of iron coated with a thin layer of tin which protects the iron.
Corrosion can also be controlled by connecting the object to be protected to another object made of a metal that is corroded even more easily. When an iron nail is wrapped with a strip of zinc and exposed to water, the zinc (being a more active metal than iron) is oxidized while the iron remains intact. This technique, called cathodic protection, is commonly used to prevent the hulls of steel ships from rusting. Blocks of zinc are attached to the underside of the hull (see Figure below). The zinc blocks preferentially corrode, keeping the hull intact. The zinc blocks must be periodically replaced to maintain the protection of the iron.
Small blocks of zinc are attached to the steel hull of this ship.
- What is corrosion?
- How does aluminum avoid complete corrosion?
- What is the purpose of cathodic protection?