Rusting is the name given to the oxidation of iron or steel in damp air. It is also called Corrosion. 


Rust is hydrated iron (III) oxide. It is a soft, crumbly solid and hence weakens the structure of iron and steel. During rusting, iron reacts with oxygen to form brown iron (III) oxide.

At the same time the iron (III) oxide reacts with water to form hydrated iron (III) oxide (or rust):

Note: The x in the equation indicates that the number of water molecules in the hydrated iron (III) oxide can vary. So, both oxygen and water are needed to cause rusting of iron.

Rusting is a serious economic problem. Large sums of money are spent each year to replace damaged iron and steel structures, or protecting structures from such damages. Rusting of bridges, corrugated iron sheets on house roofs, containers, articles, etc. require an expenditure of big sums of money as well as labour for replacement. 

Rust weakens structures such as car bodies, iron railings, and ships' hulls, and shortens their useful life. Preventing it can cost a lot of money. All efforts must be made to stop iron or steel items from rusting. This can be achieved if we know the conditions necessary for iron to rust.

The Conditions Necessary for Iron to Rust

When iron is left in contact with both water and oxygen (or air), it reacts to form hydrated iron (III) oxide. Iron will not rust on exposure to dry air or air-free water (water that has been boiled to expel all dissolved air) only. 

However, iron will easily and readily rust in water that has dissolved air in it. In figure 6.8, only the iron nail that is in contact with both water and air rusts. Therefore, rusting will only occur in the presence of both water and oxygen. If one of the two conditions is excluded, in one way or another, rusting will not take place at all.

Findings
Nails in tube 1 will rust. Nails in tubes 2 and 3 will not rust.

Reasons
In tube 1, nails are in contact with both water and air (oxygen). In tube 2, the water has been boiled to expel the dissolved air. In addition, any air above the water is prevented from dissolving in boiled water by a layer of oil. So, the nails are completely shielded away from air. Therefore, rusting is impossible. In tube 3, nails are in contact with air only. 

The moisture present in air is absorbed by anhydrous calcium chloride. Any moisture that might have been absorbed by the anhydrous calcium chloride is prevented from reaching the nails by a tuft of cotton wool. The cotton wool also absorbs some moisture directly from the air. 

Therefore, tube 3 will always carry dry air (moisture-free air). Hence, no rusting of iron nails occurs. This experiment demonstrates the fact that for iron to rust, both water and air (oxygen) must be present. If one of these conditions is controlled, no rusting can take place.

Similarity between rusting and burning

Chemically, rusting and burning are similar processes in that they both require oxygen. 

Consider the burning of magnesium to give magnesium oxide.

In this process, magnesium combines with the oxygen of the air to form magnesium oxide.
During rusting, iron combines with oxygen of the air in the presence of water to form brown hydrated iron (III) oxide, "rust."

In addition, the two processes, burning and rusting, are exactly similar in that they both generate heat. The only difference is in the time required for each of the two processes to take place. During rusting heat is given out, but without being noticed because of its slower rate of production. Burning produces noticeable heat and light.

The Different Methods of Preventing Iron from Rusting

We have learned that for iron to rust there must be direct contact between the iron and both water and oxygen from the air. Therefore, in order to stop rusting we must protect iron from either water (moisture) or oxygen (air) or both. The following are some of the methods used to prevent iron from rusting:

Painting

Painting the iron article creates a waterproof and airproof cover over the surface of the iron. This method is widespread for objects ranging in size from ships and bridges to garden gates. Paints that contain lead or zinc are mostly used. These paints are especially good for preventing rusting. 

For example, "red lead" paints contain an oxide of lead.

As oxygen and water cannot reach the iron, it does not rust. However, if the paint layer is scratched off rusting may occur. So, regular repainting is necessary to keep this protection intact.

Oiling and greasing

The oiling and/or greasing of the moving parts of machinery forms a protective film, preventing rusting. Moving parts cannot be painted since the paint layer can be easily scratched off during movement. Again, the treatment must be repeated to continue the protection.

Plastic coating

Steel is coated with plastic for use in garden chairs, refrigerators, bicycle baskets, dish racks, etc. The plastic PVC (polyvinyl chloride), a trade name for polychloroethene, is often used for this purpose. Plastic is cheap and can be made to look attractive.

Electroplating

Electroplating is the coating of one metal with a layer of another metal by means of electrolysis, where the metal to be coated is the cathode and the coating metal the anode.
An iron or steel object can be electroplated with a layer of chromium or tin to protect against rusting.

A tin can is made of steel coated on both sides with a fine layer of tin. Tin is used because it is unreactive and non-toxic. However, if protective layer is broken, then the steel beneath will begin to rust. So, proper handling of tin-plated items is needed.

Galvanizing

An iron object may be covered with a layer of zinc. This is called galvanizing. Even if the zinc is scratched to expose the iron, the iron does not rust. This is because zinc is higher in the reactivity series than iron. So, zinc reacts with water and oxygen in preference to iron.
The zinc layer can be applied by several different methods. 

These include electroplating or dipping the object into molten zinc. When an iron or steel article is dipped into molten zinc and then removed, it becomes coated with a thin layer of zinc. The zinc forms a protective coat over the surface of iron. This process is used for dustbins, car bodies, barbed wires and motorway crash barriers.

Sacrificial protection

This is a method of rust protection in which blocks of a metal more reactive than iron are attached to the iron surface. Zinc and magnesium are more reactive than iron. When blocks of zinc or magnesium are attached to the hull of a steel ship or oil rig, it corrodes in preference to iron. 

This is called sacrificial protection because the zinc or magnesium is sacrificed to protect the iron. When the blocks are nearly eaten away, they can be replaced by fresh blocks. Underground gas and water pipes are connected by wire to blocks of magnesium to obtain the same protection.

It is not necessary to cover the whole surface of a steel article with the more reactive metal for sacrificial protection to work. A ship may have magnesium blocks riveted to its hull every few metres to prevent rusting of the whole hull.

Alloying

Alloys are mixtures of metals. For example, iron can be mixed with small quantities of much less reactive metals to form an alloy called stainless steel. Stainless steel contains iron mixed with chromium, nickel and manganese. Stainless steel does not rust. It also has a very attractive appearance. It is used to make cutlery and kitchen equipment.

Use of silica gel

Silica is a common name for silicon dioxide (SiO2). Silica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid. It is used as a desiccant, which absorbs moisture to prevent rusting of iron items or articles. Most often, a small bag of silica gel is put inside bags or boxes used for storing or carrying iron items to absorb any moisture that may cause rusting.