Rusting of iron

Rusting of iron

Iron is a metal that is used extensively in making buildings, bridges, rails, machinery and many other articles.

However, the problem with iron is that, when it comes in contact with moist air, it slowly reacts with oxygen to form hydrated ferric oxide, a brown powdery substance, commonly called rust. Rust is flaky, non-sticky in nature and hence, easily crumbles from the surface of the metal. Thus, a fresh iron layer is exposed for attack of moist air to form more rust. Rust not only corrodes iron, but weakens the iron structures, thereby causing great economic loss.

Both air and water are necessary for rusting

This can be shown by an experiment.

In an absolutely clean and dry test tube [Fig.(c)] place 2 g of anhydrous calcium chloride and then a shining iron nail. Put a stopper on the test tube and place it aside for a week. The anhydrous calcium chloride is used to absorb moisture from the enclosed air. It is seen that the nail does not rust. Thus, we can conclude that dry air does not cause rusting.

Fill three/fourth of another test tube with water [Fig. (b)] which is already boiled, so that it does not contain any dissolved air. Over the water, pour 1 mL of any oil so as to cut off the atmospheric air. In the water place a shining nail and put a stopper on the test tube. After a week, it is seen that the nail does not rust. Thus, we conclude that pure water does not cause rusting.

Place a shining nail in a test tube [Fig.(a)] and half fill it with tap water. Stopper the test tube. After a week it is seen that the nail rusts. Thus, we can conclude that air and water (moist air) both are necessary for rusting.

Prevention of rusting

Rusting of iron can be prevented, if moist air is not allowed to come in contact with its surface. Following methods are employed in the prevention of rusting :

By painting: Articles, such as iron doors, windows, bodies of buses, cars, motorcycles, etc., are first given a coating of red lead oxide, followed by a coat of paint of desired colour. This effectively cuts out the moist air and prevents rusting.

By coating with red lead oxide paint (Pb₃O₄) or tar: The underside of ships, bridges, electric poles, etc., is coated with tar or red lead oxide paint, which does not allow the moist air to come in contact with iron. Thus, rusting is prevented. It is a fairly inexpensive method and is widely used.

By enamelling: Enamelling is the process of baking a mixture of silicates on the surface of iron at high temperature. Articles, such as bodies of cooking stoves, refrigerators, cups, plates, wash basins, etc., are enamelled.

By oil and grease: Moving parts of machines cannot be given a protective layer, because it wears off quickly. These moving parts of the machines are protected from rusting by coating them with a thin layer of oil or grease, which not only prevents them from rusting but also lubricates them.

Plastic coating: Iron furniture and fixtures are given a thin coating of plastic, which cuts off the moist air and prevents rusting.

By galvanising: In this process iron sheets are dipped in molten zinc and then passed through heavy rollers, when zinc metal forms a protective layer over iron. The galvanised iron is extensively used in making roofs of sheds, buckets, tubs, iron trunks and suitcases.

By tinning: In this process iron sheets are dipped in molten tin and then passed through hot and heavy rollers, when tin forms a protective layer over iron. Tinned iron sheets are extensively used for making containers for edible substances, such as fruits, vegetables, meat, etc.

By coating with chromium or nickel electrolytically: This method is highly expensive, but gives a very bright finish. Chromium or nickel is deposited on iron articles by electroplating. Articles such as, bicycle handles, rims, bumpers of the cars, etc., are coated with nickel or chromium.

By converting iron into stainless steel: When 12 to 20% chromium is incorporated in iron with 0·1 to 0·7% of carbon, it develops a special property due to which it does not rust in moist air. Such a sample of iron is called stainless steel. Stainless steel is used for making surgical instruments, kitchen utensils, cutlery, tools, etc.