Why does iron rust?

Iron rusts, or corrodes, in air because it dissolves in the acid solution provided by the moisture and the carbon dioxide of the air to form hydrous oxide. In an atmosphere which is rich in sulfur compounds – for instance, the iron will rust  faster in a very smoky air.

Our industrial system is largely based on the use of iron, and its alloys. It is the world’s most important metal but rarely found in a pure state. Iron ore has to be smelted to separate the metal from other elements. The molten iron which flows from the blast furnace after smelting is called pig iron; smelting is the process of extraction of metal from its ore by heating and melting.

Cast iron is made from pig iron by resmelting it with coke in a blast furnace. The coke raises the temperature and helps remove the impurities. The molten cast iron is tapped from the base of the furnace and poured into moulds. Although it is brittle, cast iron melts and moulds easily. Because it does not distort when red hot and corrodes only slightly in water, cast iron is used for stoves, fireplaces, manhole covers, water pipes and rain gutters.

If cast iron is resmelted, the result is wrought iron – an almost pure form of iron, which is not much made today. It can be easily shaped (or wrought) and, until the 1870’s, was used for bridge building and engineering. Nowadays it is used mostly for decorative purposes. It gave away to the master-metal steel, which was first mass produced in the 1870’s and which combined the easy-to-handle qualities of wrought iron with the toughest of cast iron.