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| Corrosion of Iron |
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| Iron (or steel) corrodes more quickly than most other transition metals and readily does so in the presence of both oxygen (in air) and water to form an iron oxide. Rusting is speeded up in the presence of salt solution. |
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| Rusting is Fe(s) + O2(g)
+ H2O(I) Þ
Fe2O3 . x H2O(5)
i.e., rust is hydrated iron (III) oxide (the equation is not meant to be
balanced and the amount of water x is variable, from dry to soggy). Rusting
is oxidation because it involved iron gaining (Fe
Þ Fe2O3) or iron atoms losing electrons (Fe -
3e-
Þ Fe3+) |
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| The rusting of iron is a major problem in its use as a structural material. |
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| Iron and steel (alloy of iron) are most easily protected by paint which provides a barrier between the metal and air/water. Moving parts on machines can be protected by a water repellent oil or grease layer. |
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| This 'rusting' can be prevented by connecting iron to a more reactive metal (e.g., zinc or magnesium). This is referred to as sacrificial protection or sacrificial corrosion, because the more reactive protecting metal is preferentially oxidised away, leaving the protected metal intact. Iron or steel can also be protected by mixing in other metals (e.g., chromium) to make non-rusting alloys called stainless steel. The chromium, like aluminium, forms a protective oxide layer. |
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| Coating iron or steel with a thin zinc layer is called 'galvanizing'. The layer is produced by electrolytic deposition by making the iron/steel the negative cathode or dipping the iron/steel object in molten zinc. The zinc preferentially corrodes or oxidises to form a zinc oxide layer that doesnot flake off like iron oxide rust. Also, if the surface is scratched, the exposed zinc again corrodes before the iron and continues to protect it. |
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| Steel cans are protected by relatively unreacted tin and works well as long as the thin tin layer is complete. |
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| Aluminium does not oxidise (corrode) as quickly as its reactivity would suggest. Once a thin oxide layer of (Al2O3) has formed on the surface, it forms a barrier to oxygen and water and so prevents further corrosion of the aluminium. |
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| Aluminium is a useful structural metal. It can be made harder, stronger and stiffer by mixing it with small amounts of other metals (e.g magnesium) to make alloys. |
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| Copper and lead are both used in roofing situations because neither are very reactive. The compounds formed do not flake away as easily as rust does from iron. Lead corrodes to a white lead oxide or carbonate and copper corrodes to form a basic green carbonate (combination of the hydroxide Cu(OH)2 and carbonate CuCO3). |
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| Both metals have been used for piping but these days lead is considered too toxic. Copper is usually used as the stronger, but equally unreactive alloy brass. |
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| The Group 1 Alkali Metals rapidly corrode in air and need to be stored under oil. |
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