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Prevention of rusting
Since rust is only formed in the presence of oxygen and water, iron can be protected from rusting by putting a coating on the surface.
A. Paint or grease
Painting is the most cost effective method for large iron object but if the painted surface is scratched, iron will be vulnerable. Grease or oil is used in the moving part of machinery where the paint will be scratched.
B. Galvanising (zinc dipping)
Iron is dipped into a tank of molten zinc or molten zinc is spray onto iron so that the surface of the iron is coated with a layer of zinc. The process is called galvanising or galvanization. The iron is called galvanised iron. Although zinc also reacted with oxygen in the air to form zinc oxide, zinc oxide has a very tight structure and protects the zinc underneath from further corrosion. The body of most automobiles are galvanized before the paint is applied. This make it more resistant to rusting.
Since zinc is toxic, it is not used in protecting the food can from rusting. Instead, a layer of tin is coated on the surface of iron can to exclude the air and water. However, if the tin-plating is scratched and the iron underneath is exposed, the tin-plated can will rust even faster than an ordinary iron can. This is because iron is more reactive than tin and iron will lose electrons through tin quickly.
D. Electroplating of other metal
Chromium is another metal which is coated on iron to prevent rusting. Chromium is frequently used because of its silvery appearance. e.g. Most water taps are electroplated with chromium.
b) Sacrificial Protection
Iron is first changed to iron(II) ion in the rusting process. If the formation of iron(II) ion can be suppressed, iron can be protected from rusting.
Fe(s) <==> Fe2+ (aq) + 2e-
A reactive metal loses electron more readily than a less reactive metal. By connecting iron to a more reactive metal, formation of Fe2+ (aq) can be retarded. Once a Fe2+ (aq) ion is formed, it will acquire the electrons from the more reactive metal and changes back to Fe. Consequently, iron is protected from corrosion but the more reactive metal is sacrificed instead.
This method is preferable for the iron object which is hard to be repainted e.g. iron pipe under the ground, hull of ship.
Sacrificial protection by galvanization
Besides isolating iron from air and water, Zn(s) also protects iron by sacrificial protection because Zn(s) is more reactive than Fe(s).
For the same reason, the body of a car is always connected to the negative terminal of the car battery. This make the car body negatively charged and less vulnerable to rusting.
Pure iron rust easily in moist air. But if appropriate amounts of carbon, chromium and nickel are added, iron will become stainless steel which is more resistant to rusting.資料來源： AL notes