Another coating that protects steel is to use a metal that is further down the activity series, a metal that does not oxidize in the air. The ideal metals are gold, silver and copper, which are towards the bottom of the activity series.

Galvanized buckets and dustbins that used to be commonplace did not rust even though they were made from steel. The metal surface of these containers appears to have irregular shiny crystals.

Galvanizing is the process by which a metal surface is given a coating of zinc. Zinc melts at 420°C, and objects to be galvanized are chemically cleaned and then dipped into a bath of molten zinc. The process of galvanizing does seem to work; it is more expensive than conventional paint protection but is finding increasing use in the motor industry.

Where does zinc come in our activity series? We have argued that to coat steel with a metal that does not oxidize in air makes sense, so it seems odd to use a metal that oxidizes even more readily than does iron.

Zinc, like aluminium, oxidizes easily. It also has an oxide that clings well to the metal and is not porous to water or oxygen. So the surface of the steel is protected by zinc metal overlain with zinc oxide. But there is a further advantage to using zinc. If the zinc coating is damaged and iron exposed, one might expect the iron to rust just as if a protective paint surface had been damaged. However, as zinc is more easily oxidized than iron, zinc is oxidized in preference and the iron remains in the metallic form as we can see here.

Representation of the protection of a galvanized surface. Electrons produced when zinc is oxidized to Zn²⁺ inhibit further oxidation of the underlying iron to the metal cations.

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