Page 33 - Curriculum Visions Dynamic Book

Galvanising
Galvanising is the process of coating iron with a surface of zinc metal. For some applications the metal is dipped in a bath of molten zinc; for others the zinc is applied in an electrolytic bath.
Zinc protects iron because it forms
a gastight oxide. If the zinc coating is scratched, it still protects the iron because zinc is more reactive than iron.
anode: the negative terminal of a battery or the positive electrode of an electrolysis cell.
cathode: the positive terminal of a battery
or the negative electrode of an electrolysis cell.
cathodic protection: the technique of making
the object that is to be protected from corrosion into the cathode of a cell. For example, a material, such as steel, is protected by coupling it with a more reactive metal, such as magnesium. Steel forms the cathode
and magnesium the anode. Zinc protects steel in the same way.
electrolysis: an electrical–chemical process that
uses an electric current to cause the break up of
a compound and the movement of metal ions in a solution. The process happens in many natural situations (as for example in rusting) and is also commonly used
in industry for purifying (refining) metals or for plating metal objects with a fine, even metal coating.
electroplating: depositing a thin layer of a metal
onto the surface of another substance using electrolysis.
reactivity: the tendency of a substance to react with other substances. The term is most widely used in comparing the reactivity of metals. Metals are arranged in a reactivity series.
Tin-plating
Steel sheet is dipped in tin to make tin-plate. Tin is an unreactive metal, which resists chemical reactions that would result in corrosion. However, because iron is more reactive than tin, if the tin-plate is scratched, the iron will corrode very quickly.
 Galvanised wire used in a fence.
Cathodic protection
Cathodic protection is a form of corrosion- proofing. It relies on the fact that when two metals are placed in a liquid, they behave like a battery and electricity flows. During this process metal is lost from the negative electrode, called an anode, and it corrodes, or oxidises; at the positive electrode
(called the cathode), reduction occurs and corrosion is impossible. By choosing metals carefully, a chemist can decide which will be the corroding anode and which the protected cathode.
A block of magnesium, zinc or other suitable metal is fastened on to the hull of a steel ship, or buried in the ground with the steel pipe and connected to it by conducting cables. Because magnesium and zinc are more reactive than iron, the blocks will
act as anodes and corrode instead of
the ship or pipe. Of course, they have to
be replaced from time to time as part of routine maintenance.
 Tin-plated can.
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