Question

To prevent a piece of iron from rusting, which one of the following metals will provide the best coating?
A) Zinc
B) Copper
C) Aluminium
D) Lead

Answer 1

Hint: Rusting is an electrochemical process. It is the process of conversion of metals into their undesirable compounds (usually oxides) by reaction with moisture and other gases present in the atmosphere. We can prevent it by preventing the surface of the metal from being in contact with moisture, carbon dioxide etc.

Complete step by step answer:
There are a number of theories about the mechanism of rusting. The most widely accepted theory is the electrochemical theory. According to this theory, the impure iron surface behaves like a small electrochemical cell in the presence of water containing dissolved oxygen or carbon dioxide. Such a cell is also called a corrosion cell or corrosion couple. In these miniature corrosion cells, pure iron acts as anode and impure surfaces act as cathode. Moisture having dissolved oxygen or carbon dioxide in it constitutes electrolytic solution. The net reaction can be shown as;
\[{\text{Fe + 2}}{{\text{H}}^{\text{ + }}}{\text{ + }}\dfrac{{\text{1}}}{{\text{2}}}{{\text{O}}_{\text{2}}} \to {\text{F}}{{\text{e}}^{{\text{2 + }}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}\]
There are several methods for preventing metals from rusting. One of the main methods is sacrificial protection. In this method, the surface of iron is covered with a layer of more reactive metal than iron in the reactivity series like zinc. This active metal loses electrons in preference to iron and hence, prevents the rusting of iron. This process of covering the metal surface with a layer of zinc is called galvanization. Galvanized iron sheets maintain their shine due to the formation of a thin protective layer of basic zinc carbonate due to the reaction between zinc, oxygen, carbon dioxide and moisture in air.

So, the correct answer is Option A .

Note: The covered metal in the galvanizing process gets consumed in due course of time, but so long as it is present, even the nearly uncovered surfaces of iron do not get rusted. In this way, scratches in the protective layer of this type are not harmful.

Rust is formed by moisture, carbon dioxide and oxygen of air. It is a non-sticking brown coloured material which can be easily removed by scratching. We can prevent rusting if we provide some coating over the surface of iron which prevents any one of the factors from being in contact with the surface of iron.