Question

Which of the following is true?
A.Rusting will take place in absence of water.
B.Rusting will take place in absence of air.
C.Rusting will not take place in the presence of water and air.
D.None of the above.

Answer 1

When acid comes into contact with metals, metal starts forming its ions if another reaction is also occurring which consumes electrons formed in the former reaction then rusting is favoured.

Complete step-by-step answer: Rusting is an electrochemical phenomenon in which iron acts as anode and non-metallic gases cathode. At anode oxidation takes place and at cathode reduction takes place. An electrolyte is ${H_2}C{O_3}$ which is formed when water and carbon dioxide come in close proximity on the surface of rough iron.
Rusting in coastal areas increases as the concentration of salt and moisture in air comparatively greater. Rusting also increases in presence of acid as it provides ${H^ + }$ ion that will enhance the cathodic reactions while decreasing the presence of base as it will absorb ${H^ + }$.
Hence for rusting both water and air is required. So the correct option is (D).
Rusting can prevent by following methods;
Barrier protection- By using oil, grease, paints electroplating with tin and nickel we can prevent rusting. Tin is most economical and nickel is most durable.
By sacrificial or cathodic protection- Electroplating by zinc is known as galvanisation which is most effective.
By using anti rust solution- solution of chromates and phosphate of alkali metals is known as anti rust solution which is alkaline in nature, so stop cathodic reaction as form a passive layer of iron chromates and iron phosphate.

Note: The Reaction that takes place on the surface of iron in presence of water and iron is given as
At anode –
$Fe \to F{e^{ + 2}} + 2{e^ - }$
At cathode –
$\dfrac{1}{2}{O_2} + {H^ + } + {e^ - } \to O{H^ - }$
$F{e^{ + 2}}$ in presence of air further oxidised into $F{e^{ + 3}}$ which can react with chromates and phosphate to form a passive layer.