Question

Why is magnesium block is attached to an underground iron pipeline?

Answer 1

Answer: To prevent iron from getting corroded.

Explanation:

The iron is connected to a more reactive metal such as magnesium directly or through a wire where magnesium acts as anode and iron acts as cathode. The anode is used up due to the oxidation of metal to its ions due to loss of electrons.

$E^0_{[Mg^{2+}/Mg]}= -2.38V$

$E^0_{[Fe^{2+}/Fe]}= -0.49V$

Where both $E^0$ are standard reduction potentials.

The electrons are transferred through the wire to $H^+$ ions present around the iron object and thus protect it from rusting. As iron acts as cathode, the method of protection is called as cathodic protection.