Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metal. The rusting of iron, tarnishing of silver, development of green coating on copper and bronze are some of the examples of corrosion. In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as anode:
Anode: 2Fe(s) → 2Fe2+ + 4e−; E°(Fe2+/Fe) = −0.44 V
Electrons released at the anodic spot move through the metal and go to another spot on the metal and reduce oxygen in the presence of H+ (which is believed to be available from H2CO3 formed due to dissolution of carbon dioxide from air into water). This spot behaves as cathode:
Cathode: O2(g) + 4H+(aq) + 4e− → 2H2O(l); E° = 1.23 V
The overall reaction is:
2Fe(s) + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2H2O(l); E°cell = 1.67 V
The ferrous ions are further oxidised by atmospheric oxygen to ferric ions, which come out as rust in the form of hydrated ferric oxide (Fe2O3.xH2O), with further production of hydrogen ions.
Which of the following cells cannot be recharged?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Primary cell
In a primary cell the cell reaction takes place only once; when the reactants are used up the cell is dead and cannot be recharged. The dry cell and the mercury cell are examples. A secondary cell can be recharged by passing current through it in the opposite direction, which reverses the cell reaction.
Why the other options are incorrect
Option (1) Nickel-cadmium cell — Incorrect. It is a secondary cell, with a longer life than the lead storage cell, so it can be recharged.
Option (2) Lead storage cell — Incorrect. It is the most important secondary cell; on charging, \( \ce{PbSO4} \) at the electrodes is converted back into Pb and \( \ce{PbO2} \).
Option (4) Secondary cell — Incorrect. A secondary cell is by definition one that can be recharged and used again.
NCERT (2026-27 edition):
2.6.1 Primary Batteries – “In the primary batteries, the reaction occurs only once and after use over a period of time battery becomes dead and cannot be reused again.”