Which of the statements given below are correct with reference to corrosion:
(A) Corrosion covers the surfaces of metallic objects with oxides or other salts.
(B) In corrosion, a metal is oxidised by the loss of electrons to oxygen and results in the formation of oxides.
(C) Rust is hydrated ferric oxide.
(D) A sacrificial electrode is used to save an object from corrosion.
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B), (C) and (D)
Corrosion is the slow destruction of a metal by its surroundings; for iron (rusting) it needs both water and oxygen and works as a tiny electrochemical cell. At one spot iron is oxidised and acts as the anode; the electrons travel through the metal to another spot, the cathode, where oxygen is reduced in the presence of \( \ce{H+} \) (from carbonic acid in the water):
\( \displaystyle \text{Anode: } \ce{2Fe(s) -> 2Fe^2+ + 4e^-} \)
\( \displaystyle \text{Cathode: } \ce{O2(g) + 4H^+(aq) + 4e^- -> 2H2O(l)} \)
\( \displaystyle \text{Cell reaction: } \ce{2Fe(s) + O2(g) + 4H^+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \)
The \( \ce{Fe^2+} \) ions are then oxidised further by air to \( \ce{Fe^3+} \), which separates as rust, \( \ce{Fe2O3.xH2O} \).
(A) Corrosion covers the surfaces of metallic objects with oxides or other salts. — Correct. Rust on iron, the black tarnish on silver and the green coating on copper and bronze are such layers.
(B) In corrosion, a metal is oxidised by the loss of electrons to oxygen and results in the formation of oxides. — Correct. The metal gives up electrons (the anode reaction above), oxygen takes them up, and oxides form.
(C) Rust is hydrated ferric oxide. — Correct. Rust is \( \ce{Fe2O3.xH2O} \), iron(III) oxide with water of hydration.
(D) A sacrificial electrode is used to save an object from corrosion. — Correct. A more reactive metal such as Mg or Zn connected to the iron corrodes itself instead, because it is oxidised more easily, and so protects the object.
NCERT (2026-27 edition):
Corrosion – “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. … 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) and with further production of hydrogen ions. … An electrochemical method is to provide a sacrificial electrode of another metal (like Mg, Zn, etc.) which corrodes itself but saves the object.”