Which of the following metals can be used as a sacrificial electrode to protect an iron object from corrosion?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Mg
A sacrificial electrode is a metal joined to the iron object that is oxidised more easily than iron, so it corrodes itself and saves the iron. Its standard electrode potential must be lower than that of \( \ce{Fe^2+/Fe} \) (−0.44 V).
For magnesium, E°(\( \ce{Mg^2+/Mg} \)) = −2.36 V, far below −0.44 V. Magnesium is therefore oxidised in place of iron:
\( \displaystyle \ce{Mg(s) -> Mg^2+(aq) + 2e^-} \)
Why the other options are incorrect
Option (1) Cu — Incorrect. E°(\( \ce{Cu^2+/Cu} \)) = +0.34 V, above iron, so copper is harder to oxidise than iron and cannot corrode in its place.
Option (2) Sn — Incorrect. Tin is used to cover the surface of iron, but with E°(\( \ce{Sn^2+/Sn} \)) = −0.14 V it is harder to oxidise than iron and does not corrode in place of iron.
Option (4) Ag — Incorrect. E°(\( \ce{Ag^+/Ag} \)) = +0.80 V, well above iron, so silver is far harder to oxidise than iron and cannot corrode in its place.
NCERT (2026-27 edition):
2.8 Corrosion – “… An electrochemical method is to provide a sacrificial electrode of another metal (like Mg, Zn, etc.) which corrodes itself but saves the object.”