Why is zinc coating preferred for sacrificial protection of iron?

The correct answer is Option (2) → The reduction potential of zinc is less than that of iron.

When two metals are in contact in the presence of moisture, they form a small galvanic cell. The metal with the lower (more negative) standard reduction potential is oxidised more easily, so it becomes the anode and corrodes. The other metal becomes the cathode and is protected.

    \( \displaystyle E^\circ_{\ce{Zn^2+/Zn}}=-0.76\ \mathrm{V} \quad\quad E^\circ_{\ce{Fe^2+/Fe}}=-0.44\ \mathrm{V} \)

Zinc has the lower reduction potential, so in a zinc coating on iron (galvanisation) zinc is oxidised in preference to iron:

    \( \displaystyle \ce{Zn -> Zn^2+ + 2e^-} \)

Zinc is used up to save the iron, so it is called sacrificial protection. The iron stays protected even when the zinc layer is scratched, because the exposed iron remains the cathode. So zinc is preferred because its reduction potential is less than that of iron.

Why the other options are incorrect

• Option (1) Zinc is less reactive than iron. — Incorrect. Zinc is more reactive than iron. A less reactive coating (such as tin) protects iron only while the layer is unbroken; once scratched, the iron corrodes faster.

• Option (3) The reduction potential of zinc is higher than that of iron. — Incorrect. If it were higher, iron would be the anode and would corrode first.

• Option (4) Zinc is a better oxidizing agent than iron. — Incorrect. Zinc protects iron by losing electrons, so it acts as a better reducing agent, not an oxidising agent.

NCERT (2026-27 edition):
Corrosion – …