Question

The iron pillar near the Qutub Minar in Delhi is famous for the following facts. Which of these facts is responsible for its long stability?

• A. It is more than $7$ metres high.
• B. It weighs about $6000$ kg.
• C. It was built more than $1600$ years ago.
• D. It has not rusted after such a long period.

Answer 1

The correct option is D

It has not rusted after such a long period.

Explanation for the correct option

Option D:

1. The Qutub Minar of Delhi has not rusted after such a long period.
2. The iron pillar is majorly made of wrought iron and the high presence of phosphorus and absence of sulfur, and magnesium in the iron are the main reasons for its longevity.
3. The corrosion resistance results from an even layer of crystalline iron(III) hydrogen phosphate hydrate forming on the high-phosphorus-content iron, which serves to protect it from the effects of the Delhi climate.
4. Rusting is a redox reaction that forms a layer of iron oxides on the surface of iron resulting in the gradually decaying of iron in the presence of air and water(present as moisture in the atmosphere).

Hence, option D is correct.