Which one of the following elements corrodes rapidly?

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Aluminium

Iron

Zinc

Silver

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This question was previously asked in

UPSC NDA-1 – 2017

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Iron corrodes rapidly, especially in the presence of oxygen and moisture, through a process known as rusting. While all the listed metals can corrode, iron’s corrosion product (rust) is porous and flakes off, exposing fresh metal to further corrosion, leading to rapid deterioration compared to the others.

Corrosion is an electrochemical process. Iron corrodes to form hydrated iron oxides, which are commonly called rust. Unlike aluminium and zinc, which form protective oxide layers (passivation), rust does not adhere strongly to the surface and is permeable to air and water, allowing corrosion to continue unchecked. Silver tarnishes by reacting with sulfur, forming a thin layer of silver sulfide, but this is a surface phenomenon and not as destructive as the rusting of iron.

Aluminium is highly reactive but forms a protective, adherent oxide layer. Zinc is more reactive than iron but also forms a protective layer (e.g., zinc carbonate or oxide) that inhibits further corrosion, and it is used in galvanization to protect iron sacrificially. Silver is a relatively noble metal and corrodes slowly, primarily through tarnishing. Therefore, iron corrodes most rapidly among the given options under typical atmospheric conditions.

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