Which of the following method cannot prevent corrosion?

The correct answer is Option (4) → Covering the surface with salt water

Corrosion of iron is an electrochemical process: at one spot iron is oxidised (anode) and at another oxygen is reduced (cathode), and the water film on the metal acts as the electrolyte. Methods of prevention either keep air and moisture away from the surface or make another metal corrode instead.

• Option (1) Covering the surface with paint — Prevents corrosion. A barrier method: the paint keeps air and moisture away from the metal.

• Option (2) Covering the surface with other more reactive metal — Prevents corrosion. For example, zinc on iron (galvanising): the more reactive zinc reacts and saves the iron.

• Option (3) Covering the surface with sacrificial electrode of another metal — Prevents corrosion. Mg or Zn connected to the object is oxidised in its place.

• Option (4) Covering the surface with salt water — Cannot prevent corrosion. Salt water is a good electrolyte; it helps the electrochemical reactions of corrosion, so iron rusts faster (as in coastal areas).

NCERT (2026-27 edition):
Corrosion – “… Prevention of corrosion is of prime importance. It not only saves money but also helps in preventing accidents such as a bridge collapse or failure of a key component due to corrosion. One of the simplest methods of preventing corrosion is to prevent the surface of the metallic object to come in contact with atmosphere. This can be done by covering the surface with paint or by some chemicals (e.g. bisphenol). Another simple method is to cover the surface by other metals (Sn, Zn, etc.) that are inert or react to save the object. An electrochemical method is to provide a sacrificial electrode of another metal (like Mg, Zn, etc.) which corrodes itself but saves the object.”