Identify the correct statement:

We begin by recalling that rusting of iron is an electro-chemical process. At anodic areas iron is oxidised

$$Fe \rightarrow Fe^{2+}+2e^-$$

while, at cathodic areas, dissolved oxygen is reduced in the presence of water

$$\tfrac12\,O_2+H_2O+2e^- \rightarrow 2OH^-.$$

The electrons produced at the anode travel through the metal to the cathode, combine with oxygen, and the overall process finally produces hydrated iron(III) oxide $$Fe_2O_3.xH_2O$$, commonly called rust. Any measure that breaks this chain—either by cutting off the supply of electrons, oxygen, or water, or by forcing some other metal to supply the electrons—will slow or stop corrosion.

Let us now analyse one by one the four statements given in the problem.

Option A: “Corrosion of iron can be minimised by forming a contact with another metal with a higher reduction potential.” The standard reduction potential of iron is

$$E^\circ(Fe^{2+}/Fe) = -0.44\;V.$$

For cathodic (sacrificial anode) protection we deliberately connect iron to a metal whose reduction potential is lower (more negative) than that of iron. According to the electro-chemical series, zinc

$$E^\circ(Zn^{2+}/Zn) = -0.76\;V$$

and magnesium

$$E^\circ(Mg^{2+}/Mg) = -2.37\;V$$

are typical choices. Such a metal becomes the anode and corrodes preferentially, forcing the iron structure to behave as the cathode and stay protected. If we were to couple iron with a metal having a higher (more positive) reduction potential, iron itself would continue to act as the anode and corrode. Hence Option A is incorrect.

Option B: “Iron corrodes in oxygen-free water.” From the cathodic half-reaction written earlier we see that dissolved oxygen is essential:

$$\text{Cathode:}\; \tfrac12\,O_2 + H_2O + 2e^- \rightarrow 2OH^-.$$

If there is no dissolved oxygen, the cathodic reaction cannot proceed, electrons cannot be consumed, and the anodic dissolution of iron ceases. Therefore, in oxygen-free (deaerated) water iron corrosion is negligible. Thus Option B is false.

Option C: “Corrosion of iron can be minimised by forming an impermeable barrier at its surface.” Paint, oil films, plastic coatings, enamel, galvanising, etc., all work on exactly this principle. They block the access of both water and oxygen to the metal surface, thereby preventing the anodic and cathodic reactions from taking place. Because this statement accurately describes a well-known protective method, it is correct.

Option D: “Iron corrodes more rapidly in salt water because its electrochemical potential is higher.” Sea or salt water indeed accelerates rusting, but the reason is the increased electrical conductivity and the aggressive action of $$Cl^-$$ ions that promote pit formation, not any inherent increase in the standard reduction potential of iron (which remains the same). Therefore Option D is wrong.

We have found only Option C to be correct.

Hence, the correct answer is Option C.