1. Understand what is being asked

We need to identify the correct statement about corrosion of iron.

Corrosion of iron is an electrochemical process involving oxidation of iron: $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$

Typically, dissolved oxygen and moisture help the corrosion process.

2. Examine each option

Option A: Iron corrodes in oxygen-free water.

In ordinary rusting of iron, both water and oxygen are required. In oxygen-free water, corrosion is greatly hindered and ordinary rusting does not proceed normally.

So, this statement is incorrect.

Option B: Iron corrodes more rapidly in salt water because its electrochemical potential is higher.

Iron does corrode faster in salt water, but the reason given is wrong.

Salt water increases corrosion mainly because:

• it is a better electrolyte,
• ionic conduction becomes easier,
• electrochemical corrosion cells form more readily.

It is not because the electrochemical potential of iron becomes higher.

So, this statement is incorrect.

Option C: Corrosion of iron can be minimized by forming a contact with another metal with a higher reduction potential.

A metal with higher reduction potential is more noble than iron. If iron is in contact with such a metal, iron becomes the anode and corrodes faster.

To protect iron by sacrificial protection, it should be connected to a metal with lower reduction potential (like Zn or Mg), not higher.

So, this statement is incorrect.

Option D: Corrosion of iron can be minimized by forming an impermeable barrier at its surface.

Yes. If an impermeable coating is formed, it prevents contact of iron with:

• oxygen,
• moisture,
• corrosive environment.

Examples: painting, plastic coating, enamel coating.

So, this statement is correct.

3. Final conclusion

The correct option is: $\boxed{\text{D}}$