In corrosion:

A. A metal is oxidized by loss of electrons to oxygen.

B. It is an electrochemical phenomenon

C. The object under corrosion behaves as anode.

D. Coating the metal with \(Zn\) can save iron object from corrosion.

Choose the correct answer from the options given below:

A, B, C, D only

The correct answer is option 3. A, B, C, D only.

Let us break down each statement in detail regarding corrosion:

A. A metal is oxidized by loss of electrons to oxygen.

In corrosion, a metal reacts with its environment, leading to the formation of oxides or other compounds. The metal loses electrons (oxidation) in this process. For example, in the case of iron:

\(Fe \longrightarrow Fe^{2+} + 2e^-\)

The electrons lost by iron are typically picked up by oxygen (or another oxidizing agent), leading to the reduction of oxygen and the formation of iron oxides (rust) in the presence of water:

\(O_2 + 4H^+ + 4e^- \longrightarrow 2H_2O \)

Hence, in corrosion, the metal is oxidized by losing electrons, and oxygen acts as an electron acceptor.

B. It is an electrochemical phenomenon.

Corrosion is essentially an electrochemical process. It involves two simultaneous half-reactions:

Anodic Reaction (Oxidation): The metal loses electrons (oxidation), typically forming metal ions.

\(Fe \longrightarrow Fe^{2+} + 2e^-\)

Cathodic Reaction (Reduction): The electrons lost by the metal are gained by oxygen or another species, resulting in the reduction.

\(O_2 + 4H^+ + 4e^- \longrightarrow 2H_2O\)

This creates a local electrochemical cell on the metal surface, where the anode is where oxidation occurs and the cathode is where reduction occurs. Corrosion involves electron flow from the anodic to the cathodic site, driven by differences in electrochemical potential.

C. The object under corrosion behaves as anode.

The site of corrosion acts as the anode in the electrochemical process. At the anode, the metal loses electrons and undergoes oxidation:

\( Fe \longrightarrow Fe^{2+} + 2e^-\)

The anode is where the corrosion occurs, as the metal is sacrificed by giving up electrons. The electrons then move to the cathode, where reduction reactions take place. The corrosion product, like rust, forms at or near the anode.

D. Coating the metal with \(Zn\) can save iron object from corrosion.

Coating iron with zinc is a common method to prevent corrosion. This is known as galvanization. Zinc is more reactive than iron and has a higher tendency to oxidize (lose electrons). When zinc is coated on iron, it acts as a sacrificial anode:

\(Zn \longrightarrow Zn^{2+} + 2e^-\)

Zinc corrodes instead of iron, protecting the iron beneath it. Even if the zinc coating is damaged, zinc will continue to act as the anode and corrode in place of the iron. This sacrificial protection prevents the iron from oxidizing and forming rust.

Conclusion: All four statements (A, B, C, and D) are correct, which makes option 3 the correct choice.