Statement I: When iron rod is immersed in NaCl solution such that half of its length is corroded and other half part when exposed to air is not corroded

Statement II: Metal portion with less access to O₂ corrodes

Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I

The correct answer is option 1. Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I.

Let us break down the explanation in detail:

Corrosion of metals, particularly iron, involves electrochemical reactions. This process can be influenced by various factors, including the presence of electrolytes, oxygen, and differences in aeration. Here’s how these factors come into play:

Statement I: When an iron rod is immersed in NaCl solution such that half of its length is corroded and the other half part when exposed to air is not corroded

Iron Rod in NaCl Solution:

The NaCl solution acts as an electrolyte, enhancing the conductivity of the solution and facilitating the electrochemical corrosion process. The part of the iron rod immersed in the \(NaCl\) solution is in contact with the electrolyte but has limited access to oxygen compared to the part exposed to air. In the absence of sufficient oxygen, this immersed part becomes anodic, where the oxidation reaction occurs:

\(\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^-\)

The iron ions \((Fe^{2+})\) formed at the anodic region then react with chloride ions in the solution, leading to corrosion.

Iron Rod Exposed to Air:

The part of the rod exposed to air has better access to oxygen, creating a more oxygen-rich environment. This region becomes the cathodic area where the reduction reaction occurs:

\(\text{O}_2 + 4\text{H}^+ + 4\text{e}^- \rightarrow 2\text{H}_2\text{O}\)

Since reduction (gain of electrons) occurs here, this part does not corrode as quickly as the anodic part.

Statement II: Metal portion with less access to \(O_2\) corrodes

Differential Aeration Cells:

When different parts of a metal object have varying access to oxygen, differential aeration cells are formed. The part with less access to oxygen (less aerated) becomes anodic and undergoes oxidation, leading to corrosion. The part with more access to oxygen (more aerated) becomes cathodic and remains protected from corrosion.

Linking Statements I and II

Statement I describes an observation where the part of the iron rod immersed in \(NaCl\) solution corrodes, while the part exposed to air does not. This is because the immersed part has less access to oxygen and thus becomes anodic, leading to corrosion.

Statement II provides the underlying principle that explains this observation: the metal portion with less access to oxygen corrodes due to the formation of anodic and cathodic regions based on differential aeration.

Conclusion

Both statements are correct:

Statement I correctly describes the observed phenomenon of selective corrosion based on exposure to NaCl solution and air. Statement II accurately explains why this selective corrosion occurs: due to the differential access to oxygen, creating anodic (corroding) and cathodic (non-corroding) regions.

Thus, the correct answer is: Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I