Corrosion is a process of deterioration and consequent loss of solid metallic materials through an unwanted chemical or electrochemical attack by its environment starting at the surface. The chemical corrosion occurs due to the direct chemical action of the environment (e.g., inorganic liquid) or atmospheric gases as O₂, H₂S , SO₂, halogens and ammonia. The extent of chemical corrosion depends on the chemical affinity of solid metal with the corrosive environment and ability of the reaction product to form protective film on the metal surface. The electrochemical corrosion occurs when a metal is in contact with the conducting liquid or when two dissimilar metals or alloys are dipped partially or completely in a solution. The electrochemical corrosion occurs due to the existence of separate anodic or cathodic areas between which there occurs a flow of current through the conducting solution.  The corrosion always occurs at the anodic areas. The rusting of iron occurs due to corrosion. The electrode reactions in the rusting process are as follows:

Anode: \(Fe \rightarrow Fe^{2+}  +  2e^{-};\) \(E^0 = - 0.44V\)

Cathode: \(O_2  +  2H_2O  +  4e^-  \rightarrow  4OH^-;\) \(E^0 = 1.23 V\)

The overall reaction is

\(Fe  +  O_2  +  H_2O  \rightarrow  Fe^{2+}  +  2OH^- ;\) \(E^0 = 1.67 V\)

The Fe2⁺ and OH⁻ ions combine to form Fe(OH)₂ which is oxidized to Fe(OH)₃ in excess of oxygen. The product formed corresponds to Fe₂O₃.xH₂O. If the supply of oxygen is limited, the corrosion product is black magnetite (Fe₃O₄).

When lead storage battery is charged

sulphuric acid is regenerated

The correct answer is option 2. When a lead storage battery is charged, sulfuric acid is regenerated.

When a lead storage battery is charged sulphuric acid is regenerated.

During the charging process of a lead storage battery, the reverse reactions of the discharge process occur, leading to the regeneration of the original reactants. Here’s what happens during charging:

Charging Reactions:

1. At the Anode (Negative Electrode):

Lead sulfate \((PbSO_4)\) on the anode is converted back to lead \((Pb)\) and sulfuric acid \((H_2SO_4)\) is regenerated.

\(\text{PbSO}_4 + 2\text{e}^- \rightarrow \text{Pb} + \text{SO}_4^{2-}\)

2. At the Cathode (Positive Electrode):

Lead sulfate \((PbSO_4)\) on the cathode is converted back to lead dioxide \((PbO_2)\) and sulfuric acid \((H_2SO_4)\) is regenerated.

\(\text{PbSO}_4 + 2\text{H}_2\text{O} \rightarrow \text{PbO}_2 + \text{SO}_4^{2-} + 4\text{H}^+ + 2\text{e}^-\)

Overall Charging Reaction:

Combining the reactions at the anode and cathode, the overall reaction during charging is:

\(2\text{PbSO}_4 + 2\text{H}_2\text{O} \rightarrow \text{Pb} + \text{PbO}_2 + 2\text{H}_2\text{SO}_4\)

Key Points:

1. Regeneration of Sulfuric Acid \((H_2SO_4)\):

During charging, sulfuric acid \((H_2SO_4)\) is regenerated as the lead sulfate \((PbSO_4)\) on both electrodes is converted back to lead \((Pb)\) and lead dioxide \((PbO_2)\), respectively.

2. Lead Dioxide and Lead Electrodes:

Lead dioxide \((PbO_2)\) does not dissolve; instead, it is re-formed on the positive electrode. The lead electrode (anode) gets restored from lead sulfate back to lead \((Pb)\).

3. Amount of Sulfuric Acid:

The amount of sulfuric acid actually increases (is regenerated) during charging, as it is produced from the conversion of lead sulfate.

Incorrect Options Clarified:

Lead dioxide dissolves: This is incorrect. During charging, lead dioxide \((PbO_2)\) is formed on the cathode, not dissolved.

The lead electrode becomes coated: This is incorrect in the context of the charging process. During discharge, the lead electrode becomes coated with lead sulfate, but during charging, this lead sulfate is converted back to lead.

The amount of sulphuric acid decreases: This is incorrect. During charging, sulfuric acid is regenerated, so its amount actually increases.

During the charging process of a lead storage battery, sulfuric acid is regenerated. Therefore, the correct answer is sulphuric acid is regenerated.