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).

A depolarizer used in a dry cell is

manganese dioxide

The correct answer is option 2. Manganese dioxide .

In a dry cell battery, the depolarizer is a crucial component that helps to maintain the efficiency and performance of the battery during its operation. The depolarizer plays a role in preventing the accumulation of gases that can hinder the battery's function.

Here is how manganese dioxide \((MnO_2)\) serves as the depolarizer in a dry cell battery:

Absorption of Hydrogen Ions \((H^+)\):

During the discharge process of the battery, chemical reactions occur at the electrodes, leading to the production of electrons and hydrogen ions \((H^+)\). Without a depolarizer, the hydrogen ions can accumulate at the cathode, forming hydrogen gas \((H_2)\) through the reduction reaction. This buildup of hydrogen gas can create a layer on the surface of the cathode, inhibiting further electron flow and reducing the battery's efficiency.

Prevention of Hydrogen Gas Accumulation:

Manganese dioxide acts as a depolarizer by absorbing the hydrogen ions produced during the discharge process. By absorbing the hydrogen ions, manganese dioxide prevents the buildup of hydrogen gas and ensures that the cathode remains active and available for further electrochemical reactions.

Maintenance of Battery Performance:

By preventing the accumulation of hydrogen gas, manganese dioxide helps to maintain the performance and efficiency of the dry cell battery over its operational lifespan. This allows the battery to deliver a consistent and reliable supply of electrical energy for various applications, such as powering electronic devices, flashlights, and toys.

In summary, manganese dioxide serves as the depolarizer in a dry cell battery by absorbing hydrogen ions, preventing the buildup of hydrogen gas, and ensuring the continuous and efficient operation of the battery. This makes manganese dioxide a critical component in the design and functionality of dry cell batteries.