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).

Fuel cells convert

chemical energy into electrical energy

The correct answer is option 3. Fuel cells convert chemical energy into electrical energy.

The other options are incorrect. Fuel cells do not convert electrical energy into chemical energy, mechanical energy into chemical energy, or electrical energy into heat.

Fuel cells are electrochemical devices that convert the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. The chemical energy of the fuel is used to create an electric current, which can be used to power devices or to generate heat.

Here is a detailed explanation of how fuel cells work:

A fuel cell consists of two electrodes, a negative electrode (or anode) and a positive electrode (or cathode). The anode is where the fuel is oxidized, and the cathode is where the oxidizing agent is reduced.

The electrodes are separated by an electrolyte, which is a substance that allows ions to pass through it. The electrolyte is usually a solid, liquid, or polymer membrane.

When a fuel cell is operating, the fuel is oxidized at the anode, and the oxidizing agent is reduced at the cathode. The electrons produced at the anode flow through an external circuit, creating an electric current. The ions produced at the anode and cathode travel through the electrolyte, completing the circuit.

The overall reaction in a fuel cell is the oxidation of the fuel and the reduction of the oxidizing agent. The products of the reaction are water and heat.

Fuel cells are a promising technology for clean energy production. They are efficient, produce no emissions, and can be used to generate electricity on-site. However, fuel cells are still relatively expensive, and they have not yet been widely adopted.