Question

Electrochemical corrosion can occur only if _____________ is present in contact with the metal.

• A. air
• B. oxygen
• C. liquid medium
• D. gaseous medium

Hint:

Think of electrochemical corrosion like a battery. A battery needs two electrodes (anode, cathode) and an electrolyte to work. For corrosion, the metal provides its own electrodes (due to impurities or stress differences), but it needs an external liquid (like water or moisture) to act as the electrolyte. No electrolyte, no corrosion battery.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Electrochemical corrosion, also known as wet corrosion, is a process that involves the formation of a galvanic cell on the surface of a metal. A galvanic cell requires four components to function: an anode, a cathode, a metallic path for electron flow, and an electrolyte for ion flow.

Step 2: Detailed Explanation:
The mechanism of electrochemical corrosion involves: 1.

Anode: A site on the metal surface where oxidation occurs (the metal loses electrons, e.g., \( \text{Fe} \rightarrow \text{Fe}^{2+} + 2e^- \)). 2.

Cathode: A site on the metal surface where reduction occurs (a species gains electrons, e.g., \( \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- \)). 3.

Metallic Path: The bulk metal itself provides a pathway for electrons to flow from the anode to the cathode. 4.

Electrolyte: This is a crucial component. An electrolyte is a medium that contains ions and can conduct electricity through the movement of those ions. It completes the electrical circuit by allowing ions to move between the anode and cathode. Let's evaluate the options: - (A) Air and (B) Oxygen: Oxygen is a very common cathodic reactant (depolarizer) in corrosion, but it is not the electrolyte. Corrosion can occur in the absence of oxygen if another species can be reduced (e.g., H⁺ ions in an acid). So, while often present, oxygen is not the fundamental requirement that defines electrochemical corrosion. - (D) Gaseous medium: A purely gaseous medium generally does not have sufficient free ions to act as an effective electrolyte. This type of corrosion is usually termed dry or chemical corrosion. -

(C) Liquid medium: A liquid medium, especially water containing dissolved salts, acids, or bases, acts as the electrolyte. This liquid allows for the movement of ions (like Fe²⁺ and OH⁻ in the rusting of iron) between the anodic and cathodic regions, which is essential to complete the electrochemical cell and sustain the corrosion process. Without this conductive liquid medium (the electrolyte), the circuit is incomplete, and electrochemical corrosion cannot occur.

Step 3: Final Answer:
Electrochemical corrosion requires an electrolyte, which is a conductive liquid medium, to be in contact with the metal. This corresponds to option (C).