Question

The formation of a passive layer decreases corrosion rate because it

• A. increases current flow
• B. decreases open-circuit potential
• C. reduces metal-electrolyte contact
• D. promotes uniform dissolution

Hint:

Passivation is a kinetic phenomenon:
- It shifts the corrosion potential (\( E_{\text{corr}} \)) in the noble direction.
- It reduces the corrosion current (\( I_{\text{corr}} \)) to extremely small values.
The key is the physical blocking of the metal-electrolyte interface.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
This question asks for the primary mechanism by which the formation of a passive layer on a metal surface reduces its corrosion rate in an electrochemical environment.

Step 2: Key Formula or Approach:
Corrosion is an electrochemical process requiring an anode (metal oxidation), a cathode (reduction reaction), an electrical connection, and an electrolyte in direct contact with the metal to facilitate ion transport.
Passivation involves the spontaneous formation of an ultra-thin, highly adherent, and non-reactive oxide or hydroxide layer on the metal surface.

Step 3: Detailed Explanation:

• Barrier Mechanism:
- The passive film (such as chromium oxide \( \text{Cr}_2\text{O}_3 \) on stainless steel or aluminum oxide \( \text{Al}_2\text{O}_3 \) on aluminum) acts as a physical barrier.
- This barrier physically isolates the underlying reactive metal atoms from direct contact with the corrosive aqueous electrolyte.
- Since the electrolyte cannot access the metal surface, both the anodic dissolution reaction (\( \text{M} \rightarrow \text{M}^{n+} + n e^{-} \)) and the cathode reduction reactions are severely blocked.

• Ion and Electron Transport Resistance:
- The passive layer is a highly insulating ceramic film.
- It exhibits very low ionic conductivity, which restricts the diffusion of metal ions outward and oxygen/corrosive ions inward.
- Consequently, this reduction in metal-electrolyte contact reduces the corrosion current density by several orders of magnitude.
- It does not increase current flow, nor does it promote dissolution.


Step 4: Final Answer:
The passive layer acts as a physical barrier that drastically reduces metal-electrolyte contact, thereby halting electrochemical corrosion.
Thus, option (C) is the correct choice.