Question

Activation polarization is controlled by

• A. temperature
• B. charge transfer kinetics
• C. fluid flow
• D. diffusion

Hint:

Hydrogen evolution on iron is a classic example of an activation-controlled reaction, where the slow step is the formation of diatomic hydrogen molecules from adsorbed hydrogen atoms on the metal surface.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks to identify the rate-controlling step or factor associated with activation polarization in electrochemistry and corrosion.

Step 2: Key Formula or Approach:
The relationship between activation polarization overpotential (\( \eta_{\text{act}} \)) and current density (\( i \)) is mathematically described by the Tafel equation:
\[ \eta_{\text{act}} = \pm \beta \log\left(\frac{i}{i_0}\right) \]
where:
\( \beta \) is the Tafel slope, and
\( i_0 \) is the exchange current density.

Step 3: Detailed Explanation:

• Definition of Activation Polarization: Activation polarization refers to an electrochemical reaction rate that is limited by an inherent, slow step in the reaction sequence at the metal-electrolyte interface.
This slow step requires an energy barrier (activation energy) to be overcome.

• Charge Transfer Rate-Limiting Step: The rate-determining step is typically the transfer of electrons (charge transfer) between the electrode and the reacting species, or the adsorption/desorption of species on the electrode surface.
Therefore, activation polarization is fundamentally controlled by charge transfer kinetics.

• Comparison with Concentration Polarization:
-

Diffusion and

fluid flow (Options C and D) control concentration polarization, where the reaction rate is limited by the mass transport of reactants to or products away from the electrode surface.
- Although

temperature (Option A) influences reaction rates, it is an environmental variable rather than the physical mechanism controlling the polarization class.


Step 4: Final Answer:
Thus, activation polarization is controlled by charge transfer kinetics, matching Option (B).