Focus on corrosion events in metallic objects using mixed potential theory.
Previous lecture covered corrosion of zinc in HCl and activation polarization.
Current lecture will cover case studies focusing on activation and concentration polarization.
Key Concepts
Polarization
Activation Polarization:
Described by the equation:
[ \eta = \beta \log \frac{I}{I_0} ]
Concentration Polarization:
Described by the equation:
[ \eta = 0.0591 \log \left(1 - \frac{I}{I_{max}}\right) ]
Focus will be on cathodic polarization, not anodic polarization._
Reasons for Focusing on Cathodic Polarization
Depletion of Metal Ions: Anodic polarization requires depletion which is uncommon as metal dissolves into ions.
Passivation: During anodic polarization, metal can become passivated, a concept that can be explained using Pourbaix diagrams.
Case Studies
Case Study 1: Galvanic Coupling of Active and Noble Metals
Example: Zinc and platinum in acidic medium.
Anodic Reaction: Zinc dissolution.
Cathodic Reaction: Hydrogen evolution on platinum.
Current Density:
Hydrogen evolution has a higher exchange current density on platinum than on zinc.
Platinum acts as the cathode; zinc acts as the anode.
Mixed Potential Theory:
For net current to be zero, total cathodic current must equal total anodic current.
Current Density Calculation
Current densities from both cathodic reactions (zinc and platinum) are summed to find new current density.
The corrosion current density increases significantly due to the galvanic coupling of active with noble components.
Case Study 2: Effect of Strong Oxidizer on Corrosion Rate
Oxidizer Example: Fe^{3+} in acidic medium.
Presence of Fe^{3+} causes strong cathodic polarization leading to increased corrosion rates.
Events Diagram Analysis
Mixed potential exists between the anodic reaction of metal dissolution and the cathodic reactions (hydrogen and Fe^{3+} reduction).
When Fe^{3+} is added:
The overall current density increases due to the additional cathodic reaction, which reduces the polarization effect of hydrogen evolution.
Depolarization Effect:
Presence of Fe^{3+} decreases the over-voltage for hydrogen evolution, thus enhancing the dissolution of the active metal.
Conclusion
Both case studies demonstrate how coupling with noble metals or the presence of strong oxidizers can significantly increase the corrosion rates of active metals.
Further discussions and case studies will continue in future lectures.