Understanding Corrosion Using Mixed Potential Theory
1. Corrosion Is a Coupled Reaction
Corrosion does not occur because of oxidation alone. It occurs when oxidation and reduction reactions operate simultaneously at the metal–electrolyte interface.
For every electron released by anodic dissolution, a reduction reaction must consume that electron elsewhere on the surface.
2. Anodic and Cathodic Curves
Reaction rates vary with potential. As potential shifts, anodic dissolution and cathodic reduction change in magnitude.
When plotted, these behaviors form polarization curves. The anodic curve represents oxidation current; the cathodic curve represents reduction current.
3. The Mixed Potential
The corrosion potential is the value at which total anodic current equals total cathodic current.
At this intersection point, the net current is zero, but both reactions are active. This condition defines the operating potential of the corrosion cell.
4. Corrosion Rate
Although net current at corrosion potential is zero, the magnitude of the intersecting currents represents the corrosion rate.
Higher intersection current density corresponds to higher metal loss rate. Lower intersection current density corresponds to slower corrosion.
5. How CP Alters the Intersection
Cathodic protection shifts the structure potential in the negative direction by supplying external current.
This shift increases cathodic current and decreases anodic dissolution, effectively lowering corrosion rate by altering the mixed potential condition.
6. The Complete Logic Chain
Anodic Reaction + Cathodic Reaction → Polarization Curves → Intersection Defines Corrosion Potential → Intersection Current Defines Corrosion Rate → CP Shifts Intersection → Corrosion Suppressed
Mixed potential theory provides the mathematical and graphical framework for understanding corrosion control.
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- Why Instant-Off Matters
- Understanding Corrosion Using Mixed Potential Theory (Current Guide)
- Why AC Can Still Cause Corrosion
- Current Distribution and Attenuation Explained