From Driving Voltage to Polarization: How CP Actually Works
1. Driving Voltage: The Starting Point
Driving voltage is the applied electrical potential supplied by a galvanic anode or rectifier. It exists to overcome circuit resistance and force current to flow.
Driving voltage alone does not protect a structure. It must create current flow through the metallic path and the electrolyte path to influence reactions at the metal–electrolyte interface.
2. Resistance Controls Current
According to Ohm’s Law (V = I × R), current is determined by driving voltage and total circuit resistance. Resistance exists in the structure, connections, soil, and anode system.
If resistance is high, more voltage is required to deliver the same current. Without sufficient current, meaningful polarization cannot occur.
3. Current Density at the Interface
When current reaches the structure surface, it distributes across exposed areas. The amount of current per unit area is current density.
Current density directly controls the rate of reduction reactions and suppresses oxidation at anodic locations. Protection effectiveness depends on achieving adequate current density at the metal–electrolyte interface.
4. Polarization: The Protective Mechanism
As reduction increases, the structure potential shifts in the negative direction. This shift is polarization.
Polarization reduces the rate of anodic dissolution by altering reaction balance. The objective of CP design is not simply to apply voltage, but to produce sufficient polarization across the structure.
5. Attenuation and Distribution
Current decreases with distance due to resistance in both the metallic and electrolyte paths. This reduction is attenuation.
Uneven current distribution produces areas of lower polarization, which may become underprotected. Proper anode spacing and system output are required to maintain uniform protection.
6. The Complete Chain
Driving Voltage → Overcomes Resistance → Produces Current → Establishes Current Density → Creates Polarization → Suppresses Oxidation
Cathodic protection is therefore a controlled manipulation of electrochemical reaction behavior through circuit design.
- From Driving Voltage to Polarization (Current Guide)
- Why Instant-Off Matters
- Understanding Corrosion Using Mixed Potential Theory
- Why AC Can Still Cause Corrosion
- Current Distribution and Attenuation Explained