Cathodic Protection Coupons
A CP coupon is a small metal specimen, usually similar to the protected structure, installed in the electrolyte and electrically connected to the structure. It represents a controlled bare-metal area for measurement and interpretation.
Coupons are useful because they can be disconnected or interrupted locally, allowing measurements that may better represent the polarized condition of an exposed structure surface.
What Coupons Help Evaluate
- Local instant-off potential with reduced voltage-drop error.
- Depolarization and 100 mV polarization behavior.
- Current density to a known exposed area.
- Protection effectiveness near buried structures where direct surface access is impossible.
- Influence of nearby CP sources, bonds, and interference currents.
Coupon Limitations
A coupon is not automatically the structure. Its validity depends on material, surface condition, area, depth, placement, connection, electrolyte exposure, shielding, and whether it represents the coating holiday or structure surface being evaluated.
A poorly placed coupon can give a precise number that is not representative. Treat coupon readings as strong evidence only when the installation and measurement method support the intended conclusion.
Coupon Measurement Types
| Measurement | Purpose |
|---|---|
| Coupon ON potential | Shows potential while connected and influenced by CP current. |
| Coupon instant-off potential | Approximates polarized potential after locally interrupting the coupon connection. |
| Coupon current | Allows current density calculation when coupon area is known. |
| Depolarization | Supports 100 mV polarization evaluation when measured over time. |