Why Instant-Off Matters: Separating Polarization from IR Drop
1. The Problem with ON Potential
When CP current is flowing, measured structure-to-electrolyte potential includes both polarization at the metal–electrolyte interface and voltage loss caused by resistance in the electrolyte.
This resistive component is IR drop. ON potential therefore reflects circuit behavior and surface condition combined.
2. What IR Drop Represents
IR drop is the voltage produced when current flows through resistance. In soil environments, resistance between the structure and reference electrode can create significant measurement error.
IR drop does not represent electrochemical reaction intensity. It is a circuit artifact superimposed on true surface potential.
3. Interrupting Current
When CP current is interrupted, current flow immediately stops. Because IR drop depends on current, the resistive voltage component collapses almost instantly.
Polarization, however, does not disappear instantly. The surface remains polarized for a short period before depolarizing.
4. Instant-Off Measurement
Instant-off potential is measured in the brief moment after current interruption and before significant depolarization occurs.
This value more closely represents true polarization at the metal–electrolyte interface because the IR component has been removed.
5. Why It Matters for CP3 Logic
CP criteria are based on polarization, not circuit voltage. Evaluating ON potential alone can misrepresent protection levels if IR drop is significant.
Instant-off testing isolates electrochemical behavior from circuit losses, allowing accurate assessment of protection effectiveness.
6. The Measurement Chain
Driving Voltage → Current Flow → IR Drop + Polarization → Interrupt Current → IR Collapses → Instant-Off Reflects Polarization
Understanding this sequence is essential for interpreting CP field data correctly.