Structure-to-Electrolyte Potential
A structure-to-electrolyte potential is the voltage difference between a buried or submerged metallic structure and a reference electrode in contact with the electrolyte near that structure.
The meter is not reading the pipe by itself. It is reading the difference between the structure connection and the reference electrode through the full measurement circuit: structure, test lead, meter, reference electrode, and electrolyte contact.
In pipeline work the reading is often called a pipe-to-soil potential, but the more general term is structure-to-electrolyte potential because the same concept applies to tanks, casings, piping, reinforcing steel, and other buried or submerged structures.
What the Measurement Can Tell You
The measurement can support CP criteria evaluation, show relative shifts caused by CP current, reveal possible stray-current influence, and help locate areas where the structure may be underprotected or overprotected. It is not a direct corrosion-rate measurement by itself, and one voltage reading should not be treated as the whole CP decision.
The value depends on reference electrode type, placement, contact resistance, current flow in the electrolyte, current flow on the structure, coating condition, and whether the CP system is energized, interrupted, depolarized, or not under CP influence.
The electrolyte may be soil, water, concrete pore solution, or another conductive environment. Poor contact, dry surface conditions, rock, pavement, frozen soil, standing water, or an unrepresentative electrode location can affect what the meter displays.
Common Reading Conditions
| Condition | Meaning | Primary Caution |
|---|---|---|
| Native / free corrosion | Potential before CP influence or after sufficient depolarization | May vary with soil, coating condition, and time. |
| ON potential | Measured while CP current is flowing | Includes voltage-drop error unless properly considered. |
| Instant-off potential | Measured immediately after interrupting CP current | Requires interruption of all significant current sources affecting the location. |
| Depolarized potential | Measured after polarization has decayed | Time required can vary widely by structure and environment. |
Measurement Practices
- Connect the positive meter lead to the structure and the negative lead to the reference electrode unless project convention states otherwise.
- Record the sign, magnitude, reference electrode type, and measurement condition.
- Record where the reference electrode was placed and whether the contact surface was soil, water, concrete, pavement opening, or another electrolyte exposure.
- Place the reference electrode as close as practical to the structure surface location being evaluated.
- Note unusual conditions such as dry soil, frozen soil, poor wetting, nearby current sources, unstable readings, or suspected interference.
- Do not compare ON, instant-off, depolarized, and native values as if they are the same kind of measurement.
- Use survey context, reading history, current status, and nearby conditions to interpret whether a local reading represents the area of concern.