The 100 mV criterion is one of the accepted ways to demonstrate that a buried or submerged metal structure is receiving adequate cathodic protection. Instead of comparing the structure-to-electrolyte potential against a fixed negative threshold, it measures the amount of polarization that cathodic protection has actually produced. If turning the current on causes at least 100 mV of cathodic polarization, or turning it off allows at least 100 mV of decay back toward the native potential, the criterion is satisfied. It is especially useful where the more familiar negative-voltage threshold is difficult to apply.
100 mV CP Criterion in one line: The 100 mV polarization criterion is a cathodic protection adequacy test that requires a structure to show at least 100 millivolts of cathodic polarization formation or polarization decay. It is measured by interrupting the CP current and recording how far the potential shifts, and it serves as an alternative to fixed negative-potential criteria.
The 100 mV criterion is based on polarization, which is the change in a structure's potential caused by cathodic protection current flowing through it. A technician establishes a baseline and then either watches polarization build up as current is first applied, or watches it decay away after the current is switched off. Because the goal is to isolate true polarization from the voltage drop in the surrounding soil, the reading is taken at the moment current is interrupted, giving what is often called an instant-off value, and then again after the structure has been allowed to relax.
In the polarization decay method, the rectifier and any other current sources are interrupted simultaneously, the instant-off potential is recorded, and the structure is left to depolarize over a period that can range from a few hours to several days depending on the coating and soil. The difference between the instant-off potential and the more positive depolarized potential is the polarization shift. If that difference is 100 mV or greater, the structure met the criterion during the protected condition.
Getting a valid number depends on eliminating stray influences. All galvanic and impressed sources acting on the structure must be off during the decay, contact between the reference electrode and the soil must be consistent, and the test point must genuinely represent the structure segment of interest. Poor reference placement, an unaccounted current source, or an incomplete depolarization period will all distort the measured shift.
The fixed instant-off negative-potential criterion works well on clean carbon steel in ordinary soils, but there are conditions where it becomes unreliable or overly conservative. The 100 mV criterion is frequently chosen for structures made of mixed metals, for older bare or poorly coated lines, for high-resistivity soils, and for situations where achieving a very negative instant-off potential everywhere would require excessive current and risk coating damage or hydrogen effects.
Because it looks at the change in potential rather than an absolute value, the 100 mV method is more forgiving of the naturally different corrosion potentials that different metals exhibit. It also tends to be practical on complex networks where a single fixed threshold does not fit every segment. Operators still apply engineering judgment, since a structure that only just reaches 100 mV of shift may warrant closer follow-up than one that polarizes strongly.
It is important to treat the criteria as alternatives rather than requirements to satisfy all at once. A structure only needs to meet one recognized criterion at a given location, and choosing the 100 mV polarization approach can be the most defensible option where the electrochemistry or the soil environment makes fixed-potential comparisons ambiguous.
A depolarization survey is time consuming to run manually because it requires synchronized interruption of every current source and repeated potential readings spread across hours or days. Remote cathodic protection units and coupon-based monitors reduce that burden by carrying built-in current interrupters and by logging structure potential continuously. When a depolarization test is initiated, the units switch to off together and stream the decay curve back to a central platform without a crew standing at each test station.
Because the data lands in a monitoring system, the polarization shift can be calculated automatically by subtracting the stabilized depolarized potential from the instant-off value, and the result can be flagged if it falls short of 100 mV. The full decay curve is retained, so an engineer can confirm the structure actually reached a stable endpoint rather than being read too early, which is a common source of false failures in manual surveys.
This is where SCADA-style field monitoring adds the most value for cathodic protection. Synchronized GPS-timed interruption, continuous potential logging, and cloud dashboards turn what was an episodic manual test into a repeatable, auditable measurement. Trends across successive depolarization tests also reveal whether a structure's ability to polarize is degrading over time, which points to coating deterioration or growing current demand before it becomes a corrosion problem.
Neither is universally better; they are alternatives, and a location only needs to satisfy one of them. The 100 mV polarization method is often preferred for mixed metals, older bare structures, and high-resistivity soils where a fixed negative threshold is hard to apply or would require excessive current. On clean coated steel in ordinary soil the fixed criterion is usually simpler.
It varies widely with coating quality and soil conditions. Well-coated steel can depolarize in a matter of hours, while bare or poorly coated structures in high-resistivity soil may take a day or more to stabilize. The key is to wait until the potential stops moving so the recorded shift reflects full polarization decay rather than a partial reading.
While current is flowing, the measured structure-to-soil potential includes a voltage drop through the soil that is not real polarization. Interrupting the current removes that drop almost instantly, so the instant-off reading and the subsequent decay reflect actual polarization. Comparing those values gives the true 100 mV shift rather than an inflated number.
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