Overview
The 100 mV polarization criterion is a cathodic protection criterion based on the amount of polarization shift observed on a structure. Instead of requiring a specific final potential value, this criterion evaluates whether the structure has shifted at least 100 millivolts in the negative direction due to cathodic protection current.
In plain English, this criterion asks whether the cathodic protection system has changed the electrical condition of the structure enough to demonstrate effective polarization.
Because this approach focuses on the change in potential rather than a single absolute voltage value, it can sometimes be useful where interpretation of fixed potential criteria may be complicated by measurement conditions.
Standard Summary
What the Criterion Is Referring To
The 100 mV polarization criterion is based on the concept that cathodic protection reduces corrosion by shifting the electrochemical potential of the structure. If the structure’s potential moves at least 100 millivolts in the negative direction compared to its native condition, the structure may be considered adequately polarized for corrosion control under this criterion.
This approach evaluates the change in potential rather than focusing only on a specific final potential value. The goal is to confirm that cathodic protection current has altered the electrochemical behavior of the structure.
Plain-English Explanation
Every metal structure has a natural electrical potential when it is not receiving cathodic protection current. When a cathodic protection system is operating, current flows to the structure and pushes its potential in the negative direction.
The 100 mV criterion evaluates whether that change is large enough to indicate meaningful polarization. If the structure becomes at least 100 millivolts more negative than its native condition, the criterion may be considered satisfied under appropriate conditions.
Instead of relying on one target voltage value, this method looks at the difference between two conditions: the natural state and the polarized state.
Why This Criterion Exists
The reason this criterion exists is that interpreting a single absolute potential measurement can sometimes be difficult. Environmental conditions, electrode placement, IR drop, and other measurement factors can influence the number displayed on the meter.
By focusing on the change in potential rather than the final number alone, the polarization shift approach can provide evidence that cathodic protection current has actually affected the electrochemical condition of the structure.
This does not mean the criterion is always easier to apply. It simply provides another method for evaluating whether adequate polarization has occurred.
How It Is Evaluated in the Field
To evaluate the 100 mV polarization criterion, the technician or engineer must determine the difference between the structure’s native potential and its polarized potential under cathodic protection.
In practice, this usually involves measuring the potential under different electrical conditions. Techniques such as current interruption may be used to help isolate the polarized condition of the structure and reduce measurement errors caused by IR drop.
The key concept is that two conditions must be compared in order to determine whether the required polarization shift has occurred.
What Can Affect the Measurement
Several factors can influence how polarization shift is measured and interpreted.
IR Drop: Voltage gradients in the electrolyte can influence potential measurements.
Reference Electrode Placement: The electrode location determines the electrolyte side of the measurement.
Coating Condition: Current distribution and polarization behavior depend on coating quality and defect locations.
Current Distribution: Polarization may not occur uniformly across the structure.
Electrical Interference: Foreign currents or nearby structures may affect measurements.
Measurement Timing: Depolarization may begin once current is interrupted, affecting the observed value.
Field Interpretation
In field interpretation, the 100 mV criterion should be viewed as evidence that cathodic protection current has changed the electrochemical condition of the structure. It does not eliminate the need to understand how the measurement was obtained.
Proper interpretation still requires considering electrode placement, IR drop effects, measurement timing, and the electrical behavior of the structure being evaluated.
The key question is whether the structure has experienced a meaningful negative shift in potential that indicates polarization caused by cathodic protection current.
Common Misunderstandings
Mistake 1: Assuming polarization shift can be evaluated from a single measurement.
Mistake 2: Ignoring IR drop when determining the polarized condition.
Mistake 3: Forgetting that the criterion depends on comparing two electrical conditions.
Mistake 4: Assuming polarization occurs uniformly along the entire structure.
Mistake 5: Treating polarization shift as independent of measurement geometry and electrode placement.
Example Scenario
A buried pipeline has a native potential of −0.62 V vs CSE before cathodic protection current is applied. After the system operates and the structure becomes polarized, the measured potential is −0.75 V vs CSE.
The difference between these values is 130 mV in the negative direction. Under the appropriate testing conditions, this shift may satisfy the 100 mV polarization criterion.
The important point is that the evaluation is based on the difference between two electrical conditions rather than a single voltage reading.
Standards Context
The concepts discussed on this page originate from AMPP SP0169 — Control of External Corrosion on Underground or Submerged Metallic Piping Systems.
These explanations are simplified educational summaries intended to help readers understand the concepts used in cathodic protection standards. They are not a substitute for the complete standard or for professional engineering training and judgment.
The official standard can be obtained from the AMPP Knowledge Hub.