Technical Practice Guide

Depolarization Testing and the 100 mV Polarization Criterion

Depolarization testing is used to evaluate how much cathodic polarization a structure achieved because of cathodic protection current. The method is commonly used when evaluating the 100 mV polarization criterion.

Overview

Depolarization testing evaluates whether cathodic protection current shifted a structure's potential by at least 100 mV in the protective direction.

This is different from simply checking whether an instant-off potential is more electro-negative than −850 mVCSE.

The 100 mV polarization criterion is useful because some structures may fail to satisfy the −850 mVCSE polarized potential criterion while still demonstrating adequate cathodic polarization.

Core concept:

The question is not only “What is the potential?” The question is also “How much did the structure polarize because of CP current?”

Technical Basis

Cathodic protection causes a structure's potential to shift in the electro-negative direction.

When CP current is interrupted, the structure gradually loses polarization and shifts back in the electro-positive direction. This process is called depolarization.

Calculated polarization is commonly determined by comparing:

  • instant-off potential to depolarized potential, or
  • native/static potential to polarized potential, where applicable.

Example

  • Instant-off potential: −820 mVCSE
  • Depolarized potential: −690 mVCSE
  • Calculated polarization: 130 mV

The calculation is based on the amount of potential shift, not whether the numerical values appear “large” or “small.”

ON potentials are not normally used by themselves to calculate polarization because ON values may include IR drop caused by current flow through the electrolyte.

When Depolarization Testing Is Used

  • annual CP surveys
  • tank bottom surveys
  • UST surveys
  • pipeline surveys
  • interference investigations
  • troubleshooting questionable criterion results
  • confirming CP effectiveness after adjustment
  • validating protection when instant-off readings are less electro-negative than −850 mVCSE
  • documenting compliance with the 100 mV polarization criterion

Equipment Typically Used

EquipmentPurpose
High-impedance voltmeterMeasures structure-to-electrolyte potentials
Reference electrode, commonly CSEProvides a stable reference potential
Test leadsConnect the meter to the structure and reference electrode
Current interrupter or disconnect pointAllows CP current to be interrupted
Rectifier, bond, coupon, or anode accessProvides current source control or measurement access
Field log or survey softwareRecords time-based potential data
Timer or data loggerTracks depolarization timing and potential decay

General Field Method

  1. Identify the structure and test location.
  2. Confirm the reference electrode type and condition.
  3. Record the ON potential if CP current is applied.
  4. Interrupt CP current or disconnect the applicable CP source.
  5. Record the instant-off potential immediately after interruption.
  6. Allow the structure to depolarize.
  7. Record the depolarized potential after the selected depolarization period.
  8. Calculate the amount of polarization.
  9. Compare the result to the 100 mV polarization criterion.
  10. Document interruption method, timing, field conditions, and abnormal influences.
Important:

Depolarization time can vary depending on structure type, coating condition, environment, and survey specification.

Valid Data Conditions

  • correct structure contact
  • correct reference electrode type
  • stable reference electrode contact with the electrolyte
  • known current condition
  • proper current interruption or disconnection
  • sufficient depolarization time for the survey objective
  • consistent reference electrode placement
  • documentation of elapsed time
  • no unintended CP current remaining on the structure
  • awareness of foreign CP systems or interference
  • stable meter readings
  • confirmation that values are assigned to the correct test point

Moving the reference electrode between instant-off and depolarized readings can make the comparison questionable unless the method accounts for it.

Common Errors and Misinterpretations

ErrorWhy It Matters
Comparing an ON potential directly to a depolarized potential without considering IR dropCan exaggerate calculated polarization
Recording the instant-off value too lateMay understate the polarized potential
Not allowing enough depolarization timeMay understate or misrepresent the total polarization shift
Moving the reference electrode between readingsCan make the comparison invalid
Leaving another CP source energizedCan prevent true depolarization
Assuming failure of −850 mVCSE means failure of CPThe 100 mV criterion may still be satisfied
Calculating polarization with mislabeled readingsProduces an invalid conclusion
Ignoring unstable reference electrode contactCan make small differences meaningless
Treating one test point as representative of the whole structurePolarization may vary by location

Interpretation

The 100 mV polarization criterion is satisfied when the structure demonstrates at least 100 mV of cathodic polarization.

ResultGeneral Interpretation
Calculated polarization ≥ 100 mVMay satisfy the 100 mV polarization criterion
Calculated polarization < 100 mVDoes not satisfy the 100 mV polarization criterion
Instant-off less electro-negative than −850 mVCSE but polarization ≥ 100 mVFails −850 criterion but may satisfy 100 mV criterion
Questionable readingsVerify data before making a criterion conclusion

Worked Example

ReadingValue
Native potential−560 mVCSE
ON potential−1,030 mVCSE
Instant-off potential−825 mVCSE
Depolarized potential−700 mVCSE

The ON value is not used by itself to calculate polarization because it may include IR drop.

The instant-off value of −825 mVCSE failed to satisfy the −850 mVCSE polarized potential criterion.

825 mV − 700 mV = 125 mV

The test point satisfied the 100 mV polarization criterion.

Correct conclusion:

The test point failed to satisfy the −850 mVCSE polarized potential criterion but satisfied the 100 mV polarization criterion, assuming the data is valid.

Practice Questions

Question 1

What does depolarization testing evaluate?

  1. Coating thickness
  2. Rectifier efficiency
  3. The amount of cathodic polarization achieved because of CP current
  4. Pipeline operating pressure

Answer: C

Question 2

Why is an ON potential not normally used by itself to calculate polarization?

  1. Because ON values may include IR drop
  2. Because ON values cannot be measured in the field
  3. Because ON values always satisfy the −850 criterion
  4. Because ON values are static potentials

Answer: A

Question 3

A test point has an instant-off potential of −815 mVCSE and a depolarized potential of −690 mVCSE. What is the calculated polarization?

  1. 95 mV
  2. 105 mV
  3. 690 mV
  4. 125 mV

Answer: D

Question 4

What condition can make a depolarization comparison questionable?

  1. Stable meter readings
  2. Moving the reference electrode between readings
  3. Correct structure contact
  4. Properly documented elapsed time

Answer: B

Question 5

How should a technician interpret a failed −850 criterion but passed 100 mV polarization criterion?

  1. The survey data must always be rejected
  2. The reference electrode has definitely failed
  3. The structure may still demonstrate adequate cathodic protection depending on the applicable standard and data quality
  4. The ON potential automatically overrides all other readings

Answer: C

Related Pages

Tank Bottom CP Systems Ring anodes, grid anodes, reference cells, liners, and current distribution. AST CP Testing Tank-bottom reference cells, rectifier interruption, criteria, and interpretation. UST CP Testing Direct-connect systems, coupons, criteria, and UST CP interpretation. CP Coupons Coupon measurements, instant-off readings, depolarization, and interpretation. Interference Testing Foreign current, stray current, bonds, and CP current distribution. Electrical Isolation Testing Continuity checks, isolation failures, bonds, shorts, and CP current distribution. Current Interruption Rectifier interruption, synchronization, and instant-off timing. Rectifier InspectionICCP rectifier output evaluation and interpretation. Reference Electrode Use Reference electrode placement, electrolyte contact, and common measurement errors. Instant-Off Potential Pipe-to-Soil Potential Surveys What Is IR Drop? Close Interval Surveys