Technical Practice Guide

Pipe-to-Soil Potential Surveys: Field Method, Measurement Quality, Interpretation, and Common Errors

Pipe-to-soil potential surveys are one of the most common methods used to evaluate cathodic protection on buried pipelines. The survey is simple in concept, but the data can be badly misread if the technician does not know what condition was measured.

Overview

A pipe-to-soil potential survey measures the electrical potential difference between a buried pipeline and a reference electrode in contact with the electrolyte.

Although the common field phrase is “pipe-to-soil potential,” the broader technical concept is a structure-to-electrolyte potential. The meter is not directly measuring corrosion rate, coating quality, or remaining wall thickness. It is measuring voltage between the pipeline connection and the reference electrode.

The reading only becomes useful when the technician knows the current condition at the time of measurement. A native potential, CP applied ON potential, instant-off potential, and depolarized potential can all be measured at the same test location, but they do not mean the same thing.

Field rule:

A potential value without measurement condition is incomplete. Always document whether the reading is native/static, ON, instant-off, or depolarized.

Technical Basis

The voltmeter measures a potential difference between the pipeline and a stable reference electrode in the electrolyte. For buried steel pipelines in soil, the reference electrode is commonly a copper-copper sulfate reference electrode, abbreviated CSE.

Pipeline potentials are influenced by several interacting conditions:

  • native corrosion behavior of the steel in the environment
  • cathodic protection current applied to the pipeline
  • coating condition and exposed metal area
  • soil or electrolyte resistivity
  • reference electrode placement and contact quality
  • electrical continuity of the pipeline section
  • nearby CP current sources, bonds, foreign structures, and interference
  • test lead integrity and correct structure identification

An ON potential is measured while CP current is applied. It can show the influence of the CP system, but it may include IR drop caused by current flow through the electrolyte. Because of this, an ON potential is not automatically the same as a polarized potential.

Instant-off and depolarized potentials are used when the survey objective requires evaluation of the −850 mVCSE polarized potential criterion or the 100 mV polarization criterion.

When Pipe-to-Soil Surveys Are Used

Pipe-to-soil potential surveys are commonly used during:

  • annual CP surveys
  • baseline surveys
  • troubleshooting low protection levels
  • close interval surveys
  • current requirement investigations
  • rectifier adjustment verification
  • interference investigations
  • post-repair verification
  • commissioning
  • compliance documentation

The same basic measurement can support many different survey objectives, but the interpretation changes depending on whether the reading is native, ON, instant-off, or depolarized.

Equipment Typically Used

EquipmentPurpose
High-impedance voltmeterMeasures potential difference between pipe and reference electrode.
Copper-copper sulfate reference electrodeProvides a stable reference potential in soil.
Test leadsConnect the meter to the pipeline and reference electrode.
Test stationProvides access to pipeline wires, coupons, bonds, or casing wires.
Current interrupterAllows instant-off measurements where current interruption is required.
Field data logger or survey formRecords readings, locations, reading type, timing, and field conditions.
GPS or stationing referenceDocuments where readings were obtained.

General Field Method

  1. Identify the pipeline, test station, and intended measurement point.
  2. Verify the test lead connection to the pipeline.
  3. Inspect the reference electrode condition.
  4. Place the reference electrode in contact with the soil or electrolyte near the pipeline or test point.
  5. Connect the meter with correct polarity.
  6. Record the potential reading and identify the reading type.
  7. If applicable, record ON, instant-off, native, and/or depolarized values.
  8. Document abnormal field conditions such as dry soil, paving, poor access, unstable readings, or questionable contact.
  9. Confirm questionable readings before relying on them.
  10. Restore any interrupted CP current sources.
Do not treat the number alone as the answer.

A reading of −900 mVCSE could mean different things depending on whether it is an ON potential, an instant-off potential, or a depolarized value.

Valid Data Conditions

Reliable pipe-to-soil potential data generally requires:

  • correct structure contact
  • correct reference electrode type
  • good reference electrode contact with the electrolyte
  • stable meter reading
  • proper meter polarity
  • known current condition
  • correct test point identification
  • awareness of nearby CP current sources
  • documentation of abnormal soil, paving, dry conditions, or access limitations
  • synchronized interruption where required
  • verification of questionable or unusual readings

Common Errors and Misinterpretations

ErrorWhy It Matters
Recording an ON reading without labeling it as ONCan lead to incorrect criterion evaluation.
Treating an ON potential as a polarized potentialMay overstate CP because of IR drop.
Poor reference electrode contactCan create unstable or inaccurate readings.
Measuring the wrong test leadCan assign data to the wrong structure.
Ignoring nearby current sourcesMay leave IR drop or interference effects in the reading.
Assuming one test point represents the whole pipelineCP levels can vary along the pipeline.
Failing to document abnormal conditionsMakes later interpretation difficult.
Comparing values without knowing the reference electrode typeDifferent reference electrodes have different potential scales.
Assuming more electro-negative is always betterExcessive CP can create other problems depending on the structure and coating.

Interpretation

Pipe-to-soil potentials are interpreted based on the survey objective and the applicable criterion.

Reading TypeWhat It Can Help ShowMain Limitation
Native/static potentialBaseline condition without CP influence.May not represent protected condition.
ON potentialPotential with CP current applied.May include IR drop.
Instant-off potentialPolarized potential immediately after interruption.Requires proper interruption and timing.
Depolarized potentialPotential after polarization decay.Requires time and stable conditions.
Calculated polarizationAmount of polarization achieved.Depends on valid instant-off and depolarized readings.

An instant-off potential is commonly used for the −850 mVCSE polarized potential criterion. A depolarized potential may be used with the instant-off value to calculate polarization. A structure may fail to satisfy the −850 mVCSE polarized potential criterion but still satisfy the 100 mV polarization criterion.

Worked Example

ReadingValue
Native potential−545 mVCSE
ON potential−1,060 mVCSE
Instant-off potential−830 mVCSE
Depolarized potential−695 mVCSE

The ON value is more electro-negative than −850 mVCSE, but it may include IR drop.

The instant-off value of −830 mVCSE failed to satisfy the −850 mVCSE polarized potential criterion.

830 mV − 695 mV = 135 mV

The calculated polarization is 135 mV, so 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 a pipe-to-soil potential survey measure?

  1. Pipeline wall thickness loss over time
  2. The electrical potential difference between the pipe and a reference electrode
  3. The exact corrosion rate of the pipeline
  4. The amount of coating remaining on the pipe

Answer: B

Question 2

Why can ON potentials be misleading when evaluating CP criteria?

  1. They are always less electro-negative than instant-off potentials
  2. They do not require a structure connection
  3. They directly measure coating thickness
  4. They may include IR drop from current flowing through the electrolyte

Answer: D

Question 3

Which reading is commonly used for evaluating the −850 mVCSE polarized potential criterion?

  1. Instant-off potential
  2. Pipeline operating pressure
  3. Rectifier input voltage only
  4. Coating holiday count only

Answer: A

Question 4

Which field condition can make a pipe-to-soil potential reading questionable?

  1. Correct test lead identification
  2. Proper meter polarity
  3. Poor reference electrode contact with the electrolyte
  4. Documented current condition

Answer: C

Question 5

A test point failed to satisfy the −850 mVCSE polarized potential criterion but demonstrated more than 100 mV of calculated polarization. What should the technician understand?

  1. The ON potential should automatically replace all other readings
  2. The test point may satisfy the 100 mV polarization criterion depending on the applicable standard and data quality
  3. The reference electrode has definitely failed
  4. The survey data must always be discarded

Answer: B

Related Pages

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. ACVG Survey Applied AC signal coating defect detection and interpretation. DCVG Survey Coating defect detection, voltage gradients, and field interpretation. Current Requirement Testing Temporary current application, current distribution, and CP design interpretation. Soil Resistivity Testing Wenner four-pin method, data quality, and CP interpretation. Galvanic Anode Testing Anode output, current measurement, anode life, and interpretation. Interference Testing Foreign current influence, bonds, stray current, and interpretation. Casing Testing Carrier pipe isolation, casing shorts, and electrolytic contact interpretation. Electrical Isolation Testing Isolation joints, continuity checks, 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. Depolarization Testing Learn how calculated polarization is used to evaluate the 100 mV polarization criterion. Instant-Off PotentialField method, purpose, interpretation, and common errors. −850 mVCSE CriterionPolarized potential criterion interpretation and limitations. 100 mV Polarization CriterionCalculated polarization and polarization-based criterion evaluation. IR DropWhy current flow can distort structure-to-electrolyte potential readings. Reference ElectrodesReference electrode type, contact, and placement. Close-Interval SurveyInterrupted field survey method for higher-resolution potential data.