Technical Practice Guide

CP Coupons: Coupon Measurements, Instant-Off Readings, Depolarization, and Interpretation

CP coupons are controlled bare-metal test surfaces used to evaluate how a small exposed area responds to cathodic protection current.

Overview

CP coupons are small metal specimens installed in the electrolyte and electrically connected to the protected structure.

A coupon can simulate a small bare-metal area or coating holiday.

When bonded to the structure, the coupon receives CP current.

When disconnected, the coupon can be measured immediately to estimate a polarized potential with reduced IR drop associated with current flowing to the coupon.

Coupon measurements can help evaluate CP effectiveness where full system interruption is difficult.

Coupon measurements are not automatically representative of the whole structure.

Coupon location, exposure area, electrolyte contact, bond condition, and current distribution affect interpretation.

Core concept:

A CP coupon is a controlled bare-metal test surface. It can help show how a small exposed area responds to cathodic protection current.

Technical Basis

A coupon is electrically bonded to the protected structure so it polarizes along with the structure.

When the coupon is disconnected from the structure, current to the coupon stops.

A potential measured immediately after disconnection is often treated like a coupon instant-off potential.

  • The coupon is usually made of material similar to the protected structure.
  • The coupon has a known exposed surface area.
  • The coupon is placed in the electrolyte near the structure.
  • When connected, the coupon receives CP current.
  • When disconnected, the current path is interrupted locally at the coupon.
  • The immediate disconnected potential can reduce IR drop effects associated with current flowing to the coupon.
  • If the coupon remains disconnected, it begins to depolarize.
  • The difference between the coupon instant-off potential and depolarized potential can be used to calculate polarization.

Coupons can reduce the need to interrupt all current sources in some applications, but they do not eliminate the need for valid technique and interpretation.

Coupon measurements should be interpreted with structure-to-electrolyte potentials, coupon placement, coupon exposure, current density, electrolyte conditions, and current distribution.

When CP Coupons Are Used

  • Evaluating CP effectiveness where full current interruption is difficult
  • UST CP testing
  • Tank bottom CP monitoring
  • Pipeline test station monitoring
  • Complex facility piping systems with multiple current sources
  • Estimating polarized potential at a controlled bare-metal surface
  • Evaluating 100 mV polarization using coupon depolarization
  • Monitoring current density to a known exposed area
  • Checking CP performance near suspected shielding or current distribution concerns
  • Supporting long-term CP monitoring programs

Equipment Typically Used

Equipment Purpose
CP coupon Provides a controlled exposed metal surface in the electrolyte.
Coupon test station or junction box Provides access to coupon and structure leads.
High-impedance voltmeter Measures coupon-to-soil and structure-to-electrolyte potentials.
Copper-copper sulfate reference electrode Provides a stable reference potential for soil measurements.
Coupon disconnect switch or test lead Allows the coupon to be disconnected for instant-off or depolarization measurements.
Shunt or current measurement device, where installed Allows coupon current or current density evaluation.
Timer or data logger Tracks immediate disconnect readings and depolarization timing.
Field log or survey software Documents readings, timing, coupon status, and field conditions.
Caution:

Do not disconnect coupons, structure leads, bonds, or facility wiring unless qualified, authorized, and permitted by site procedures. Restore the required coupon connection after testing.

General Field Method

  1. Identify the coupon, protected structure, test station, and reference electrode location.
  2. Confirm whether the coupon is bonded to the structure before testing.
  3. Measure the coupon-to-soil potential while the coupon is bonded, if required.
  4. Measure the structure-to-electrolyte potential for comparison where appropriate.
  5. Disconnect the coupon from the structure using the test station switch or lead arrangement.
  6. Measure the coupon instant-off potential immediately after disconnection.
  7. Leave the coupon disconnected only as required by the test method or survey objective.
  8. Measure coupon depolarized potential after the selected depolarization period, where applicable.
  9. Calculate coupon polarization when instant-off and depolarized values are available.
  10. Reconnect the coupon to the structure after testing unless the test plan requires otherwise.
  11. Document coupon status, timing, reference electrode placement, field conditions, and any abnormal observations.

Exact procedures vary by owner specification, coupon design, structure type, and applicable standard.

Valid Data Conditions

  • Correct identification of coupon and structure leads
  • Known coupon status: bonded, disconnected, or depolarizing
  • Stable reference electrode contact
  • Consistent reference electrode placement for comparative readings
  • Immediate measurement after coupon disconnection when collecting instant-off data
  • Documented depolarization time where depolarized readings are used
  • Known coupon surface area if current density is evaluated
  • Known current path and bond condition
  • Awareness of nearby current sources, shielding, soil conditions, and structure geometry
  • Confirmation that the coupon is reconnected after testing where required

If the coupon is not actually bonded, the bonded reading is meaningless.

If the coupon is not measured immediately after disconnection, the value may not represent instant-off.

If the reference electrode is moved between readings, calculated polarization may be questionable.

If the coupon is shielded or poorly exposed to electrolyte, it may not represent the intended condition.

Common Errors and Misinterpretations

Error Why It Matters
Assuming a coupon represents the entire structure A coupon represents a local exposed surface, not every location on the protected structure.
Recording the disconnected value too late The coupon may have already begun depolarizing.
Failing to verify that the coupon is bonded before testing An unbonded coupon will not polarize with the structure as intended.
Leaving the coupon disconnected after testing The coupon will no longer monitor the bonded CP condition.
Moving the reference electrode between instant-off and depolarized readings Can make calculated polarization unreliable.
Ignoring coupon surface area when evaluating current density Current density depends on exposed area, not current alone.
Assuming coupon data replaces structure potentials Coupon data supports interpretation but should be evaluated with structure survey data.
Using a shielded or poorly exposed coupon as representative The coupon may not receive the same electrolyte exposure or current distribution as intended.

Interpretation

Coupon data should be interpreted as a local measurement of CP response at a controlled exposed surface.

  • A bonded coupon potential shows the coupon condition while connected to the protected structure.
  • A coupon instant-off value can help estimate the polarized potential at the coupon surface.
  • A depolarized coupon value can be compared to the instant-off value to calculate polarization.
  • If calculated polarization is at least 100 mV, the coupon may satisfy the 100 mV polarization criterion at that location.
  • If the coupon instant-off value is more electro-negative than −850 mVCSE, it may satisfy the −850 mVCSE polarized potential criterion at the coupon location.
  • Coupon data should not be overextended to areas with different coating condition, soil condition, current distribution, or shielding.
Observation General Interpretation
Coupon instant-off more electro-negative than −850 mVCSE May satisfy the polarized potential criterion at the coupon location, assuming valid data.
Coupon polarization at least 100 mV May satisfy the 100 mV polarization criterion at the coupon location.
Bonded coupon potential very different from structure potential Verify coupon lead identity, bond condition, location, and field setup.
Coupon depolarizes quickly after disconnection May indicate limited polarization retention or local exposure/current distribution effects.
Coupon current density unusually high May indicate high local current demand or a small exposed area receiving concentrated current.

Worked Example

A CP coupon is evaluated at a pipeline test station:

Measurement Value
Structure ON potential −1,040 mVCSE
Coupon bonded potential −1,020 mVCSE
Coupon instant-off potential after disconnection −875 mVCSE
Coupon depolarized potential −720 mVCSE

The coupon bonded potential is similar to the structure ON potential, suggesting the coupon was connected and receiving CP current.

The coupon instant-off potential of −875 mVCSE satisfied the −850 mVCSE polarized potential criterion at the coupon location.

Calculated polarization is 875 mV − 720 mV = 155 mV.

The coupon also satisfied the 100 mV polarization criterion at that location.

The conclusion applies to the coupon location and should be interpreted with structure potentials, coupon placement, current distribution, and field conditions.

Practice Questions

Question 1

What does a CP coupon simulate?

  1. A rectifier output terminal
  2. A controlled bare-metal area or coating holiday
  3. A permanent reference electrode only
  4. A soil resistivity pin spacing

Answer: B

Question 2

Why is the coupon instant-off reading taken immediately after disconnection?

  1. Because the coupon must be fully depolarized first
  2. Because the reference electrode cannot be used after disconnection
  3. Because coupon current density must be zero for 24 hours
  4. Because the coupon may begin depolarizing if the reading is delayed

Answer: D

Question 3

Why should a coupon not be assumed to represent the whole structure?

  1. Because a coupon represents a local exposed surface and local conditions
  2. Because coupons cannot be connected to structures
  3. Because coupons only measure AC voltage
  4. Because coupon readings never include polarization

Answer: A

Question 4

What is calculated polarization if coupon instant-off is −875 mVCSE and depolarized is −720 mVCSE?

  1. 45 mV
  2. 100 mV
  3. 155 mV
  4. 1,595 mV

Answer: C

Question 5

Why must the coupon be reconnected after testing when required?

  1. To make the coupon permanently native
  2. So the coupon continues monitoring the bonded CP condition
  3. To remove the structure from the CP circuit
  4. So the reference electrode can depolarize

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. Instant-Off Potential Instant-off measurements, polarized potentials, and IR drop reduction. Depolarization Testing 100 mV polarization criterion evaluation and calculated polarization. Current Interruption Interruption timing, instant-off measurements, and current source control. Reference Electrode Use Reference electrode placement and measurement quality. Pipe-to-Soil Potential Surveys Structure-to-electrolyte potential measurements and interpretation. Electrical Isolation Testing Isolation, bonds, shorts, and current distribution. Casing Testing Carrier pipe isolation, casing contact, and electrolytic contact interpretation. −850 mV Criterion Common CP criterion used for structure-to-electrolyte potential evaluation. 100 mV Polarization Criterion How calculated polarization is used to evaluate CP effectiveness. CP Coupons Measurement Page Coupon purpose, placement, instant-off readings, and current density. Learn CP Coupons Beginner-friendly explanation of coupon purpose and interpretation. Underground Storage Tanks UST CP applications and survey interpretation context. Aboveground Storage Tanks Tank bottom CP applications and survey interpretation context.