Technical Practice Guide

AST Cathodic Protection Testing: Tank Bottom Reference Cells, Rectifier Interruption, Criteria, and Interpretation

AST cathodic protection testing evaluates whether aboveground storage tank bottoms and associated metallic components are receiving adequate cathodic protection.

Overview

AST CP testing is used to evaluate whether a tank bottom is receiving adequate cathodic protection.

AST tank bottoms are often tested using permanent reference cells installed under or near the tank bottom.

Impressed current CP systems commonly protect AST bottoms, but galvanic systems may also be used in some configurations.

Rectifier output does not prove the tank bottom is adequately protected.

Tank-bottom potentials must be interpreted using the correct reference electrode, current condition, and applicable criterion.

Permanent reference cell condition and placement strongly affect interpretation.

AST CP testing may be limited by tank construction, secondary containment, liners, access, and whether the tank is in service.

Core concept:

AST CP testing is not just rectifier inspection. It is an evaluation of how the tank bottom responds to CP current at the available reference-cell locations.

Technical Basis

Tank-bottom cathodic protection supplies protective current to the underside of a steel tank bottom.

For impressed current AST systems, the rectifier supplies DC current, anodes distribute current below or around the tank, and current reaches the tank bottom through the electrolyte.

Tank-to-electrolyte potentials are measured using reference cells.

AST CP testing differs from ordinary pipe-to-soil testing because the tank bottom is not directly visible and many readings come from permanent reference cells.

  • The reference cell location is fixed.
  • Reference cell condition may degrade or become questionable.
  • Under-tank electrolyte conditions may vary.
  • Liners, sand pads, concrete ringwalls, containment systems, and repairs can affect current distribution.
  • Annular ring and chime areas may respond differently than interior tank-bottom areas.
  • Tank bottom replacement or repair work can change current demand and current distribution.
  • In-service and out-of-service conditions can affect what can be safely measured and how readings should be interpreted.

The measured potential depends on tank bottom polarization, reference cell type and condition, current condition, IR drop, CP current distribution, electrolyte contact, tank bottom coating or replacement history, and whether the tank is in service or undergoing repairs.

Zinc reference cells may be used in some AST installations, but −850 mVCSE criterion statements must be tied to CSE-equivalent readings or the proper reference scale.

When AST CP Testing Is Used

  • Annual or periodic AST CP surveys
  • AST CP system commissioning
  • Testing impressed current tank-bottom CP systems
  • Testing galvanic tank-bottom CP systems where installed
  • Evaluating permanent under-tank reference cells
  • Checking tank-bottom instant-off potentials
  • Evaluating 100 mV polarization using depolarization data
  • Troubleshooting low or inconsistent tank-bottom potentials
  • Verifying CP performance after tank repairs or bottom replacement
  • Evaluating CP performance after rectifier adjustment
  • Supporting compliance documentation and engineering review

Equipment Typically Used

Equipment Purpose
High-impedance voltmeter Measures tank-to-electrolyte potentials and reference-cell readings.
Permanent under-tank reference cell Provides a fixed measurement point beneath or near the tank bottom.
Copper-copper sulfate reference electrode Provides a common reference scale for soil/electrolyte CP measurements.
Tank junction box or reference cell test station Provides access to tank, reference cell, and CP system leads.
Rectifier or RMU access Allows output inspection, current interruption, and remote status verification.
Current interrupter Cycles CP current output for instant-off measurements.
Digital multimeter Supports voltage, continuity, shunt, and troubleshooting checks where appropriate.
Field log or survey software Documents readings, current condition, reference cell identity, and field observations.
Caution:

Do not access energized rectifiers, tank electrical systems, or classified/hazardous areas unless qualified, authorized, and permitted by site procedures. AST facilities may include electrical, fuel, vapor, confined-space, hot-work, and operational hazards.

General Field Method

  1. Review tank records, CP system drawings, rectifier data, reference cell locations, and prior survey history.
  2. Identify the tank, CP system, rectifier, reference cell test station, and each permanent reference cell.
  3. Confirm reference cell type where known, such as CSE or zinc.
  4. Record rectifier output and operating condition before testing.
  5. Measure tank-to-electrolyte ON potentials at available permanent reference cells.
  6. Interrupt applicable CP current sources where instant-off data is required and practical.
  7. Record instant-off potentials immediately after current interruption.
  8. Where depolarization testing is required, record depolarized potentials after the specified depolarization period.
  9. Evaluate questionable reference cells by comparing history, adjacent cells, stability, and field observations.
  10. Document reference cell identity, current condition, timing, rectifier status, tank service status, and access limitations.
  11. Restore interrupted CP current sources to normal operation after testing.
  12. Interpret results according to the applicable criterion, reference scale, current condition, and data quality.

Exact procedures vary by owner requirement, standard, tank construction, CP system design, and available test access.

Valid Data Conditions

  • Correct tank and CP system identification
  • Known reference cell identity and type where possible
  • Stable reference cell reading
  • Known current condition: ON, instant-off, depolarized, or native
  • Correct rectifier interruption where instant-off data is used
  • Documented timing for instant-off and depolarized readings
  • Known tank service status, repair status, and CP system operating status
  • Awareness of tank-bottom repairs, secondary containment, liners, and pad conditions
  • Comparison to prior reference cell history where available
  • Verification of unusual or unstable readings before making a criterion conclusion

A failed or unstable permanent reference cell can make the conclusion questionable.

A single reference cell may not represent the entire tank bottom.

An ON reading may include IR drop.

If the rectifier is off for repairs, readings should be interpreted as current-off or outage-condition data, not normal operating CP data.

Reference electrode type matters; CSE and zinc scales are not directly interchangeable without proper conversion or criterion basis.

Common Errors and Misinterpretations

Error Why It Matters
Assuming rectifier output proves tank-bottom protection Rectifier output does not show whether tank-bottom potentials satisfy criteria.
Using a questionable permanent reference cell without qualification A bad reference cell can make good CP look bad or bad CP look good.
Comparing zinc reference cell readings directly to CSE criteria Different reference scales require correct interpretation.
Calling an ON reading an instant-off reading ON readings may include IR drop and may not represent polarized potential.
Ignoring tank repair or out-of-service conditions Readings during repairs or outages may not represent normal CP operation.
Assuming one reference cell represents the entire tank bottom Current distribution and electrolyte conditions may vary under the tank.
Ignoring reference cell history Historical trends can help identify failed, shifted, or questionable cells.
Failing to restore rectifier output after interruption The AST CP system may be unintentionally left out of service.

Interpretation

AST CP test results should be interpreted according to reference cell type, current condition, tank-bottom response, and applicable criterion.

  • Instant-off tank-bottom potentials may be compared to the −850 mVCSE polarized potential criterion where valid.
  • Depolarization data may support the 100 mV polarization criterion.
  • ON readings can be useful operational data but may include IR drop.
  • If a CSE reference cell is questionable, conclusions based on that cell should be qualified.
  • If one reference cell fails while others pass, the result should be evaluated spatially and historically.
  • If all reference cells fail, the CP system may be off, underpowered, poorly distributed, or affected by tank-bottom/pad conditions.
  • Rectifier output, reference cell readings, tank history, and field conditions should be interpreted together.
Observation General Interpretation
Instant-off potential more electro-negative than −850 mVCSE May satisfy the polarized potential criterion at that reference cell location, assuming valid CSE-equivalent data.
Calculated polarization at least 100 mV May satisfy the 100 mV polarization criterion at that reference cell location.
One cell fails while nearby cells pass Evaluate current distribution, reference cell condition, history, and location-specific conditions.
All cells shift electro-positive compared with prior years May indicate reduced CP output, outage, repair condition, system change, or broad reference/data issue.
Reference cell reading is unstable or historically abnormal Reference cell condition should be questioned before making a final criterion conclusion.

Worked Example

An impressed current AST CP system is evaluated during a periodic survey. Rectifier output is interrupted for instant-off readings:

Reference Cell ON Potential Instant-Off Potential Depolarized Potential Comment
CSE 1 −1,050 mVCSE −910 mVCSE −760 mVCSE Stable reading
CSE 2 −980 mVCSE −845 mVCSE −730 mVCSE Near annular area
CSE 3 −1,020 mVCSE −875 mVCSE −755 mVCSE Stable reading

CSE 1 and CSE 3 satisfied the −850 mVCSE polarized potential criterion.

CSE 2 failed to satisfy the −850 mVCSE polarized potential criterion because its instant-off potential was less electro-negative than −850 mVCSE.

Calculated polarization at CSE 2 is 845 mV − 730 mV = 115 mV.

CSE 2 satisfied the 100 mV polarization criterion.

The correct conclusion is not simply that the tank bottom failed. The correct conclusion is that one location failed the −850 mVCSE polarized potential criterion but satisfied the 100 mV polarization criterion, assuming the readings and reference cell are valid.

The result should be interpreted with reference cell condition, tank history, rectifier output, and applicable standard.

Practice Questions

Question 1

What does AST CP testing evaluate?

  1. Only whether the rectifier cabinet is painted
  2. Whether the tank bottom is responding to CP current at available reference-cell locations
  3. Only fuel level inside the tank
  4. Only roof corrosion conditions

Answer: B

Question 2

Why does reference cell condition matter in AST CP testing?

  1. Because reference cells control rectifier output automatically
  2. Because zinc cells can always be read as CSE values without interpretation
  3. Because tank bottoms are visible only through reference cells
  4. Because a bad reference cell can make good CP look bad or bad CP look good

Answer: D

Question 3

Why should zinc reference cell readings not be directly compared to CSE criteria without proper basis?

  1. Because zinc and CSE are different reference scales
  2. Because zinc reference cells cannot be installed under tanks
  3. Because zinc readings are always invalid
  4. Because CSE criteria apply only to pipelines

Answer: A

Question 4

What does rectifier interruption support during AST testing?

  1. Fuel inventory measurement
  2. Automatic reference cell replacement
  3. Instant-off potential measurement
  4. Tank roof inspection

Answer: C

Question 5

What is the correct interpretation when one reference cell fails the −850 mVCSE criterion but satisfies the 100 mV polarization criterion?

  1. The entire tank bottom automatically fails without qualification
  2. The location failed one criterion but satisfied another, so the result should be interpreted with reference cell validity, history, and applicable standard
  3. The rectifier output reading alone decides the result
  4. The zinc reference cell scale should be ignored

Answer: B

Related Pages

Tank Bottom CP Systems Ring anodes, grid anodes, reference cells, liners, and current distribution. UST CP Testing Direct-connect systems, coupons, criteria, and UST CP interpretation. Rectifier Inspection Rectifier output evaluation and troubleshooting concepts. Current Interruption Interruption timing and synchronized instant-off testing. Instant-Off Potential Instant-off measurements, polarized potentials, and IR drop reduction. Depolarization Testing 100 mV polarization criterion evaluation and calculated polarization. Reference Electrode Use Reference electrode placement and measurement quality. Pipe-to-Soil Potential Surveys Structure-to-electrolyte potential measurements and interpretation. CP Coupons Coupon measurements, instant-off readings, depolarization, and interpretation. Galvanic Anode Testing Anode output, current measurement, and CP 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. Aboveground Storage Tanks AST application context, components, and corrosion-control concerns. Impressed Current Cathodic Protection How rectifier-powered CP systems protect buried structures. Galvanic Cathodic Protection How sacrificial anode CP systems protect buried structures.