Cathodic Protection for Underground Storage Tanks

Why UST CP Matters

Underground storage tanks and associated piping are buried in soil and may be exposed to moisture, salts, coating damage, dissimilar metals, and other corrosion risks.

UST systems often include multiple components: tanks, product piping, flex connectors, submersible turbine pump components, vent lines, risers, and test stations. Electrical continuity and isolation conditions affect CP interpretation.

UST CP surveys require disciplined testing because a single tank-to-soil reading may not represent every protected component.

Core Concept

Galvanic UST systems

Many UST systems use galvanic anodes. Magnesium anodes are common in soil applications, especially where the protected structure is coated and current demand is manageable.

Impressed current UST systems

Impressed current systems may be used where current demand is higher, multiple structures are protected, or adjustable control is required.

Test stations and access points

Test stations provide access to tank leads, piping leads, anode leads, coupons, and reference electrode test locations. Proper wire identification is essential.

Coupons

Coupons may be used to evaluate polarization and current density at a controlled exposed metal surface. Coupon connected and disconnected status must be documented.

Criteria selection

UST CP criteria depend on the applicable standard, regulation, test method, reference electrode, structure type, and whether readings are ON, instant-off, depolarized, or coupon-based.

Field Application

UST CP surveys may include tank-to-soil potentials, piping-to-soil potentials, coupon potentials, anode current measurements, continuity testing, rectifier inspection, and depolarization testing.

Galvanic anodes may need to be disconnected during certain tests when the test station design allows it. Impressed current systems may require interruption to collect instant-off readings.

The survey should identify which structures were tested, which reference electrode was used, which CP sources were connected or interrupted, and which criterion was evaluated.

Common Mistakes

1. Assuming one tank reading represents all UST piping.
   Why it is wrong: Piping and tank components may have different continuity, coating, or current distribution conditions.
2. Ignoring galvanic anode connection status.
   Why it is wrong: Connected, disconnected, and depolarized readings have different meanings.
3. Failing to identify the tested structure.
   Why it is wrong: Tank, piping, coupon, and anode readings are not interchangeable.
4. Assuming all buried metal at a UST facility is protected.
   Why it is wrong: Some metallic components may be isolated, unbonded, shielded, or outside the CP system.
5. Ignoring test station wiring errors.
   Why it is wrong: Mislabeled or broken wires can invalidate conclusions.

Field Example

A UST system includes a coated steel tank, buried steel piping, magnesium anodes, and a coupon test station. The tank satisfies an applied potential criterion, but the coupon fails a depolarization criterion.

The data should not be merged into one vague pass/fail conclusion. The tank and coupon measurements represent different test conditions and must be interpreted according to the applicable criterion and structure being evaluated.

Practice Questions

1. Why may a tank-to-soil reading not represent all UST piping?
2. What is a common galvanic anode material for UST soil applications?
3. Why must coupon connected or disconnected status be documented?
4. Why is test station wire identification important?
5. Why should tank and coupon readings not be merged without interpretation?

Related Pages

• Galvanic Cathodic Protection
• Cathodic Protection Coupons
• Test Stations
• 100 mV Polarization Criterion
• AMPP NACE SP0285 Overview
• CP Coupons