Cathodic Protection for Aboveground Storage Tanks

Why AST CP Matters

The underside of an aboveground storage tank bottom is often inaccessible after construction. Corrosion can occur where the tank bottom contacts a conductive pad, soil, moisture, or contaminated foundation material.

Tank-bottom CP is difficult to verify because the protected surface is beneath the tank. Fixed reference cells, perimeter measurements, coupons, and specialized survey methods may be used depending on design and access.

A tank may have a functioning CP system but still have areas that are difficult to protect because of shielding, poor electrolyte continuity, tank pad conditions, or current distribution limitations.

Core Concept

Protected surface

The protected surface is the external underside of the tank bottom. CP current must reach exposed metal areas through the electrolyte beneath the tank.

Galvanic tank systems

Galvanic anodes may be installed beneath or around smaller tanks or specific designs where current demand is relatively low.

Impressed current tank systems

Impressed current systems may use distributed anodes, ring anodes, linear anodes, or other configurations designed to improve current distribution beneath the tank.

Fixed reference cells

Permanent reference electrodes may be installed beneath the tank bottom to provide repeatable monitoring points. Their reliability must be evaluated because permanent cells can drift, fail, dry out, or become questionable.

Current distribution

Tank-bottom current distribution is affected by pad material, moisture, anode layout, tank diameter, bottom contact, shielding, and foundation conditions.

Field Application

AST CP surveys may include rectifier inspection, fixed reference cell readings, perimeter potentials, coupon readings, current interruption, depolarization testing, and review of historical monitoring data.

Data from fixed reference cells should be reviewed for consistency and reasonableness. A single abnormal reference cell may indicate an actual CP issue, but it may also indicate reference cell failure or poor contact.

Tank repair, out-of-service conditions, disconnected systems, or intentional shutdowns must be documented because they affect interpretation of CP data.

Common Mistakes

1. Assuming perimeter readings represent the entire tank bottom.
   Why it is wrong: Perimeter readings may not represent the center or shielded areas beneath the tank.
2. Trusting permanent reference cells without review.
   Why it is wrong: Permanent reference cells can fail, drift, or lose reliable electrolyte contact.
3. Ignoring tank pad conditions.
   Why it is wrong: Moisture, sand, concrete, liner systems, and contaminants affect current flow.
4. Assuming rectifier output proves tank-bottom protection.
   Why it is wrong: Output does not prove current reached all tank-bottom areas.
5. Ignoring out-of-service or repair conditions.
   Why it is wrong: Tank status can explain disconnected systems or abnormal CP readings.

Field Example

A tank has three fixed copper-copper sulfate reference cells beneath the bottom. Two cells show stable protective readings, while one shifts unusually electropositive compared with historical data.

The abnormal reading should not be accepted blindly. The technician should evaluate CP system operation, compare historical trends, review reference cell reliability, and determine whether the reading indicates a tank-bottom CP issue or a failed reference cell.

Practice Questions

1. What surface is protected by AST tank-bottom CP?
2. Why are fixed reference cells commonly used beneath tank bottoms?
3. Why can a permanent reference cell become unreliable?
4. Why may perimeter readings not represent the whole tank bottom?
5. Why does rectifier output not prove full tank-bottom protection?

Related Pages

• Reference Electrodes
• 100 mV Polarization Criterion
• Rectifiers
• Groundbeds
• AMPP NACE SP0193 Overview
• CP Coupons