What Is Cathodic Protection?

Why Cathodic Protection Matters

Metallic structures exposed to soil, water, concrete, or another electrolyte can corrode when electrochemical corrosion cells form on the metal surface. In those cells, corrosion occurs at anodic areas where metal atoms give up electrons and enter the electrolyte as ions.

Cathodic protection reduces corrosion by supplying protective current to the structure. When properly designed, operated, and monitored, CP can significantly reduce external corrosion on pipelines, tank bottoms, underground storage tanks, marine structures, and other metallic assets.

Misunderstanding cathodic protection leads to bad field decisions. A structure may appear protected because a meter reading looks sufficiently negative, while the reading may actually include voltage drop, interference effects, poor reference electrode placement, or other measurement errors.

Core Concept

The corrosion cell

For corrosion to occur, four elements are required: an anode, a cathode, an electrolyte, and a metallic path. The anode is where metal loss occurs. The cathode is where reduction reactions occur. The electrolyte allows ionic current flow, and the metallic path allows electronic current flow.

How cathodic protection changes the corrosion cell

Cathodic protection does not remove the electrolyte, coating defects, or all corrosion risks. Instead, it changes the electrochemical behavior of the protected structure by supplying current to the metal surface. The goal is to reduce or suppress anodic activity on the structure.

In practical terms, CP current is discharged from an anode, travels through the electrolyte, and enters the protected structure at exposed metal surfaces. The protected structure receives current and is polarized in the cathodic direction.

Two basic CP system types

Cathodic protection systems are commonly grouped into two main types: galvanic cathodic protection and impressed current cathodic protection.

A galvanic CP system uses sacrificial anodes, such as magnesium, zinc, or aluminum, that naturally have a more active electrochemical potential than the protected structure. These anodes corrode preferentially and provide protective current.

An impressed current CP system uses an external DC power source, usually a rectifier, to drive protective current from impressed current anodes to the protected structure. ICCP systems are commonly used where higher current output or adjustable control is required.

Field Application

Cathodic protection is used on many structure types, including buried pipelines, aboveground storage tank bottoms, underground storage tanks, marine structures, buried facility piping, well casings, and reinforced concrete structures.

Field personnel evaluate CP by taking electrical measurements with reference electrodes, voltmeters, test stations, coupons, rectifier readings, and current interruption equipment. These measurements are used to determine whether the system is operating and whether the structure satisfies applicable CP criteria.

A rectifier output reading by itself does not prove that a structure is protected. It only shows that the rectifier is producing voltage and current. The protected structure must still be evaluated using appropriate structure-to-electrolyte potential measurements, current measurements, continuity testing, and other applicable survey methods.

Common Mistakes

1. Assuming CP means corrosion has stopped completely.
   Why it is wrong: Cathodic protection reduces corrosion activity, but it does not eliminate every corrosion risk or replace coatings, inspection, or maintenance.
2. Assuming a negative potential reading always proves adequate protection.
   Why it is wrong: Potential readings can include voltage drop, interference, reference electrode errors, or readings taken at locations that do not represent the structure condition.
3. Assuming rectifier output proves the structure is protected.
   Why it is wrong: A rectifier can be operating while current distribution is poor, anodes are failing, cables are damaged, isolation is shorted, or some parts of the structure remain underprotected.
4. Ignoring the difference between galvanic and impressed current systems.
   Why it is wrong: These systems behave differently, are tested differently in some situations, and have different failure modes.
5. Treating CP criteria as universal rules without context.
   Why it is wrong: Criteria depend on structure type, environment, reference electrode, survey method, applicable standard, and project requirements.

Field Example

A buried steel pipeline has an impressed current CP system. During inspection, the rectifier is found operating at 12.0 volts and 4.5 amps. That confirms the rectifier is producing DC output, but it does not prove the entire pipeline is adequately protected.

To evaluate protection, the technician must measure structure-to-electrolyte potentials at appropriate test locations, consider whether readings include voltage drop, verify interruption timing if instant-off readings are required, and determine whether the applicable CP criterion is satisfied.

Practice Questions

1. What are the four required parts of a corrosion cell?
2. In a corrosion cell, where does metal loss occur: the anode or the cathode?
3. Why does rectifier output alone not prove that a buried pipeline is adequately protected?
4. What is the basic difference between galvanic cathodic protection and impressed current cathodic protection?
5. Why can an ON potential reading be misleading during CP evaluation?

Related Pages

• How Cathodic Protection Works
• Corrosion Fundamentals
• Galvanic Cathodic Protection
• Impressed Current Cathodic Protection
• Cathodic Protection Criteria
• Structure-to-Electrolyte Potential