Cathodic protection (CP) is one of the most effective methods for controlling corrosion on buried and submerged metallic structures. It is used on pipelines, storage tanks, marine structures, and other assets where corrosion is driven by electrochemical reactions in an electrolyte (soil, water, or concrete pore solution).
This article explains how cathodic protection works, the difference between galvanic and impressed current systems, how protection criteria are interpreted in practice, and what engineers should consider when designing and testing CP systems.
1) Why corrosion occurs (the electrochemical model)
Corrosion is an electrochemical process. For corrosion to occur, four components must exist:
- Anode (oxidation occurs; metal dissolves)
- Cathode (reduction occurs; electrons are consumed)
- Electrolyte (ion path through soil/water/concrete)
- Metallic path (electron path through the structure/metal)
At anodic sites, metal atoms lose electrons and enter the electrolyte as ions. Corrosion occurs strictly at the anode. At cathodic sites, a reduction reaction consumes electrons (often oxygen reduction in neutral, aerated environments).
2) What is cathodic protection?
Cathodic protection reduces corrosion by shifting the structure’s electrochemical potential in the negative direction so that anodic metal dissolution is reduced or effectively suppressed. In practical terms, CP supplies electrons to the structure and changes the driving conditions for the corrosion reactions.
3) Two primary CP system types
3.1 Galvanic (sacrificial anode) CP
A galvanic CP system attaches a more active metal (commonly magnesium, zinc, or aluminum) to the structure. Because the anode has a more negative potential, it corrodes preferentially and provides protective current to the structure.
- Pros: simple, no external power, low maintenance
- Cons: limited current output, reduced effectiveness in high-resistivity environments, finite anode life
3.2 Impressed current cathodic protection (ICCP)
ICCP uses an external DC power source (rectifier) and inert or semi-inert anodes. Output is adjustable and can provide higher current for large, poorly coated, or complex structures.
- Pros: high capacity, adjustable, suitable for large assets
- Considerations: monitoring, rectifier maintenance, interference management
4) Protection criteria and what they really mean
CP effectiveness is commonly evaluated using structure-to-electrolyte potentials measured with a reference electrode. A widely referenced criterion for steel in soil environments is:
- −850 mV with respect to a Cu/CuSO₄ reference electrode (context and technique matter)
Other approaches include demonstrating a specified amount of cathodic polarization (for example, 100 mV polarization). The correct interpretation depends on the standard being applied and the measurement method used.
5) Polarization and IR drop (why “instant off” matters)
When CP current flows through an electrolyte, a voltage loss occurs due to electrolyte resistance. This is commonly referred to as IR drop. Some “on” potential readings include both polarization effects and IR effects, which can mislead interpretation.
Instant-off measurements are often used to reduce IR drop influence and better represent the polarized condition of the structure. The goal is to evaluate protection without being fooled by measurement artifacts.
6) Soil resistivity and current distribution
Soil resistivity strongly influences:
- anode bed resistance
- required driving voltage
- current distribution along the structure
In high-resistivity environments, galvanic systems may be current-limited and ICCP designs often require careful anode bed placement (including deep anode beds where appropriate) to achieve effective current distribution.
7) Interference and AC considerations
CP systems can interact with other metallic structures and nearby power systems. Stray DC current can cause unintended corrosion, and induced AC from parallel transmission corridors can create both corrosion and safety concerns.
Common mitigation tools include bonding strategies, solid-state decouplers, grounding approaches (including zinc ribbon), and gradient control mats at locations where personnel safety is a concern.
8) Monitoring and testing methods
Effective CP programs rely on routine monitoring and periodic surveys, such as:
- structure-to-electrolyte potential surveys
- instant-off testing (where applicable)
- rectifier inspections and output checks
- interference testing
- close-interval survey (CIS) where needed
Trending data over time is often as important as any single reading. Changes can indicate coating damage, shielding, interference, contact issues, or equipment drift.
9) Common CP design and interpretation mistakes
- Ignoring coating condition and assuming “bare steel” current demand
- Using poor reference electrode placement and trusting misleading readings
- Failing to account for IR drop and not using appropriate test methods
- Overlooking shielding and assuming current reaches all areas equally
- Not evaluating interference risks early in design
10) Standards and compliance
CP criteria, test methods, documentation practices, and design assumptions are defined by widely recognized industry standards. Always confirm the governing standard(s) and project specifications for your asset and jurisdiction, and apply criteria using the proper measurement technique.