CORROSION PROTECTION FOR INDUSTRIAL COATING CONTRACTORS
Article 19: How Coatings and Cathodic Protection Work Together
Understanding the relationship between protective coatings, exposed steel, and electrical corrosion control
The Big Idea
A protective coating and a cathodic protection system perform different jobs, but they are often designed to work together. The coating isolates most of the steel from the corrosive environment. Cathodic protection supplies protective electrical current to the steel exposed at coating holidays, defects, and damaged areas.
Pipelines, storage-tank bottoms, marine structures, buried vessels, water systems, and other steel assets may use both protective coatings and cathodic protection. When both systems are properly designed, installed, inspected, and maintained, each one reduces the burden placed on the other.
Industrial coating contractors do not normally design or adjust cathodic protection systems. However, their work can directly affect whether those systems operate successfully. Surface preparation, coating application, holiday repair, and protection of electrical components are therefore important parts of the overall corrosion-control program.
1. Two Systems With One Objective
Corrosion requires electrochemical activity at the steel surface. A protective coating limits contact between the steel and water, oxygen, electrolytes, and other corrosive substances. A well-applied coating greatly reduces the amount of steel that is exposed.
Cathodic protection changes the electrochemical condition of exposed steel so that corrosion is reduced. It does not need to protect every square inch of a sound coated surface. Instead, protective current is directed primarily toward areas where the steel is electrically connected to the system and exposed to the electrolyte.
- The coating provides the primary physical barrier.
- Cathodic protection provides electrochemical protection at exposed areas.
- Inspection confirms that the combined system is functioning as intended.
Field Rule
Cathodic protection is not permission to accept poor surface preparation, thin coating, holidays, contamination, or defective repairs. The coating must still meet the project specification.
2. Coating Quality Controls Current Demand
A cathodic protection system must deliver enough protective current to the steel that is exposed to the surrounding electrolyte. A sound coating leaves only a very small exposed area, so the required current is usually much lower than it would be for bare steel.
As the number or size of coating defects increases, more steel becomes exposed. The cathodic protection system may then need to supply more current. Extensive coating damage can exceed the system's intended capacity and can make it difficult to achieve adequate protection throughout the structure.
Conditions that can increase current demand include:
- Missed areas and incomplete coating coverage
- Pinholes, holidays, and mechanical damage
- Coating deterioration caused by aging or chemical exposure
- Blistering, cracking, peeling, or delamination
- Damage during handling, transportation, installation, or backfilling
- Poorly completed field joints and repairs
- Damage caused by excavation or maintenance work
High-quality coating work therefore improves more than appearance and barrier protection. It can reduce electrical demand, conserve anode material, lower operating requirements, and improve the distribution of protective current.
3. What Happens at a Coating Holiday?
A holiday is a discontinuity in the coating that exposes the substrate or creates a path through the coating film. It may be a pinhole, thin spot, crack, cut, scrape, or missed area.
When the coated structure is placed in soil, water, or another electrolyte, protective current can flow to exposed steel at the holiday. A properly operating cathodic protection system can reduce corrosion at that location.
This does not make holidays acceptable. A larger number of holidays increases current demand and introduces more opportunities for corrosion, coating undercutting, contamination, and localized failure. Required holiday testing, repairs, and retesting must be completed before the structure enters service.
4. Galvanic and Impressed-Current Systems
Galvanic cathodic protection
A galvanic system uses anodes made from a metal that is more electrochemically active than the protected structure. The anodes corrode preferentially and provide protective current without an external power supply.
Impressed-current cathodic protection
An impressed-current system uses an external direct-current power source, commonly a rectifier, together with installed anodes. The system can provide greater and more adjustable current, but it requires qualified monitoring and control.
Both types depend on electrical continuity, correct connections, suitable anodes, an electrolyte, and a structure that has not been electrically isolated from the intended system.
5. Underprotection
Underprotection occurs when parts of the structure do not receive adequate cathodic protection. The exact acceptance criteria and test procedures must come from the governing specification, applicable standard, and qualified cathodic protection personnel.
Possible causes include:
- More exposed steel than the system was designed to protect
- Coating deterioration or widespread mechanical damage
- Depleted or poorly performing galvanic anodes
- Rectifier failure or incorrect impressed-current output
- Broken cables, leads, bonds, or electrical connections
- Loss of electrical continuity
- Electrical interference from nearby structures or systems
- High-resistance environmental conditions
- Shielding that prevents current from reaching the steel
Visual coating inspection alone cannot determine whether the structure meets cathodic protection criteria. Electrical measurements and professional interpretation are required.
6. Overprotection
Excessive cathodic polarization may create conditions that contribute to coating disbondment, blistering, or other coating-related problems. Hydrogen-related concerns may also exist for certain metals, high-strength steels, welds, or service conditions.
A blister or disbonded coating does not prove that overprotection caused the failure. Similar symptoms can result from contamination, moisture, poor surface preparation, solvent entrapment, application errors, incompatible materials, excessive film thickness, or service exposure.
Contractors should document the observed condition and notify the owner or inspector. They should not adjust rectifiers or other cathodic protection equipment unless they are qualified and specifically authorized to do so.
7. Cathodic Disbondment
Cathodic disbondment is the loss of adhesion between a coating and the steel near a defect while the structure is under cathodic protection. Electrochemical reactions at exposed steel can change the local environment beneath or along the edge of the coating.
Resistance to cathodic disbondment depends on the complete coating system and its application. Important factors include:
- Coating chemistry and compatibility with cathodic protection
- Surface cleanliness and profile
- Soluble-salt contamination
- Application and curing conditions
- Coating thickness
- Temperature and exposure duration
- Condition and size of the original holiday
- Level of cathodic polarization
Project specifications may require laboratory testing to evaluate a coating system's resistance to cathodic disbondment. These tests compare coating performance under controlled conditions and do not replace proper field application.
8. Shielding Beneath Disbonded Coating
A serious condition can develop when water reaches the steel beneath a disbonded coating but protective current cannot reach the same area effectively. The loose coating may act as a shield, allowing a corrosive environment to exist while interfering with cathodic protection current.
Shielding risk depends on the coating, the geometry of the disbonded area, the electrolyte, deposits, backfill, and other site conditions. A coating that remains attached and performs as a barrier is preferable to relying on cathodic protection beneath a disbonded film.
Important Distinction
Cathodic protection may protect steel at an open holiday. It may not provide adequate protection beneath every disbonded coating. The condition of the coating remains critical.
9. Components the Coating Crew Must Protect
Before abrasive blasting, power-tool cleaning, washing, or coating begins, the contractor should identify cathodic protection and electrical-isolation components in the work area.
These may include:
- Anodes and anode leads
- Rectifiers and junction boxes
- Reference electrodes or reference cells
- Test stations and test leads
- Continuity bonds and bonding cables
- Insulating joints, insulating flanges, and isolation hardware
- Electrical connections and contact points
- Monitoring equipment and permanent sensors
Abrasive blasting can cut insulation, damage cables, destroy labels, contaminate terminals, or damage delicate reference electrodes. Coating can cover required contact points, obstruct testing, or interfere with the intended function of components.
The crew should use approved masking and protective covers. No component should be disconnected, relocated, coated, removed, or modified without written direction and coordination with the owner or cathodic protection specialist.
10. Working Around Energized Systems
Impressed-current systems may remain energized during maintenance activities. The contractor must determine the equipment status before work begins and follow the owner's safety procedures, electrical controls, lockout requirements, and permit system.
A coating contractor should never assume that a wire is abandoned or harmless. Damaged wiring can disable protection, create an electrical hazard, or affect other connected structures.
11. Holiday Detection and Repair
Holiday detection helps locate discontinuities before a coated structure is buried, immersed, insulated, or placed into service. The test method and voltage must be appropriate for the coating type, thickness, substrate, and project specification.
Each detected holiday should be marked, repaired using the approved procedure, allowed to cure as required, and retested. Repair records should identify the location and final acceptance status.
Excessive test voltage can damage a sound coating. Testing should be performed with properly calibrated equipment and by personnel who understand the applicable procedure.
12. Recoating Can Change the Cathodic Protection System
Recoating a deteriorated structure can substantially reduce the amount of exposed steel. As coating coverage improves, the cathodic protection current demand will often decrease.
This change may require the cathodic protection system to be evaluated and adjusted by qualified personnel. The coating contractor should notify the owner when work is complete so that post-work testing can be performed. The contractor should not independently change rectifier settings.
13. Recommended Contractor Workflow
- Review the project documents. Identify coating requirements, cathodic protection components, isolation devices, testing procedures, and responsible personnel.
- Conduct a pre-work inspection. Photograph and record the location and visible condition of components near the work.
- Coordinate system status. Confirm whether the cathodic protection system will remain energized and who has authority to disconnect or adjust equipment.
- Protect sensitive components. Use approved masking, covers, and work practices before cleaning begins.
- Perform surface preparation and coating work. Prevent damage to leads, terminals, test stations, insulating components, and reference electrodes.
- Inspect the completed coating. Verify thickness, cure, appearance, repair quality, and holiday-test results as required.
- Remove masking carefully. Confirm that required electrical contacts, labels, and test points remain accessible.
- Document the final condition. Record repairs, damage, deviations, and restored components.
- Request post-work verification. Qualified cathodic protection personnel should confirm system performance.
14. Common Field Mistakes
- Coating an anode, terminal, or required electrical contact
- Blasting directly against cables, leads, or reference electrodes
- Removing a wire without identifying its purpose
- Failing to mask test connections and component labels
- Damaging insulating joints or altering isolation hardware
- Leaving coating holidays because cathodic protection is present
- Failing to retest repaired holidays
- Adjusting a rectifier without authorization or qualification
- Failing to report damaged or disconnected components
- Completing major recoating work without requesting post-work system verification
15. Documentation Requirements
The final coating report should document any cathodic protection components located within the work area and any action taken to protect them. Useful records include:
- Pre-work and post-work photographs
- Locations of anodes, leads, test stations, and isolation devices
- Identification of masking and protective measures
- Damage discovered before or during the work
- Components disconnected or reconnected by authorized personnel
- Holiday-test settings, results, repairs, and retest results
- Requests for cathodic protection testing after completion
- Names of the owner representatives and qualified specialists involved
Bottom Line
The coating protects the majority of the surface, while cathodic protection addresses exposed steel at coating defects. Strong performance requires a sound coating, an effective cathodic protection system, careful coordination, and verification by qualified personnel.
Technical References
The following references provide additional technical guidance. The project specification and current edition of the applicable standard should always govern the work.
Knowledge Check
Answer each question before opening the response.
1. What is the primary role of the protective coating?
The coating provides a physical barrier that isolates most of the steel from the corrosive environment.
2. What is the role of cathodic protection on a coated structure?
It supplies protective current to electrically connected steel exposed at coating holidays, defects, or damaged areas.
3. How can coating damage affect current demand?
Coating damage exposes more steel to the electrolyte. The cathodic protection system may then need to supply more current.
4. Why can disbonded coating create a shielding concern?
Water may reach the steel beneath the disbonded coating while the loose coating interferes with protective current reaching that same area.
5. Should a coating contractor adjust an impressed-current rectifier?
Not unless the contractor is properly qualified, specifically authorized, and following the owner's procedures. Normally, this work belongs to qualified cathodic protection personnel.
6. Why should the cathodic protection system be checked after major recoating?
Recoating can greatly reduce exposed steel and current demand. Qualified personnel should verify system performance and determine whether adjustment is required.
Coming Next
Article 20: Final Inspection, Repair Verification, and Project Documentation
The final article brings the complete corrosion-protection process together. It will address final inspection, acceptance criteria, repair verification, closeout records, and the documentation needed to demonstrate that the coating work was completed correctly.
