When talking about protecting a buried pipeline, a storage tank, or an offshore infrastructure against corrosion, one is actually referring to two distinct and complementary strategies: passive protection and active cathodic protection. Confusing them, or assuming that only one is sufficient, is one of the most frequent design errors in industrial installations.
In this article, we explain what each one is, their technical differences, and why at Procainsa we recommend combining them in almost all the projects we undertake.
What is passive corrosion protection?
Passive protection is based on a very simple principle: physically isolating the metal from the electrolyte that causes corrosion (soil, water, seawater). If the metal is not in direct contact with that environment, the electrochemical reaction that degrades it cannot occur.
In practice, this is achieved through coatings and coverings applied directly to the metal surface:
- Fusion Bonded Epoxy (FBE) Coatings: the most common system for newly constructed steel pipelines.
- Polyethylene or Polypropylene Tapes: used for weld joints and repairs.
- Coal tar coatings: found in older installations, now no longer used for environmental reasons.
- Epoxy and Polyurethane Paints: common in storage tanks and offshore structures.
The problem with passive protection is that no coating is perfect or permanent. During manufacturing, transport, installation, or simply over time, localized defects known as holidays appear: small discontinuities in the coating where the metal is exposed to the electrolyte. A single holiday is enough for corrosion to start at that specific point, in a localized and often more aggressive way than if there were no coating at all.
What is active protection or electrochemical cathodic protection
Active protection, which we normally refer to simply as cathodic protection, does not attempt to isolate the metal from the electrolyte, but rather to change its electrochemical behavior. The metal structure becomes the cathode of an electrochemical cell, so that instead of losing electrons (oxidizing, corroding) it receives them from an external source.
That external source can be a more reactive metal that is sacrificed in its place (sacrificial anodes) or a controlled electric current supplied by a rectifier (impressed current cathodic protection). Unlike passive protection, active protection keeps working even at the points where the coating has failed, precisely because it does not depend on a physical barrier.
Why passive protection alone is not enough
No serious corrosion engineer recommends relying solely on coating for critical installations. The reasons are clear:
- Every coating, however good, develops defects over time.
- Defects are not visible from the outside on a buried or submerged pipeline.
- Corrosion at a holiday point can be faster and more localized than generalized corrosion.
- Repairing a damaged coating in a buried installation involves excavation, with the cost and operational disruption that entails.
That is why reference international standards, such as NACE SP0169 and ISO 15589, require active cathodic protection for critical buried and submerged pipelines, regardless of the quality of the applied coating.
Comparison table between passive protection and active cathodic protection
| Aspect | Passive protection (coatings) | Active protection (cathodic) |
|---|---|---|
| Operating principle | Isolates the metal from the electrolyte using a physical barrier | Converts the metal into the cathode of an electrochemical cell |
| Protects against corrosion on its own | Only while the coating remains intact | Yes, continuously throughout its design service life |
| Requires electric current | No | Only in the case of impressed current; not in sacrificial anodes |
| Vulnerable to localized defects | Yes, a single holiday can initiate localized corrosion | No, it also protects at points where the coating fails |
| Requires periodic maintenance | Visual inspection and spot repair | Potential verification and anode replacement or rectifier adjustment |
| Relative initial cost | Low to medium | Medium to high, depending on the size of the installation |
| Recommended use | As a first line of defense in any buried or submerged metallic installation | As a mandatory complement in critical installations, covering coating failures |
How passive protection and active cathodic protection are combined in practice
In modern engineering design, passive protection and active cathodic protection are not treated as alternatives, but as two layers of the same corrosion protection system:
- The coating acts as the first line of defence, drastically reducing the metal surface exposed to the electrolyte.
- The active cathodic protection acts as the second line, specifically protecting the points where the coating has failed.
This combination has a very relevant additional advantage from an economic point of view: by reducing the exposed metallic surface, the cathodic protection current needed to protect the entire installation is reduced by several orders of magnitude compared to what would be required to protect the same structure without any coating. This allows for the design of smaller, more economical cathodic protection systems with longer-lasting anodes.
When it is essential to add active cathodic protection to the coating
Although it is advisable to combine both whenever possible, there are scenarios where active cathodic protection ceases to be a recommendation and becomes a requirement:
- Buried or submerged pipelines for gas, oil, or chemical products, where a leak has serious safety or environmental consequences.
- Installations subject to regulations that explicitly require cathodic protection, such as regulated gas pipelines, oil pipelines, or storage tanks.
- Installations with a long design life (decades), where the cumulative degradation of the coating over time is inevitable.
- Buried or submerged installations with difficult access, where inspecting or repairing the coating is costly or unfeasible.
Do you need to assess whether your installation has adequate protection?
At Procainsa, we have been designing cathodic protection systems that complement existing coatings for over 35 years, with NACE certified technicians.
Frequently asked questions about passive and active cathodic protection
Does cathodic protection replace the coating?
No. Active cathodic protection and coating fulfill complementary, not substitute, functions. The coating reduces the exposed metallic surface and, with it, the necessary protection current; active cathodic protection covers the points where the coating inevitably fails over time. The combination of both is standard practice in critical industrial installations.
Is it mandatory to combine passive and active protection?
It depends on the type of installation and the applicable regulations. On buried or submerged gas, oil and chemical pipelines, standards such as NACE SP0169 and ISO 15589 require active cathodic protection in addition to the coating. In other less critical installations, the decision depends on the risk analysis and the required design life.
What happens if an installation only has a coating, without active cathodic protection?
As long as the coating remains intact, the installation is protected. However, as soon as the first defect or holiday appears, that point becomes completely unprotected, and corrosion can advance rapidly and locally, without any mechanism to stop it, until it is detected by an inspection or until a failure occurs.
How do you know whether an installation needs active cathodic protection in addition to the coating?
It is determined through an engineering analysis that considers the type of installation, the fluid transported or stored, the applicable regulations, soil resistivity, design life and operational criticality. At Procainsa, we carry out this analysis as the first step of any cathodic protection project.
Do you have a cathodic protection project or want to audit an existing one?
Our technical team analyzes your installation and proposes the most suitable combination of passive and active protection, with over 35 years of experience in the sector.
