Once it has been decided that an installation needs active cathodic protection, the next technical question is which of the two existing systems to use: sacrificial anodes or impressed current. They are not two brands or two different qualities of the same thing, but two different operating principles, each with clear advantages depending on the type of installation.
In this article, we explain how each system works, their real technical differences, and what factors determine which is appropriate in each case.
What are sacrificial anodes
Sacrificial anodes, also known as galvanic protection, are metals more reactive than the metal to be protected. When electrically connected to the structure, a galvanic current is spontaneously generated: the anode gives electrons to the protected structure and corrodes itself instead, hence the term “sacrifice”.
This system does not require any external power source. The protection current arises from the natural potential difference between the two metals, so it operates completely autonomously from the moment of its installation.
The most common sacrificial metals in industrial applications are:
- Zinc: suitable in low resistivity soils and water, such as wet soils or seawater.
- Magnesium: offers greater driving force, useful in medium or high resistivity soils.
- Aluminum: widely used in offshore and marine structures due to its good performance and lower weight.
What is impressed current cathodic protection
Impressed current protection uses an external power source, usually a rectifier connected to the mains, which drives a controlled direct current into the structure to be protected. The current flows through inert anodes, normally made of graphite, activated titanium or silicon iron, which are not significantly consumed over time.
Unlike sacrificial anodes, here the level of applied current can be adjusted in the rectifier, allowing for much more precise control of the degree of protection at all times during the installation’s life.
Differences between sacrificial anodes and impressed current
| Aspect | Sacrificial anodes | Impressed current |
|---|---|---|
| Source of protection current | Galvanic potential difference between metals | External power source (rectifier) |
| Requires a power supply | No | Yes |
| Current adjustment capability | Fixed, according to the galvanic pair installed | Adjustable in the rectifier |
| Recommended installation size | Small or medium-sized structures | Large-scale structures, such as gas pipelines or oil pipelines |
| Anode lifespan | Limited, they are consumed and require periodic replacement | Inert anodes with much longer lifespan |
| Initial cost | Lower | Higher, due to the rectifier and electrical installation |
| Operating cost | No power consumption | Continuous electrical consumption of the rectifier |
| Risk of interference with nearby installations | Low | Higher; requires an interference study of nearby installations |
When to use sacrificial anodes
- Small or medium-sized installations, such as individual tanks or short pipe sections.
- Locations without access to electrical supply, such as remote or marine structures.
- Low resistivity soils or environments, where the galvanic couple performs efficiently.
- Projects where a low initial cost and simple installation are prioritized, without the need for electrical maintenance.
When to use impressed current
- Large-scale installations, such as gas pipelines, oil pipelines, or complete tank farms.
- High resistivity soils, where sacrificial anodes would require an impractical amount of material.
- Installations where precise and adjustable control of the protection level over time is required.
- Projects with a very long design life, where the limited lifespan of sacrificial anodes would necessitate frequent replacements.
Is it possible to combine both systems in the same installation
Yes. In complex installations, it is common to use impressed current as the main system for the bulk of the structure, and reinforce with sacrificial anodes specific points of difficult access, electrically isolated areas, or sections added later to the original installation. The final decision always depends on an engineering study that considers the extent of the installation, soil resistivity, electrical availability, and the required design life.
Frequently asked questions about sacrificial anodes and impressed current
What is the difference between sacrificial anodes and galvanic protection
None. Galvanic protection is the technical name for the same system commonly known as sacrificial anode protection. Both terms describe the use of a more reactive metal that corrodes in a controlled manner to protect the main structure.
Can sacrificial anodes be used in long buried pipelines
Technically it is possible, but it is rarely the most efficient option. For large-scale pipelines, impressed current allows protecting the entire length with fewer installation points and more precise control, whereas sacrificial anodes would require a very high number of units distributed along the route.
What maintenance does each system need
Sacrificial anodes require periodic inspection of their consumption and replacement when they are depleted. Impressed current requires periodic checks of the rectifier and of the applied current and voltage levels, as well as verification that the inert anodes and cabling remain in good condition.
Which of the two systems is more cost-effective in the long term
It depends on the installation. Sacrificial anodes have a lower initial cost but require periodic replacement of the consumed material. Impressed current has a higher initial cost but a lower maintenance cost per protected unit in large installations, as well as a longer service life for its components. In large-scale installations, impressed current usually proves more cost-effective in the long term.
