In the design of a Cathodic Protection scheme, a decision must be made as to whether the scheme should be a sacrificial anode or impressed-current system or a mixture of the two systems.
Sacrificial anode systems have the advantage of being
- simple to install,
- independent of any source of electric power,
- suitable for localised protection,
- less liable to cause interaction on neighbouring structures.
It is difficult to over-protect the structure and moderately easy to obtain a uniform electrode potential across the structure. The most severe limitation of the sacrificial anode is the small driving force which restricts its use to conductive environments or well-coated systems. To protect a large structure, such as a pipeline, with sacrificial anodes, a large number of them would need to be distributed along it, involving a multiplicity of electrical connections and considerable installation work.
The advantages of the impressed-current system include
- the large driving force available can protect a large, even uncoated, structure in high resistivity environments,
- comparatively few anodes are needed,
- the voltage may be adjusted to allow for environmental and coating changes.
The impressed-current system may allow considerable over-protection, however, and considerable variation of potential over the structure is difficult to avoid.
Generally, sacrificial anode schemes have found favour for small well-coated low-currentdemand structures or for localised protection, with impressed-current schemes being utilized for large complex structures which may be bare or poorly coated. However, in North Sea offshore work, it has been found economic to provide galvanic protection to large uncoated platforms and similar structures where the cost of coating is prohibitive. In addition, the galvanic anodes offer easily installed robust systems which, being independent of a power source, provide protection immediately on “float-out” of the structure.