Most pipeline cathodic protection (CP) applications involve either galvanic anode or impressed current systems installed in earth for protection of external surfaces. Of the galvanic anode installations, most use magnesium as the anode material. Rectifiers are the most common source of direct current power for impressed current systems.
For pipelines installed in the ocean bed (as for long harbor crossings and lines to offshore drilling operations), considerable use has been made of galvanic anode bracelets. These are essentially a ring of specially cast anodes encircling the pipe and attached directly to it. This permits having the anodes already attached to the pipe as it is laid. By doing so, the pipeline will be cathodically protected as soon as it becomes submerged. Used in conjunction with a good coating, sufficient anode material may be provided for long useful life. Zinc has been used most frequently for this type of installation.
Where surface soil conditions for pipelines on land are not suitable for groundbeds installed near the surface, deep groundbeds (vertical) may be installed if underlying earth resistivity is more favorable. Such groundbeds usually are installed in a single hole, with impressed current systems. Particular care must be exercised during installation to avoid premature failures of anodes or anode leads that may not be repairable and may necessitate the installation of a complete new groundbed.
Other instances where deep groundbeds are necessary include sites where right-of-way for surface groundbeds cannot be obtained. For example, a deep bed can be installed on a pipeline right-of-way. They also are used in congested distribution systems where remote groundbeds are needed, but where available sites for surface groundbeds are not sufficiently remote from the pipes to be protected or from structures of other ownership.
In some congested areas, anodes (galvanic or impressed current) are distributed along the length of pipe to be protected. This permits placing the anodes close to the pipe, with each anode protecting a short length. The effect on other structures also may be controlled more readily. This type of installation may be more expensive than remote groundbeds placed at much longer intervals but may, nevertheless, be the best solution in some instances.
Where pipelines are banked, giving rise to severe shielding, a continuous ribbon anode may be used within the bank and parallel to the pipelines to provide protective current within the bank. Such material is available in zinc or magnesium. For impressed current systems, platinum-coated wire or rod anodes are available if required.
The interiors of large pipelines carrying corrosive liquids (such as seawater or industrial waste) may be lined with a suitable coating and protected with strip-type galvanic anode material. If the pipe interior is bare, relatively large amounts of current may be needed. In this case, an impressed current system may be used with platinum-coated anodes penetrating the pipe walls at intervals.
This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 513-514.