Buried pipelines are usually protected against corrosion damage by the combination of a protective coating system and cathodic protection (CP). The coating is usually considered to represent the primary form of protection, while the CP system is designed to provide protection at coating discontinuities (defects) that are invariably present. As such, the CP system can be viewed as "additional insurance" against the corrosion risk factor.

The diagram below schematically illustrate current flow in an impressed current CP system (the principle is similar for a sacrificial anode system). Note, under these idealized conditions, current flows through the electrolyte (soil) onto the pipeline, in the form of ionic current.

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(direction of current as conventional current)

The schematic diagram below details current flow onto the pipeline at coating discontinuities, under the protective influence of the CP system.

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Stray current represents current flow in the electrolyte that does not originate from the CP system designed to protect the pipeline. Such "external" current sources can disrupt the "ideal" current distribution of the CP system and consequently can lead to corrosion problems. It can be said that the external stray current sources interfere with the normal operation of the CP system, as illustrated schematically below.

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