Stray current will tend to pass through the electrolyte (soil, water) flow onto a buried pipeline in a certain location and discharge from the pipeline in another area. In a dynamic stray current situation, the magnitude of current flow and the position of current pick-up and discharge areas will fluctuate with time.

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In terms of corrosion damage, the areas of current discharge are generally of greatest concern. It is in these areas that stray current flow represents anodic metal dissolution. Such metal dissolution tends to be localized (concentrated current, high current density) over a small surface area at coating defects / discontinuities. The localized corrosion rate, expressed as localized thickness loss per unit of time, would therefore tend to be high.

The pipeline will essentially receive a "boost" of cathodic protection in areas where current is picked up. However, this additional (uncontrolled) flow of current can lead to undesirable "overprotection" effects such as coating disbondment, hydrogen embrittlement and excessive alkalinity. Such alkaline environments can lead to corrosion damage in alloys such as aluminum, lead and zinc.

References:

Links:

Link to the basic theory section, where a corrosion cell is illustrated with current discharging at the anode.

Link to the theory section of this module, with an illustration of current flow onto a pipeline under conditions of cathodic protection.