Pipelines are protected against corrosion by a combination of a high quality coating and cathodic protection systems. They are in many cases exposed to interference from dc and/or ac currents. If the current is transferred to the pipeline in a conductive way it can be an advantage not to have a coating with very high resistance. This enables mitigations as local groundbeds or introducing a voltage drop in the pipe. To detect ac corrosion the local corrosion rate must be monitored. The local corrosion rate can be evaluated by excavating test coupons but penetration probes give an economic benefit and results are received without delay, giving the most cost effective way to monitor a highly influenced section of the pipe network.