It is now recognised that cathodically-protected pipelines (and other metal structures) may be affected by AC from nearby power lines or traction systems.
This phenomenon is thought to be of particular concern in the case of pipelines with high quality (eg fusion-bonded epoxy) coatings that run parallel to high-voltage overhead transmission lines. There is also evidence to suggest that AC-assisted corrosion can occur below the 15VAC threshold.
The MERLIN system has been used to monitor test posts with known or suspected high levels of AC. In diagnostic mode (hourly measurements) it has been possible in some cases to identify and mitigate the source of transient or regular variations in AC levels.
Abriox’s MERLIN system has been designed to enable pipeline operators to monitor levels of AC on their pipeline – and to monitor their effect (if any) on the cathodic protection system. The MERLIN CP Monitor measures simultaneous DC and AC levels on all channels.
AC parameters that can readily be monitored include:
AC voltage (pipe-to-soil)
AC current flowing through a coupon (measured across a shunt)
When the shunt value and the coupon surface area are entered, the CPSM software automatically calculates the current density in A/m2 and will present the results in both tabular and graphical form.
The British Standards Institute has issued a new draft technical specification on the Evaluation of AC corrosion likelihood of buried pipelines: application to cathodically-protected pipelines (DD CEN/TS 15280).
BSI points out that, in the presence of AC interference, the criteria given in EN 12954:2001 (Cathodic protection of buried or immersed metallic structures. General principles and application for pipelines) are not sufficient to demonstrate that the steel is being protected against corrosion.
The new technical specification provides limits, measurement procedures and information to deal with long-term AC interference and evaluating the likelihood of AC corrosion.