AC Mitigation - zinc strip anodes.

Zinc strip anodes of specific dimensions have been burried parallel to the pipeline in the ditch where it is parallel to the overhead power cables.

These are connected to the pipeline metal at intervals of about 400 meters through a switch that allows current readings to be measured.

It seems that the switch is opened (turned off) during CIPS/DCVG surveys.

These zinc strips will sacrifice to the steel of the pipeline and are therefore the anodes of corrosion cells of which the pipeline is the cathode.

The corrosion products on the zinc should be tested as they might cause a rectifying effect on AC charges in the same way that oxides of other metals are used in the production of electronic diodes.

The configuration of these strips in relation to the power lines might also have the effect of inducing AC charges into the zinc and they are directly connected to the pipeline metal.

The pipeline metal is connected to earthing mats that are adequate for discharging induced AC and stray DC current to earth.

It is unclear what benefit the zinc strip is except as a sacrificial anode to lower the potential of the pipeline metal, adding charges to the ground along it's length.

It has the danger that it will corrode to destruction at a point where the electrolyte is most aggressive and then separate the remainder of the strip from the pipeline. This would become apparent with a distinctive drop in current.

The zinc strip will be protected by the impressed current system where it is in balance and will consume current when the output is turned up too high.

This can be proved by puting an ammeter in the zinc/pipe connection and watching the polarity change.

The zinc anode will also show on a DCVG/CIPS survey as a depression where the TR is high and a peak where the TR is low. This is because of the charges in the ground potential.

The words AC Mitigation seem to have originated in the USA by a company manufacturing or distributing zinc ribbon anodes of a similar specification to that of this contract. NACE has links with this company and connections should be investigated as the claims for it's use seem contrary to the electrical logic of the problem.

It says that induced AC charges build up to a dangerous level in the pipeline due to it running parallel to overhead power lines. It does not say that this rule of nature applies to the zinc running parallel to the pipeline and therefore we now have the opportunity to test the truth of each proposition.



The exact specification for the zinc ribbon dimensions is outside of that readily available at competitive prices and it is reasonable to ask for a technical explanation.