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Methods for AC mitigation

Published by , Senior Editor
World Pipelines,

How do you stop AC corrosion? Undoubtedly, this is a question that countless pipeline operators have asked themselves when experiencing an induced AC voltage on their pipelines from high voltage transmission lines. In an excerpt from Metricorr’s article in the April 2021 issue of World Pipelines, Andreas Junker Oleson outlines various methods to stop AC corrosion.

Methods for AC mitigation

How do you stop AC corrosion?

The answer is not easily found in an ordinary Google-search, primarily because the available literature points in many directions and is strongly influenced by commercial interests.

What is causing AC corrosion?

As the name implies, AC corrosion is caused by an alternating current passing through the metal-electrolyte (pipe-soil) interface. While the actual corrosion mechanism is still debated, we have a pretty good understanding of the influencing factors.

An alternating current can pass in different ways, as outlined below:

  • Through charging and discharging of the electrochemical double layer i.e. as current passing through a capacitor, which is unlikely to contribute to any corrosion.
  • Through electrochemical reactions carrying a charge, such as oxidation and reduction reactions, which may cause corrosion.

The fraction of the current that potentially carries charge electrochemically is determined by many factors. One of these is the AC frequency. The fraction of the AC current discharge that contributes to corrosion is small for higher frequencies. At 50 - 60 Hz, which is the common AC signal in the transmission grid, the fraction is perhaps only a few percent, but this may still be sufficient to cause significant corrosion. Smaller coating defects will cause the current to be concentrated locally, thus increasing the AC corrosion risk.

Whether the electrochemical reactions actually lead to corrosion, or not, is highly dependent on the metal’s surface oxides, solution chemistry, pH, hydrogen evolution and electrochemical potential, all of which are strongly affected by cathodic protection (CP). Therefore, a fundamental part of understanding what AC corrosion is, is understanding the role of CP. AC corrosion is often observed when there is inadequate CP or excessive CP, in combination with an alternating voltage. This implies that there is an intermediate CP range where corrosion does not occur. This has been found empirically to be around -1.0 VSCE, but dependent on electrolyte properties. In this range there may still be an alternating current passing through the metal-electrolyte interface, but the surface properties are such that this does not lead to active corrosion. This is possible via redox reactions within some passive films, for example.

The most relevant industry standards on the topic are NACE SP21424-2018 or ISO 18086:2019, that correctly outline both AC and DC current densities as important parameters in the evaluation of AC corrosion risk. NACE SP21424 describes the “autocatalytic nature of AC corrosion of cathodically protected pipelines” and states that three pre-requisites are needed for AC corrosion to occur: induced AC; a small coating defect; and excessive CP. Acknowledging that corrosion is caused by AC current, this describes the self-perpetuating mechanism in which CP enhances the alternating current density and thereby the driving force for corrosion.

How to stop AC corrosion

Back to the relevant question: how to prevent pipeline corrosion caused by AC interference? There is more than one way to do this:.

Option 1 – Remove induced AC

This is the obvious solution, given that it is the driving force for corrosion, but it is an expensive solution with conventional mitigation designs, and it is practically impossible to completely remove induced AC. Even a few volts of AC might still be enough to cause AC corrosion.

Option 2 – Remove small defects

Imagine a perfect world with no coating defects at all. It would effectively stop all forms of corrosion – but it does not exist. Poorly coated pipelines with larger defects and a low coating impedance are well ‘grounded’ and less likely to suffer AC corrosion, but this raises other concerns with respect to CP.

Option 3 – Remove excessive CP

This implies aiming for the previously mentioned intermediate CP range where corrosion does not occur, despite an induced AC volt-age. This solution is readily available to most pipeline operators with rectifier-controlled CP. It may necessitate a higher level of monitoring to verify the effectiveness of this strategy.

Each of these three actions potentially eliminates AC corrosion on their own but will also be effective in combination.

To read the rest of the article, click here to read the digital version of the April 2021 issue.

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