AC-Induced Corrosion – It’s a Real Threat

Pipeline operators beware: AC-induced corrosion is threatening your newer pipelines.

Right of Way - ideal sites for pipeline and electrical utility corridors

This is the finding from numerous pipeline integrity investigators whether it be smart pig indications or direct assessment evaluations.  Astoundingly, pipelines with outstanding coating and cathodic protection systems are showing evidence of AC corrosion – and in some cases, quite severe.

In the United States we have a long history os using electrical utility corridors as ideal sites for pipeline rights of way.  The advantages of common utility rights of way are compelling.  The area has been cleared of trees where power lines run above ground, creating a cleared highway perfectly suited for installing pipelines and other buried utilities.

Pipeline operators have long assumed that AC-induced voltages do not present a corrosion risk and need only be addressed from a personnel safety perspective.  John Morgan’s well respected NACE textbook titled “Cathodic Protection (2nd Edition, 1987)” states… ‘In general, AC corrosion is not a hazard and there is little evidence…that alternating current causes corrosion.”  But recently, this conventional thinking has been turned upside down.

So why are we now seeing AC corrosion when just a few decades ago it was not considered a viable corrosion threat?

Quite simply, it is the unintended consequence of exceptional coating efficiencies.  The use of Fusion Bonded Epoxy (FBE) and three layer polyethene (3LPE) coatings have proven quite effective in protection pipelines from conventional galvanic corrosion.  These coating systems represent significant improvement over the conventional coating systems they have replaced.  Installed coating efficiencies exceeding 99.9% are common.

Surprisingly however, a newly constructed pipeline boasting an outstanding coating system is much more prone to AC-induced corrosion.  Induced or stray AC current picked up along a buried pipeline with a high-quality coating holiday has only a limited number of small coating defects through which is can dissipate to ground.  Thus, the localized AC current density at the discharge location can be quite high.  In contrast, older pipelines with larger and more frequent coating defects offer numerous sites for AC current to discharge back to ground.

AC corrosion rates can range from 2 to 500 mpy with a typical value in the range of 60 mpy.  AC-induced corrosion can occur when the AC voltage levels are well below the 15 VAC safety threshold cited by NACE.  AC current density, soil conditions, and holiday size all affect the corrosion rate.

Pipeline operators must be aware of the heightened risks of AC corrosion resulting when pipeline with advanced coatings and high efficiency is installed in electrical transmission utility rights of way.  Monies need to be allocated to thoroughly investigate, and if necessary mitigate AC risk.

For more information about MATCOR’s AC mitigation products visit: http://www.matcor.com/index.php?id=75