As industry trends are resulting in more shared right of ways between power lines and pipeline utilities, the need for mitigation of AC interference is expanding, further driving the desire for efficient solutions. Low impedance grounding systems are generally used to mitigate elevated AC potential and current density on buried pipelines. Multiple mitigation grounding techniques are in use across the pipeline industry, with varying materials, configurations, and construction practices. While there are pros and cons to each, determination of a preferred mitigation design for a given site should consider overall system efficiency, component design life, and specific material properties.
Presently, a comprehensive review of primary mitigation materials is not available to industry, as historical work generally covers a specific subset of these concepts or designs. This study presents a comparative assessment reviewing the electrochemical properties of common AC mitigation materials, and assesses common industry claims with respect to AC mitigation grounding. The intent of this work is to provide the corrosion test data and technical information to support an engineering based decision for mitigation material selection.
Mitigation of induced AC, whether for personnel safety or corrosion purposes, generally involves the use of a low impedance grounding system to pass induced AC to ground. For AC grounding to be provide effective mitigation, it must present a low impedance to AC flow to ground, relative to alternative paths, to effectively dissipate AC voltage.1,2 Materials should be selected considering expected longevity of the grounding system in the specific environment, total installation costs, and impedance reduction achieved for a given site. As the grounding systems are generally decoupled from DC, the life expectancy is based on the self-corrosion rate of the grounding material in the given environment.
Common designs follow either surface or deep grounding system design, utilizing bare copper cable, zinc ribbon, or engineered systems.3 All designs have their unique benefits and detriments, considering performance, costs, and constructability. With either surface or deep grounding designs however, there is disagreement across industry with respect to material preference for grounding. While there are certainly technical justifications for a selecting a particular material for a given application, common industry claims seem to be repeated based on a misunderstanding of the technical issues, and nuances between AC and DC corrosion. For example, arguments against zinc for AC grounding are routinely cited in reference to susceptibility to passivation or depolarization in specific environments.4,5,6,7 While both passivation and a change in electronegativity can have significant effects for a material acting as an anode in the case of DC corrosion control, the effects are only relevant to AC mitigation if they also contribute to a change in total impedance of the mitigation. In which case, the change in impedance is the property change of interest, and the passivation or electropositive shift are not relevant by itself.