Under normal operating conditions, electrical perturbations on buried, coated andcathodically protected steel pipelines from foreign electrical systems sharing closedrights of way and driving either alternative currents (a.c.) or direct currents (d.c.) cancause external localized corrosion. This corrosion results from the exchange of currentsbetween the exposed metal of the pipeline, at coating defects, and the surroundingelectrolyte, the soil.The main sources of these electrical perturbations are :1.overhead or underground high a.c. voltage power lines. Through the a.c. inducedvoltage exerted on the pipeline due to the electromagnetic field createdby the currents from the lines, they can induce a.c. current on a nearby pipeline;2.traction systems running with a.c. currents (e.g. locomotives) which are typicallypowered by a high voltage power lines at 50 Hz or 16,7 Hz, or d.c. currents(e.g. streetcars). They can induce and/or conduct a.c. or conduct d.c.current on a nearby pipeline.The corrosion phenomenon due to a.c. perturbations, so called “a.c. corrosion”, canoccur on buried pipelines which share close rights of way with these electricalsources. A.c. corrosion affects usually small localized coating defects, typically onpipelines isolated from soil by a high insulation resistance coating such as three layerspolyethylene (PE) or polypropylene (PP).It is known that the a.c. corrosion is a complex phenomenon which depends on manyfactors such as the induced a.c. voltage, the a.c. and the d.c. current densities on theexposed metal, the surface area and the shape of the coating defects, the local resistivityand chemical composition of the soil, the pH… In addition, d.c. stray currentsfrom d.c. railway systems can also superpose to a.c. perturbations, particularly in urbanareas, and can induce corrosion damages on buried pipelines but few studiesdealt with the influence of d.c. stray current on the kinetics of corrosion of steel subjectedto a.c. interferences [1;2].Therefore, Corrosion Laboratory at GDF SUEZ R&D facility, performed a parametricstudy based on corrosion tests in soil boxes on X70 steel samples subjected to a.c.voltage perturbations or to mixed perturbations which means with both a.c. voltageand d.c. stray currents. These results were also confronted to the results already existingin the open and proprietary literature.
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