Influence of d.c. stray currents interferences on a.c. corrosion phenomenon

摘要

Under normal operating conditions, electrical perturbations on buried, coated and cathodically protected steel pipelines from foreign electrical systems sharing closed rights of way and driving either alternative currents (a.c.) or direct currents (d.c.) can cause external localized corrosion. This corrosion results from the exchange of currents between the exposed metal of the pipeline, at coating defects, and the surrounding electrolyte, the soil. The main sources of these electrical perturbations are: overhead or underground high a.c. voltage power lines. Through the a.c. induced voltage exerted on the pipeline due to the electromagnetic field created by the currents from the lines, they can induce a.c. current on a nearby pipeline; traction systems running with a.c. currents (e.g. locomotives) which are typically powered 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", can occur on buried pipelines which share close rights of way with these electrical sources. A.c. corrosion affects usually small localized coating defects, typically on pipelines isolated from soil by a high insulation resistance coating such as three layers polyethylene (PE) or polypropylene (PP). It is known that the a.c. corrosion is a complex phenomenon which depends on many factors such as the induced a.c. voltage, the a.c. and the d.c. current densities on the exposed metal, the surface area and the shape of the coating defects, the local resistivity and chemical composition of the soil, the pH… In addition, d.c. stray currents from d.c. railway systems can also superpose to a.c. perturbations, particularly in urban areas, and can induce corrosion damages on buried pipelines but few studies dealt with the influence of d.c. stray current on the kinetics of corrosion of steel subjected to a.c. interferences [1; 2]. Therefore, Corrosion Laboratory at GDF SUEZ R&D facility, performed a parametric study 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. voltage and d.c. stray currents. These results were also confronted to the results already existing in the open and proprietary literature.