La corrosion induite par courant alternatif sur les canalisations enterrées sous protection cathodique.

摘要

Pipelines buried in soil are protected by a thick organic coating complemented by cathodicprotection. This double precautions permit to preserve the durability of gas or petroltransmission networks. In spite of this double protection, when these pipelines are in thevicinity of a high voltage AC electrical field, such as a power line or an electrical railway forinstance, corrosion may occur at the location of coating holidays, named “AC corrosion”.Practically, AC corrosion phenomenon is widely observed and, still, there are littlefundamentals’ studies about it.AC corrosion origin and the effects of some parameters such as: AC amplitude, CPpolarization, soil electrolyte’s conductivity, and bulk pH, have been examined, theoreticallyand experimentally, by using a carbon steel disc electrode in simulated natural soil water. Thisphenomenon may be explained by two approaches:The first one takes into account the direct current (faradic rectification) created due to thenon-linearity of interface response under AC perturbation. As a consequence, this DC currentmay make the cathodic protection (polarization) less efficient.The second approach is based on the study of the relationship between the anodic faradiccurrent and the AC phenomenon. Towards this aim, a special device has been developedwhich allows recording both the instant current and potential signals, synchronously, andthen, through signal treatments, permits to split the total measured interfacial current valueinto the faradic component (the part of the total interfacial current circulating through thefaradic branches of the circuit) and the capacitive component (the part circulating through thecapacitance). The method was used to calculate the anodic faradic current. The existence of aphase shift between faradic and capacitive current has been highlighted. Though it may have asignificant effect on the magnitude of the faradic current, the effect of the electrolyteresistance has been found to be even more important, under the experimental conditionstested. The analyses of corrosion products accumulated at metal surface showed that the ACsignal at 50 Hz may be too fast for electrochemical kinetics reasons to quantitativelytransform sufficient amount of corrosion products at each cycle so that the effect could bedetected through the experimental tools and methods adopted.