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Abstract
Underground pipelines protected with a thick coating and by cathodic polarisation may suffer a serious external corrosion damage in the presence of stray alternating current (AC) voltage induced by high voltage industrial electric transport system, such as power lines or electrified railroads. The origin of the corrosion enhancement comes from the nonlinearity of the currentpotential characteristics of the metal-soil interface. In this paper, we will theoretically evaluate the increase of the corrosion current density and the potential shift induced by a high amplitude AC signal to models of corroding systems: anodic polarisation curves obeying an exponential law with respect to the potential, and cathodic process under the mixed activation-diffusion kinetics. The originality of the present work lies in the use of a relatively small number of dimensionless variables to describe the faradaic rectification for the corrosion potential shift and the corrosion current enhancement. In this article, the effect of the electrolyte resistance was neglected.
Highlights
These two figures illustrate the correlation between, on one hand, the interfacial potential and, on the other hand, the voltage between the steel and the remote earth resulting from the inductive coupling between the pipeline and the electric power line, as well as the direct current (DC) potential from the steel to the remote earth, resulting from the presence of the cathodic protection (CP) system.[28]
Basic expressions on the faradaic rectification for an activation controlled interface The electrochemical system constituted by the steel/soil electrolyte interface is intrinsically non-linear; the current response induced by a high alternating current (AC) voltage modulation is non-linear, too
Underground steel installations, especially steel pipelines, are generally covered with thick organic coating, and in addition protected cathodically. When these assets are exposed to high AC voltage fields created by high level alternating currents transported by industrial systems in their vicinity, an enhancement of the corrosion may be observed at coating defects, if the CP system is not adequately fitted to this particular electrical constraint
Summary
In this figure, the electrochemical stability of the steel/soil system is illustrated through a simplified Pourbaix diagram in which the span of the excursions of the interfacial potential E(t) is depicted.[27] Figure 1b presents a typical electrical equivalent circuit depicting the local situation at any given coating holiday and adapted from that suggested in the recent state-of-the-art review elaborated by the National Association of Corrosion Engineers (NACE) International Society.[5] These two figures illustrate the correlation between, on one hand, the interfacial potential and, on the other hand, the voltage between the steel and the remote earth resulting from the inductive coupling between the pipeline and the electric power line, as well as the direct current (DC) potential from the steel to the remote earth, resulting from the presence of the CP system.[28]. The CP potential is set to polarise the steel somewhere between the immunity and the passivity domain.[28]
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