Abstract
AC corrosion enhancement of buried steel pipe is induced by high stray AC electromagnetic field and to the faradaic rectification due to non-linearity of current-potential characteristics. In Part I, this phenomenon was evaluated by neglecting the electrical resistance of soil electrolyte. In this part, we will examine when this resistance is no longer negligible. The mathematical model can be expressed by an ordinary nonlinear differential equation. In this part, the corrosion mechanism is modelled by two reactions obeying Tafel law, one for anodic dissolution of steel and another for cathodic process. This corrosion mechanism is valid when the corrosion current density is much higher than the diffusion limited current of dissolved oxygen. Compared with the system where the electrolyte resistance is negligible, the corrosion enhancement induced by AC signal is markedly attenuated because of the ohmic drop through the electrolyte resistance, and the double layer capacitance constitution a bypass circuit.
Highlights
In Part I of this series of papers,[1] the mathematical development and digital simulations of AC voltage enhanced corrosion on buried steel structures under the cathodic protection were reported for highly conducting electrolyte medium, that is, the contribution of the electrolyte resistance is negligible
RE value was twice as low in this medium compared with mildly aggressive medium whereas Cd value is 1.5 times greater in a highly aggressive soil. These two last parameters tend to decrease the effective interface perturbing signal ∆E, for a given DU value, decreasing the corrosion enhancement by the faradaic rectification. This series of papers addresses the effect of the faradaic rectification phenomenon on the AC enhanced corrosion of buried and coated pipelines located in the vicinity of high voltage AC power lines
It was assumed that the corrosion kinetics at steel/soil interface at a coating defect is not depending on the time wise development of the corrosion process
Summary
In Part I of this series of papers,[1] the mathematical development and digital simulations of AC voltage enhanced corrosion (or AC corrosion) on buried steel structures under the cathodic protection were reported for highly conducting electrolyte medium, that is, the contribution of the electrolyte resistance is negligible. A model describing the corrosion of metals subjected to its enhancement induced by alternating voltage, taking into account the electrolyte resistance RE (Ω cm2) and the double layer capacitance Cd (F cm–2), was developed by Lalvani and his group.[6,7] In their approach, they considered an electrochemical interface controlled by activation processes for both anodic and cathodic reactions (bi-tafelian corrosion mechanism). These authors supplied an approximate solution valid in principle for relatively small amplitude of the AC perturbation. We will see the validity limit of the so-called “linear approximation” reported by Xiao and Lalvani,[7] regarding the effect of the peak-voltage of AC perturbation, DU on the corrosion kinetics
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