Abstract
Pipelines buried in soil are protected by a thick organic coating complemented by cathodic protection. In spite of this double protection, when these pipelines are in the vicinity of a high voltage AC electrical field, such as a power line or an electrical railway for instance, corrosion may occur at the location of coating holidays. This phenomenon may be explained by a faradic rectification due to a non-linearity of electromagnetic interface behavior, and by the AC field effect. At each AC signal, the steel-soil interface may be polarized anodically, therefore inducing a dissolution process. We will examine quantitatively AC induced corrosion by harmonic analyses and the simultaneous collection of potential and current signals at 50 Hz, using a carbon steel disc electrode in simulated natural soil waters. It has been observed that in presence of an AC signal, the corrosion potential (defined at the zero-overall DC current) shifts towards a more negative direction. As a consequence, the cathodic protection (polarization) is less efficient. The more conductive is the solution, the higher is the AC corrosion rate. In addition, though the interface impedance is essentially determined by the electrolyte resistance and partly by the double layer capacitance (around 50 Hz), the faradic current density is in the order of the overall current. The alternative transformation of the steel - ferric oxide - ferrous oxide at each AC signal may therefore lead to a significant corrosion of carbon steel.
