AC Corrosion: Mechanism and Proposed Model

Corrosion caused by the discharge of 60 Hz AC current from a pipeline in a high voltage AC (HVAC) corridor has been discussed and studied over the past 20 or more years. More recent studies in Europe have specifically addressed these corrosion issues following several failures attributed to the presence of AC discharge from the pipeline. Very few corrosion failures in North America have been specifically attributed to what is termed AC-enhanced corrosion (ACEC). One missing area of research is well-controlled laboratory experiments in soil environments. This study proposed a mechanism of ACEC that is based on conventional electrochemistry using the same equivalent analog circuits used to discuss other corrosion processes. It was shown that only a small amount of the 60 Hz AC current discharge passes through the resistive component of the equivalent circuit, which results in corrosion (metal loss) reactions. The AC current passing through this resistive component produces both anodic and cathodic polarization shift (sine wave dependent) resulting in a net increase in the average corrosion rate as compared to the free-corrosion rate. The proposed model for ACEC does not invoke any new electrochemical concepts and is based on the conventional (DC) treatment of the corrosion processes; the model excludes treatment of cases with imposed cathodic protection current. The amount of ACEC is dependent on the magnitude of AC current that passes through the resistive component of the parallel resistive-capacitive electrochemical interface. ACEC is characterized by the rapid formation of a diffusion controlled (Warburg) process for corrosion in soils. Although diffusion controlled, the overall impedance decreases as the total AC current increases. The model suggests that AC currents (60Hz) cause anodic (positive) polarization shifts during the positive portion of the imposed AC sinewave along with cathodic polarization shifts in the negative portion of the AC sinewave; the net result is an increase in the average oxidation (metal loss) current as compared to the free-corrosion condition. The proposed model for the ACEC mechanism showed excellent correlation with the experimental results. The research work was made possible by the funding from PRCInternational.