Discussion on AC Corrosion Rate Assessment and Mechanism for Cathodically Protected Pipelines

Abstract

Although a lot of AC corrosion failure cases and research work on cathodically protected pipelines have been reported, the mechanism of the AC corrosion process has not been completely understood, and there still exist many debates on AC corrosion assessment criteria under cathodic protection (CP), especially under a high CP level. AC corrosion simulation experiments in three kinds of environments were conducted to study the AC corrosion behavior under different conditions. Based on the AC corrosion rates and the corresponding AC current densities, DC IR-free potentials, and DC current densities, the AC corrosion rate assessment diagrams were presented and the threshold values for AC and DC parameters corresponding to certain corrosion rates were determined in three studied environments. Besides, the AC corrosion morphologies, products, local environment parameters, and electrochemical characteristics were measured and analyzed under the combined effects of AC and CP. The effect of hydrogen evolution reaction on the AC corrosion process under a high CP level was discussed based on the electrical equivalent circuit model at the steel/electrolyte interface, the dynamic electrochemical reaction process, and the change of local environment close to the specimen surface.