Corrosion of Pipeline Steel in the Presence of Alternating Current and the New CP Recommendation

The alternating current (AC)-induced corrosion of a cathodically protected X65 pipeline steel was studied in a high pH, concentrated carbonate/bicarbonate solution. Results demonstrated that the corrosion rate of the steel increases with the AC current density, and AC interference could increase the pitting corrosion of the steel. In the absence of AC interference or at a low AC current density, i.e., 20 A/m², a cathodic protection (CP) potential of −950 mV(Cu/CuSO₄ electrode, CSE), which is 100 mV more cathodic than −850 mV(CSE) recommended by National Association of Corrosion Engineers (NACE), provides a full protection over the steel. When the AC current density is higher than 20 A/m², the NACE-recommended CP is incapable of protecting the pipeline from corrosion. A new CP standard is thus developed for recommendation to industry to avoid AC corrosion of pipelines.