Investigation of Alternating Current Corrosion of Cathodically Protected Pipelines: Development of a Detection Method, Mitigation Measures, and a Model for the Mechanism

Investigations performed in recent years regarding alternating current (AC) corrosion on cathodically protected pipelines are reported and summarized. A model for the corrosion mechanism is shown and the formation of a rust layer is experimentally demonstrated. The use of the coulometric oxidation for detecting the degree of AC corrosion on coupons by oxidizing the Fe(II) in the rust layer is introduced and experimental results from field investigations are presented. The effect of the cathodic protection level on the corrosion rate is shown. In order to obtain data for optimizing the cathodic protection parameters, the corrosion rate was investigated at various on-potentials and interfering AC voltages. Based on the obtained results it was possible to demonstrate that the corrosion rate can significantly be decreased if the off-potential is more negative than –0.85 V vs. copper/copper sulfate (Cu/CuSO4) electrode (CSE) and the on-potential is in the range of –1.2 VCSE. Moreover, it was possible to demonstrate that the high AC corrosion rate on coupons can readily be decreased if the cathodic protection level is adjusted. To obtain an understanding of the processes involved and to clarify the influence of alkalinity on the corrosion rate, the effect of cathodic current density on the pH value on the metal’s surface of coupons were analyzed with a new in situ pH measurement technique. The effects of pH, spread resistance, and electrochemical reduction are discussed with respect to the experimentally observed corrosion rate.