Cathodic protection and DC non-stationary anodic interference

DC stray currents may induce corrosion on buried carbon steel pipelines transporting methane even if in cathodic protection. According to international standard, anodic interference is unacceptable if the IR-free potential is more positive than the protection potential (−0.85 V CSE). But, duration and magnitude of the anodic peak is not defined. This paper describes results of laboratory tests performed in soil-simulating environment on cathodically protected carbon steel specimens suffering DC interference at varying peak current density, anodic potential and duration. Measured corrosion rate is always lower than the theoretical value predicted by Faraday Law. Consequently, it is possible to conclude that cathodic protection has a positive effect towards DC interference. A parameter Time Out of Protection has been defined as the period of time during interference where the potential is more positive than −0.85 V CSE; this factor has been correlated to the measured potential during DC interference.