A Modeling Approach to Understanding the Interrelated Nature of CP Current and AC Stray Current on Pipelines

The change in AC current on a pipeline as a function of cathodic protection (CP) current is well known in the industry as is the change in CP current as a function of the interfering AC current. To better understand the underlying mechanisms responsible for these observations, the interrelated nature of AC and CP was reproduced here for pipeline steel in soil-simulating environments and the results were analyzed within the context of kinetics and transport based models. The kinetics model combines the Butler-Volmer reaction kinetics for steel oxidation, oxygen reduction and hydrogen reduction with the time evolution of potential at an interface subject to alternating potentials. The critical observation from these calculations was that changes to the measured CP on a pipeline with AC interference were not due to changes in the underlying electrochemical kinetics, but rather, due to asymmetric polarization of the steel along existing Tafel slopes resulting in a change in the time-averaged DC signal. To explore the effect of CP current on the magnitude of the AC interference, a transport based model of grounding resistance at pipeline coating holidays was developed. For holiday sizes less than 20 cm², calculations revealed that at a critical CP current density of 1 A/m², local soil pH increases rapidly and the normalized grounding resistance decreases. Correspondingly, corrosion rate increases dramatically.