Understanding the electrochemical processes at the interface of cathodically protected steel pipeline and soil during anodic transients

The steel/soil interface on cathodically protected pipeline steel has been investigated using electrochemical polarisation measurements and electrochemical impedance spectroscopy, to add more insights into the electrochemical processes occurring at the interface. A critical electrode potential (CEP) was found that divides the steel corrosion reaction kinetics into two regions. When the electrode potential is below this critical value, the steel surface has a large impedance value, indicating much slower corrosion kinetics than when the electrode potential is more positive than the CEP value. This critical value is found to be related to pitting potential determined by potentiodynamic polarisation measurements. The CEP could be affected by the CP condition and environmental, including the CP potential and the soil moisture content. At a more negative CP potential, the CEP was found to be nobler; while at the same CP potential but in the soil with a higher moisture content, the CEP became more negative. The impact of the CP potential and soil moisture level on the CEP value is explained by local soil pH effects on steel passivity.