This paper presents a summary of the results obtained during 3 years of PhD studies on "AC Corrosion of Cathodically Protected Pipelines" at the Technical University of Denmark (DTU)1, which was finalized in October 2018 and defended in January 2019.1 Amongst the findings are: An improved understanding of AC corrosion as different corrosion phenomena under low and high levels of cathodic protection respectively. An enhanced understanding of the reactions in soil systems that lead to establishment of corrosive environments. Discovery of a close correlation between current density criteria and the Pourbaix diagram. An improved understanding of the influence of the CP level on the electrochemical double-layer characteristics. Presentation of a new model for AC corrosion that considers ionic dissolution and hydrogen evolution in the passive film destabilisation mechanism. The results are discussed in relation to literature and relevant cathodic protection and AC interference standards.
AC corrosion is a topic that has gained a lot of attention in recent years, particularly in the pipeline industry. Characteristic for this form of corrosion, it has been found to be enhanced by traditional corrosion protection strategies such as cathodic protection and high impedance pipeline coatings, in the presence of an alternating voltage. The primary source of AC on buried pipelines is electromagnetic interference from high voltage transmission lines above ground, when the two are sharing the same right-of-way. The Danish company MetriCorr†, has produced corrosion rate monitoring probes since 2002, specifically designed to investigate corrosion of cathodically protected structures under electrical interference conditions AC and/or DC. In 2014 an industrial PhD study was initialized in collaboration with DTU, financially supported by the Innovation Fund Denmark, to investigate the phenomenon in detail. This paper outlines the major findings of this study.1
It is well established that AC alters the DC polarisation behaviour of steel. In particular, the depolarisation of both anodic and cathodic reactions causes an increase of the cathodic current density, JDC, under cathodic protection at the same polarised potential.2-8 This behaviour is also called faradaic rectification of the alternating current.9 On the other hand, the cathodic current also influences the AC current density, JAC, at an induced AC voltage, UAC, because of alkalisation of the local environment at the steel surface that alters the spread resistance, RS, of a coating defect. RS is a parameter that is inherently dependent on the soil resistivity, ρsoil and the geometry of a coating defect.