On the Mechanism of AC Assisted Corrosion of Buried Pipelines and Its CP Mitigation

Mechanisms of AC assisted corrosion of pipelines under cathodic protection (CP), and CP criteria for its mitigating remain still subjects of controversy. This paper presents a critical review of the fundamental understanding of the AC assisted corrosion phenomenon and also some laboratory investigation; such as the analysis of corrosion products obtained under carefully controlled AC corrosion tests using Raman spectroscopy, simultaneous recording of potential and current, and separation of this experimental overall current into its faradic and charging of the double layer capacitance contributions. For the latter, a simplified equivalent circuit model of the steel / solution interface, taking account of the interfacial and transport phenomenon was used. Results obtained are discussed: • to highlight the effect on corrosion rate and corrosion feature, of some critical parameters namely AC current density, AC voltage amplitude, CP level, soil resistance between the coating defect and remote earth, and development of iron oxide layers; • to devise some aspects on the mechanistic view of the phenomenon; and • to comment on the CP criteria currently in use, with a special focus on the dependency of CP protection level with AC current density and voltage amplitude.