AC corrosion on cathodically protected pipelines is commonly experienced due to interference from high voltage transmission lines at a frequency of 50 or 60 Hz. In some cases, the source of interference may be low frequency AC transit systems (1/3 of the fundamental frequency) and in other cases higher harmonics of the fundamental frequency can be detected on pipelines. In part A of this paper a field investigation of a 120 Hz rectifier ripple, suspected to lead to AC corrosion, is investigated. In this part B of the present investigation, AC corrosion of cathodically protected steel is investigated using electrical resistance (ER) probes at various frequencies and different CP levels in an artificial soil solution. The effect of frequency on basic aspects and understanding of AC corrosion, specifically in terms of alkalization mechanisms and related time constants for involved processes, is discussed.
The existing NACE1 SP21424-2018 standard on AC corrosion addresses typical power frequencies up to 60 Hz, and the studies undertaken to sustain criteria for risk assessment, mitigation and monitoring have been based on 50 or 60 Hz1. Other frequencies such as 16 2/3 Hz, and 2nd and 3rd harmonics of the 60 Hz (i.e. 120 and 180Hz) may interfere with the CP system on buried pipelines as well, and it is necessary to address this. Already, some research has been performed on the effect of frequency on AC corrosion2-5, the earliest dating back to the beginning of the 20th century, where it was concluded that low frequency AC had the highest impact on corrosion rates2. Zhu and Du explain this by a negative shift of the corrosion potential with lower AC frequencies, and the damaging impact of AC on the stability, compactness and uniformity of the passive film on steel4. Several authors also point out another important effect from varying the frequency: At high AC frequencies, the impedance of the double layer capacitance is low, and most of the alternating current will pass via charging/discharging of the electrochemical double layer, whereas for lower frequencies, a larger portion of the AC current will pass via the polarization resistance, i.e. as electrode reactions such as corrosion5-8. Using this rationale, AC corrosion can essentially be understood as DC interference corrosion (at least during the anodic cycle), if the frequency is low enough. However, if AC corrosion could simply be treated as a periodical DC corrosion phenomenon, where the fraction of current running via electrode reactions and not the double layer capacitance is increasing with lower frequencies, then a simple mitigation measure could be to increase the level of CP as it is sometimes done in DC interference cases. This conclusion is wrong, since it is well documented that increasing the CP level will in fact accelerate corrosion rates in the presence of AC interference9-12. AC corrosion is, in other words, far more complex than just considering anodic dissolution during the positive half-cycle of an alternating voltage signal.
