Corrosion occurs when AC is conducted to a metal through an electrolyte. Alternating current produces cyclic changes in the polarity of the electrode. Each half-cell reaction has different accumulative effects. For instance, the anodic half cycle can lead to corrosion or pitting whereas the cathodic half cycle can lead to hydrogen charging and possibly cracking. AC produces a shift in the electrode potential, however the direction and magnitude of the shift depend on the intensity of the AC. Pitting is usually observed after sufficient time. Studies have been conducted to explain the phenomena and identify the possible implications for the design of corrosion protection systems such as cathodic protection. Electrochemical impedance spectroscopy (EIS), and cyclic polarization have been used to determine the effect of AC and induced magnetic fields. Characterization by scanning electron microscopy of the surface has also been performed.
Alternating current (AC) corrosion in buried pipelines has been widely studied and reported in literature in the recent years. The influence of stray currents1-5 and interference from overhead high-voltage transmission power lines which induce AC current in the pipelines5-8 have been the cause of much concern. In such cases, cathodic protection is not fully capable of protecting a pipeline when the defects in the pipeline coating subject it to AC-induced corrosion.7,9-10 The following factors have been identified in field studies as being necessary for corrosion to occur.11
• A pipeline with a coating of high dielectric strength
• A means of inducing AC onto the pipeline, e.g. an overhead power line, direct electric heating (DEH) system, etc.
• An environment of low resistivity providing a good route to ground for the current
• A high current density
• Coating defects The mechanism of AC corrosion, however, remains not fully characterized or understood.
Many different theories have been proposed to explain the mechanism by which alternating current affects the corrosion process. One of the theories is based on the asymmetry of cathodic and anodic Tafel slopes which affects the corrosion rate and potential causing a rectification effect.12 Studies have reported the effects of activation control, diffusion control and mixed control on the anodic and cathodic processes which indicate that the Tafel parameters have a strong influence on the sensitivity of a corrosion system to AC corrosion.13-17 Other authors have suggested that alternating current has the ability to depolarize cathodic and anodic reactions and degrade electrochemical passivity, thereby enhancing the corrosion rate of the metal.18-19 AC is further known to cause the corrosion potential to shift in the negative direction20-21, exchange current density to increase and the Tafel slopes to decrease.18-19 Song et al. have investigated the effect of AC voltage, AC frequency and AC current density on the corrosion rate of a metal.22 Some of the other studies have shown that AC corrosion mechanisms involve alkalization of the environment near the coating defects.23-24 Figure 1 illustrates the region on the Pourbaix diagram affected by this increase in alkalinity.
