Introduction

The phenomenon of AC corrosion has been considered by many authors since the early 1900s. However, the mechanisms of AC corrosion are still not completely understood. The body of recent (post-1980) literature indicates that AC corrosion or AC-enhanced corrosion (ACEC) is a bona fide effect (reported corrosion rates up to 20 mpy [0.5 mm/y], with pitting rate considerably higher); there appears to be a tacit agreement that at prevailing commercial current frequencies (such as 50 or 60 Hz) corrosion is possible, even on cathodically protected pipelines.

AC corrosion on cathodically protected pipelines is not well understood, despite discussion about it dating back to the late 19^th century. For many years, corrosion experts did not consider corrosion attributed to alternating currents on metallic structures very important. In 1891, Mengarini¹ concluded that corrosion ("chemical decomposition") by AC (1) is less than that caused by the equivalent direct current (DC), (2) is proportional to the AC, (3) there exists a threshold AC density below which no "decomposition of electrolyte" occurs, and (4) the extent of corrosion decreases with increased AC frequency.

In 1916, McCollum, et al.² published a research paper that concluded iron does not suffer from attack when a limiting frequency of the current (somewhere between 15 and 60 Hz) is reached. AC corrosion was not well understood for two reasons: (1) the electrochemical phenomenon of corrosion is normally attributed to DC, and (2) the instruments normally used to measure the electric parameters in direct currents cannot correctly detect the presence of AC with frequencies between 50 and 100 Hz.³ Recently, concern for AC corrosion mitigation has been increasing because AC interference has been shown to affect cathodically protected underground structures and increase safety concerns (i.e., high AC step-and-touch potentials). Factors that contribute to AC interference on pipelines include (1) the growing number of high-voltage power lines, (2) AC operated high-speed traction systems, (3) high isolation resistance of modern pipeline coatings, and (4) coating integrity.³

^{(1) American Gas Association (AGA), 400 N. Capitol St. NW, Washington, DC 20001.}

⁽²⁾ CEOCOR, c/o CIBE, rue aux laines, 70, B-1000 Brussels, Belgium.