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1. Introduction
Stray current which is not flowed in the certain conductor or the designed circle could be divided into two types, namely, direct current (DC) and alternating current (AC) (Allahkaram et al., 2015; Cui et al., 2016). Although the effect of AC on corrosion of carbon steel is much lighter compared to the equivalent DC, the corrosion failure behaviour caused by AC stray current could not be neglected (Gummow et al., 1998; Goidanich et al., 2005a, 2005b; Bertolini et al., 2007). Plenty of studies (Kim et al., 2005; Ormellese et al., 2011; Wen et al., 2015) are carried out to study the effect of AC on corrosion and recognize that the corrosion rate of metal structures is accelerated in the presence of AC interference. The corrosion damage of stray current in buried metal structures is first investigated as far back as the early 1800s, and the structure suffered a strong corrosion (Martin, 2006). It is acknowledged (Hartman, 1991; Peez, 1993; Wakelin and Sheldon, 2004; Movley, 2005) that the corrosion caused by AC stray current often occurs in the parallel areas of buried steel pipelines and high-voltage transmission lines or the rail transit systems. Particularly, a lot of buried steel pipelines corrosion cases are reported in recent years, and AC corrosion always occurs at coating defects on coated pipelines in the presence of AC interference (Ragault, 1998; Ellis, 2001; Funk and Schoneich, 2002; Hanson and Smart, 2004). For example, a natural gas pipeline which is coated with polyethylene occurs numbers of AC corrosion after being buried for 1 year because the pipeline is located in a common area with a high-voltage transmission line for approximately 6 km in France (Ragault, 1998). It is also reported in America (Hanson and Smart, 2004) that corrosion perforation phenomenon occurred only after the piping system working for six months in a section of the buried pipeline which shares right of way with six high-voltage transmission lines and the peak AC corrosion rate is as high as 400 mpy.
It is well known that AC corrosion rate on a metal structure is not easy to be predicted, and the AC corrosion mechanism is not yet completely understood. Thus, amount of works in...