The presence of alternating current (AC) may cause serious damage on metal structures even when cathodic protection (CP) is applied and off-potential matches -850 mVCSE criterion. When pipelines are in the vicinity of a high AC electrical field

such as a power line or an electrical railway

this corrosion may occur at the location of coating holidays. Although AC corrosion of metal was only a fraction of an equivalent amount of direct current

but the corrosion damage cannot be ignored. For direct current (DC) corrosion on metal structures there is large agreement on criteria to be used for corrosion mitigation. AC corrosion mechanism is not yet understood and reliable techniques for the determination of the corrosion risk are not yet available. Therefore

the basic problems on AC corrosion should be investigated in detail. An overview of the AC induced metal corrosion is presented. Increasing number of cases and laboratory test results confirmed that AC corrosion risk really exists and it is a serious factor in corrosion of pipelines. Many studies indicate that a number of variables affect the severity of AC corrosion activity

e.g. AC frequency is one of the factors affecting corrosion rate. Investigations show that corrosion rate decreases with increasing frequency firstly

but with further increase of frequency up to a threshold corrosion rate increases. Several mechanism hypotheses of AC corrosion are presented

but without consensus regarding the mechanisms. There are several criteria to evaluate the probability of AC corrosion

which are based on the AC density or the ratio between AC density and DC density

or the AC voltages or the instantaneous off-potential

furthermore

the charges needed to oxidize corrosion produces on the corroded steel surface. However

criteria based solely on one indicator are inefficient. The comprehensive evaluation indicators system considers the balance between such induction effects as electrolyte environment and cathodic protection level

may be more reasonable.