Abstract: In this paper, electrochemical techniques and weight-loss method are used to identify how AC stray current corrodes pipeline steel in two carbonate solutions (dilute and concentrated). The result of electrochemical experiment shows that the polarization behavior of pipeline steel is manifested as the current oscillation of polarization curve. Its amplitude increases with the ratio of AC density and DC density, especially near the corrosion potential of ±100 mV. The result by weight-loss method shows that the corrosion occurs equably at low rate when the AC stray current density is 0-20 A/m², but at higher rate when the AC stray current density is 20-100 A/m² in thick carbonate solution and 20-200 A/m² in dilute carbonate solution; local pitting corrosion occurs when the AC stray current density is 200-500 A/m² in concentrated carbonate solution and more than 500 A/m² in dilute carbonate solution. The corrosion volume caused by AC stray current is about 1% of the corrosion volume caused by the same DC stray current.