EFFECTS OF ALTERNATING CURRENT ON X70 STEEL MORPHOLOGY AND ELECTROCHEMICAL BEHAVIOR

Alternating current (AC) corrosion is a significant problem to metal structures in close proximity to alternating current carrying conductors. While for direct current (DC) interference there is large agreement on protection criteria to be used for corrosion mitigation, but the mechanism and relationship between AC density and corrosion rate are still not clear, although some studies have been carried out for many years. In this work, electrochemical methods, including open curves, polarization curves and electrochemical impedance spectra (EIS) on X70 steel samples, were performed in soil-simulating conditions at various AC current densities from 0 to about 100 A/m(2). The corrosion behavior and mechanism were discussed based on the electrochemical, XRD, stereo microscope and SEM results. The simulation experiment was set up to predict the effects of AC current density on corrosion potential and corrosion rates. The results indicate that the corrosion potential and corrosion rate increases with the rise of AC current density. When the alternating current goes across the X70 steel, its surface suffers from an obvious corrosion potential accompanied with the bubble crop up. It's because the hydrogen and oxygen reduction reactions happened on the surface of X70 steel. There is only one capacitive reactance arc on EIS at the corrosion potential under a low current density interference. Warburg impedance appears in a higher alternating current density of the low frequency area. It shows that the corrosion process of surface is controlled by diffusion. The amount of AC corrosion can be generally expressed as a percentage of the amount of DC corrosion with the equivalent corrosive effect. Alternating current causes less than 1% of the corrosion caused by the equivalent direct current. The X70 steel is corroded uniformly in the early AC interference, but corrosion pits appear gradually on the surface of X70 steel with the extension of time. Especially in the high current density interference, the pitting is more likely to occur. Furthermore, the higher current density increases pitting tendentiousness and depth. Alternating current has a major impact on metal polarization, and the asymmetry of positive and negative half cycle induces corrosion at alternating current interference. Alternating current could generate an "oscillating" effect in the metal/medium interface.