Corrosion Characterization of X70 Steel with Coating Damage Under Mixed AC/DC Interference

Electrified railroads, high-voltage transmission lines, and oil and gas pipelines exhibit mixed AC/DC corrosion phenomena at intersections. A comprehensive investigation into the corrosion behavior of coating-damaged X70 steel under mixed AC/DC interference was undertaken, using weight loss experiments, electrochemical tests, and morphology observation. The results demonstrate that: (1) The corrosion rate under identical current densities follows: mixed AC/DC interference > DC interference > AC interference; (2) the maximum depth of corrosion pits under identical mixed AC/DC current densities follows: specimens with a coating damage diameter of 5 mm > 2 mm > 1 mm > uncoated specimens; (3) a critical current density exists. When the mixed AC/DC current densities exceed 120 A/m2, the degree of corrosion in the anode area increases dramatically; (4) in the anode area, the mixed AC/DC interference sustains the corrosion intensity of DC interference and aggravates AC interference-induced pitting corrosion. Simultaneously, in the presence of minor coating damage, the diffusion of cations into the solution is hindered, leading to continuous migration of Cl− into the corrosion pit, aggravating the corrosion. Numerous corrosion pits and crevices form within the corrosion pit, causing an incompletely corroded metal (covered by a corrosion product film) to fall into the solution. In the cathode area, a significant increase in localized pH at the coating damage and the oscillating effect of the AC result in slight corrosion; and (5) at the mixed AC/DC current densities of 180 A/m2, the corrosion depth in the anodic area of the D = 5 mm specimen reaches 1,883 μm, and the diameter of the coating damage extends from the original 5 mm to 7 mm.