Waterline corrosion behaviors of carbon steel covered by organic coating with artificial defects in seawater

The waterline corrosion behaviors of carbon steel covered by organic coating with artificial defects at and under the waterline simultaneously in simulated artificial seawater were studied by the wire beam electrode (WBE) and electrochemical impedance spectroscopy (EIS) technologies. It was first proposed that the corrosion process of the metal substrate covered by organic coating under the waterline was mainly divided into three stages: no significant corrosion reactions of the metal substrate occurred; the cathodic reaction occurred with relatively slight corrosion on the metal substrate; cathodic delamination of the coating occurred, and the metal substrate was further corroded. Due to the different concentrations of dissolved oxygen, the corrosion reactions of the two exposed electrodes at the waterline and under the waterline were dominated by cathodic and anodic reactions, respectively. For the electrodes under the waterline, on account of the anodic dissolution reaction of the metal substrate, the cathode to anode polarity reversal occurred. In addition, for the yellow corrosion products, the main components were α − Fe2O3, β − FeOOH and γ − FeOOH, while α − Fe2O3 and Fe3O4 were the main components of the black corrosion products. γ − FeOOH participated in the cathodic reaction to form Fe3O4, which resulted in the reduction of the anodic current density at the electrode surface.