Corrosion behavior of carbon steel undera defected epoxy coating studied by scanning Kelvin probe

Scanning Kelvin probe (SKP) was used for measuring the corrosion behavior of carbon steel with a defected epoxy coating after neutral salt spray test, and the volta potential changes of carbon steel at different stages of neutral salt spray test were analyzed. It is shown that the defect formed by the damaged epoxy/carbon coating provides a way for the corrosive medium to contact the carbon steel matrix. The potential of carbon steel is more negative at the defect point than that under the coating, resulting in anodic dissolution at the defect point. After some time of neutral salt spray test, insoluble corrosion products can cover the bare defect, leading to the potential at this part shifting to more positive values. Consequently, the defect becomes a cathode, while its vicinity under the coating exists as an anode. Meanwhile, the potential difference between the new cathode and the new anode offers a driving force for continuous corrosion under the coating. Therefore, with the corrosion developing, there would be a considerable corrosive medium and corrosion products aggregating at the interface, which destroy the binding between the epoxy coating and carbon steel, leading to the appearances of bubbling and peeling.