EIS study on degradation of polymer-coated steel under ultraviolet radiation

Degradation of heavy-duty steel coatings when exposed to ultraviolet (UV) radiation was investigated by electrochemical impedance spectroscopy (EIS) and additional techniques in order to clarify the feasibility of evaluation of the UV degradation by EIS. Two coatings were considered: polyester-urethane topcoat plus epoxy primer (PU/E) and epoxy topcoat plus epoxy primer (E/E). Each was applied to a steel substrate and exposed to cyclic wetting-drying under UV radiation. The PU/E coating developed topcoat cracks but did not delaminate from the substrate; capacitive behaviour was evident, and corrosion of the underlying steel was not observed. The E/E coating showed topcoat chalking and partial disappearance, exposing the primer, but corrosion of the underlying steel was not observed. The morphology and chemical changes were compared with the results of EIS.