UV aging characterization of epoxy varnish coated steel upon exposure to artificial weathering environment

Epoxy varnish coating was exposed to artificial weathering environment produced by fluorescent UV/condensation weathering equipment for different time periods. The degradation process of epoxy varnish coating was studied by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and adhesion test. The results showed that the electrochemical behavior of aged coating was closely related to the formation and development of blisters on the surface due to coating degradation. The coating resistance, Rp, decreased to a lower value after 28 days of exposure, which indicated significant deterioration of barrier properties. Small blisters were observed on coating surface after 21 days of exposure. With increasing aging time, blisters grow up and subsequently broke. The mechanism of blisters formation and subsequent breakage were suggested. The soluble degradation products penetrate into the coating along with water to form osmotic cells leading to the form of blisters on the coating surface under the alternating wet and UV irradiation condition. The spread of degradation areas caused the growth and development of blisters. With the loss of coating material and embrittlement, cracks appear on the surface and the blisters break, which may result in the significant deterioration of bulk properties of the coatings.