Degradation Process of an Impasto Epoxy Primer Coating under Cyclic Wet-dry and Immersion Condition

The degradation process and mechanism of a thick epoxy primer coating was investigated in 3.5% NaCl solution under cyclic wet-dry and immersion condition by electrochemical impedance spectroscopy (EIS) and fourier transform infrared spectroscopy (FTIR). The degradation of the coating under the two conditions was mainly due to the hydrolysis of epoxy groups in electrolyte solution. The defects of coating such as micro-pores and the hydrophilic groups such as -OH and -NH2, which came from hydrolysis of epoxy groups, promoted the electrolyte solution penetration and accelerated the degradation process. But the degradation process of the coating under cyclic wet-dry condition was faster evidently than that under immersion condition. The main reasons were as follows. The hydrolysis degree of the coating under cyclic wet-dry condition exceeded that under immersion condition. During drying period, oxygen accelerated the reaction on the cathode, resulting in a relatively quick corrosion of metal matrix and a decrease of the coating adhesion. Besides, the inner stress, which resulted from the incongruity of shrinking and swelling between coating and substrate, accelerated the degradation process of the coating.