Structure and Microstructure Studies of Epoxy Coating After Natural Exposure Testing

In this work, visual evaluation, Scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) analysis and Fourier transform infrared (FTIR) spectroscopy were used to study the degradation of an epoxy coating system applied on mild steel test panels (10cm x 15cm) exposed at two natural exposure test sites. After surface preparation and application of coating, test panels were subjected to natural exposure at industrial and urban test sites located in Karachi, Pakistan. Visual evaluation showed that the major modes of degradation were totally different at two natural exposure test sites. Scanning electron microscopy (SEM) revealed that the testing caused changes in the surface morphology of the epoxy coating systems with natural exposure testing at urban site showing less surface degradation. Good correlation was observed between the SEM and gloss measurements. Energy dispersive X-ray (EDX) analysis indicated the increase in oxygen content and O/C (oxygen/carbon) ratio for exposed coatings as compared to unexposed coating. Increase in O/C ratio showed that the photodegradation of the coatings occurred upon weathering. EDX analysis is in good agreement with the fourier transform infrared (FTIR) spectroscopy results. Both observations indicated a high chemical degradation in the epoxy coating system tested at industrial exposure test site. Photodegradation of the epoxy coating system caused chain scission and as a result loss in intensity of the C-O stretching vibration of aryl ether near 1250cm(-1) and C=C stretching of aromatic nucleus in the region of 1500-1600cm(-1) were observed.