The delineation of local water interaction with epoxy coatings using fluorescence microscopy

Abstract

The precise mode by which water and ions enter organic coatings when subjected to corrosive environments remains unresolved. Local electrochemical methods have provided insight into the local nature of the underfilm degradation process. But these local electrochemical methods have not provided conclusive evidence as to whether the transport of water and ions occurs through discrete or regional heterogeneities within the organic film, or whether these species enter as a uniform front and then elicit localized corrosion dictated by heterogeneities on the substrate surface (e.g., intermetallic particles). The mode of water interaction with 30-μm thick epoxy coatings was investigated via the phenomenon of autofluorescence using conventional fluorescence microscopy. Epoxy coatings were spun-cast onto glass slides and exposed to distilled water, 0.1 M NaCl or 1 M NaCl at ambient pH for 1 day. Following exposure, a conventional fluorescence microscope was used to observe the coating surfaces both in bright and fluorescence fields. Autofluorescence of hydrated films indicated that water uptake occurred at discrete sites on the surface of the coatings. The number density of these sites was found to increase with water activity. These results indicate that initial water uptake in epoxy coatings occurs at discrete sites on the surface of the coatings.