(327f) Degradation of Organic Coatings In Deionized Water and NaCl Solution
Water percolation into the metal-coating interface is usually the main cause of the loss of barrier properties of coatings and leads to coating delamination and underfilm corrosion. Previous studies focus on coating degradation behaviors under electrolyte solution immersions. Reported studies using pure water (e.g. deionized or distilled water) are only for metallic materials. Traditionally, investigations on the degradation of organic coatings are performed for organic coatings immersed in stationary solutions. Recently, flowing fluid accelerated coating degradation has been received more attention since flowing fluid may enhance the transport of ions and oxygen through the coating film, abrade the coating surface with fluid shear, and degrade the barrier properties of organic coatings.
In this study, both deionized water and 3.5 wt% sodium chloride solution are chosen as the working fluids. To investigate the degradation behavior of coatings under a variety of flow rates, the organic coatings are exposed to stationary immersion as well as the laminar flow. A variety of coatings have been investigated including epoxy and polyurethane based clear coating, commercially available air force coating, and marine coating. The barrier properties of coatings are monitored inline by electrochemical impedance spectroscopy (EIS) measurements. Moreover, equivalent circuit models are developed to interpret EIS spectra and to analyze the physical behavior of coatings as they are degrading. We find that the impedance spectra of coatings decrease with the immersion time and the decrease is more substantial for flowing fluids at higher flow rate disregard of the fluid type. Different types of coatings respond differently to DI water and 3.5 wt% sodium chloride solution for their degradation rates.