•An epoxy resin-Q345 system with a sandwich structure was prepared.•Cl− ions permeated into epoxy resin coating prior to K+ ions.•Free volume size and PAL increased when the coating was immersed into the solution. An epoxy resin coating with a sandwich structure was prepared to investigate ion transport behavior in the coating. The macro- and micro- appearance of the coating immersed in 5wt.% KCl solutions was observed by stereomicroscopy, scanning electron microscopy equipped with an energy dispersive spectrometer. The electrochemical property of the coating was characterized by electrochemical impedance spectroscopy, and change of free volume after immersion was characterized by positron annihilation lifetime spectroscopy. The results indicated that Cl− ions permeated into the coating prior to K+ ions, the free volume size and positron annihilation lifetime of the coating increased during immersion.