As commercial paints have a complex composition, this work was carried out on a model epoxy system (DGEBA/DAMP) with or without pigment (TiO2). Firstly, physico-chemical properties of the composites free films were studied by using DSC and DMA and compared to those of the sole resin. Then, composites films were applied onto steel panels and were immersed in 3 wt.% NaCl solution at different temperatures ( 30, 40, 50 and 60 degrees C). The water uptake and diffusion processes were followed by Electrochemical Impedance Spectroscopy ( EIS). Using a thermodynamic approach, it was shown that the diffusion process is accelerated in presence of pigments due to internal stresses that develop in composite systems. However, these stresses are relaxed by water sorption during ageing. Finally, it appears that the monitoring of water sorption by EIS allows to characterize the mechanical effects at the coating/substrate interface during immersion. (C) 2017 Elsevier Ltd. All rights reserved.