Effect of the organic coating thickness on water uptake measurements by EIS

In this work, we studied the effect of the coating thickness on the water uptake measured by electrochemical impedance spectroscopy (EIS). A model epoxy system (DGEBA/JeffamineD230), without pigment, was chosen to obtain free-films and coatings on aluminium with thicknesses ranging between 50 and 400 μm. The polymer systems were carefully cured above the glass transition temperature and rapidly cooled down to avoid physical ageing. Then, free-films and coatings were immersed at 50 °C and water uptake was measured by gravimetry for free-films and by EIS for coatings. In parallel, the coating swelling was measured by scanning electrochemical microscopy (SECM). The results showed that water uptake values for the free-films are close to 2.8 wt% for all tested thicknesses. For the coatings, the Brasher and Kingsbury relation lead to water uptake values that were thickness-dependent, and higher than those obtained with free-films. For thin organic coatings (50 μm), it was proposed that the time analysis of EIS data must consider the time needed to reach the measuring frequency to enhance the quality of water uptake measurements by EIS. For thick organic coatings (370 μm), water uptake values obtained by EIS were found to coincide with those measured for free-films when swelling is considered (introduction of the time-dependent thickness into the Brasher and Kingsbury relation). The discrepancy with thinner coatings was explained by internal stress relaxation that is more important for thinner coatings at the metal/coating interface, leading to more important swelling and water uptake values. • The organic coating thickness has an influence on the Brasher & Kingsbury approach. • The initial coating capacitance is time-dependent. • For thin organic coatings (50 μm), swelling is higher than the absorbed water due to the overwhelming role of the interphase with the substrate. • The Brasher & Kingsbury approach has to be used with great care for thin organic coatings.