Effect of mechanical stresses on epoxy coating ageing approached by Electrochemical Impedance Spectroscopy measurements

The present work is a new illustration of the interest to use electrochemical analysis to investigate polymer coating degradation. We focus our attention on specific effect of visco-elastic (VE) stress on the hygrothermal ageing of a model polymer DGEBA/TETA coating applied on steel panels. Two different VE stress values (stretched and compressed modes) were applied on the coated panels and these samples were hygrothermally aged in NaCl 3 wt.% solution at different temperatures. Electrochemical Impedance Spectroscopy was used in order to determine the solubility, the diffusion coefficient and the initial relative permittivity of the coating. The obtained results suggested that all of these parameters appeared as thermo-activated functions of the absolute value of the applied stress and that the stress, independent of its sign, leads to a decrease of the initial relative permittivity, solubility and diffusion coefficient. A thermodynamic approach was used to separate the enthalpy and entropic contributions of each property. It is shown that the entropic contribution plays a major role on the modification of these parameters. It means that the VE applied stress modifies the polymer chain spatial distribution. Finally, the VE stress allows a better barrier effect of the coating and then to delay the corrosion process of the metallic substrates.