Effect of mechanical stresses on marine organic coating ageing approached by EIS measurements

In this work, the role of a stress-strain state (visco-elastic domain) on the protective properties of two marine epoxy coatings (with and without VOC) applied onto mild steel was studied. Different stress values were applied on coated substrates and bent samples were immersed in 3 wt.% NaCl solution at different temperatures. Non-bent coated samples were also immersed in the same conditions as references. Electrochemical Impedance Spectroscopy was used to evaluate the organic coating degradation on the compressed and the stretched sides periodically.The degradation kinetics showed that the tensile mode was very damaging for one coating while a slight effect was observed on the other coating. In the first case, the water uptake was found to be more important in the tensile mode for higher stress values. A particular attention was focussed on the initial relative permittivity which appeared as a thermo-activated function of the absolute value of the applied stress, for both coatings. Using a thermodynamic approach, the influence of the enthalpic and entropic part of the permittivity was discussed. The diffusion coefficient of the solution into the coating was also measured. The results showed that the diffusion coefficient is strongly modified by the mechanical stress but different behaviours were obtained with both coatings. It is proposed that the entropic contribution plays a major role on the modification of this coefficient.