Effect of mechanical stress on kinetics of degradation of marine coatings

Water, UV and temperature are well-known factors for organic coating degradation. Mechanical stress can also affect long-term lifetime in marine environments and probably is conducive to synergistic effects with other ageing parameters. The present work proposes a method to estimate the role of a stress–strain state on the protective properties of two marine epoxy coatings onto mild steel (with and without VOC). Preliminary mechanical measurements on free films by Dynamic Mechanical Analysis (DMA) indicated that the two coatings have a visco-elastic behaviour for a stress level lower than 3.3 MPa. Consequently, a stress equal to 3 MPa was applied on coated substrates using bent samples which were immersed in 3% NaCl solution and in natural seawater (“Les Minimes” yachting harbour in La Rochelle). This test is innovative because a visco-elastic deformation implies that the chain motion remain unchanged after a time of recovery (total restoration of strain in unloading stage) so coatings are not disturbed without applied stress. Non-bent coated samples were also immersed in the same environments as references. The coating degradation was followed by Electrochemical Impedance Spectroscopy (EIS) on both sides of the sample (compressed and stretched sides). The results allowed evaluation of the influence of mechanical state on the coatings degradation in visco-elastic regime and also demonstrate that the mechanical effect depends on the direction of the stress loading.