Application of EIS and EN techniques to investigate the self-healing ability of coatings based on microcapsules filled with linseed oil and CeO2 nanoparticles

Microcapsules with urea–formaldehyde as the shell and linseed oil as the healing agent were synthesized by a previously reported procedure. Two kinds of synthesized microcapsules, without and with CeO₂ nanoparticles, were separately added to the epoxy resin coatings. The epoxy coatings containing microcapsules were applied on carbon steel, and their self-healing effect was investigated in 0.5 M HCl solution. The amount of the released healing agent that filled up the scratch was estimated by scratch filling efficiency (SFE). The SFE values are only the theoretical estimates of the self-healing performance. The scratch sealing efficiency (SSE), which is a measure of corrosion protection performance of the damaged coating, can be measured by electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) techniques. For sake of an optimum self-healing system, two series of coatings, with and without nanoparticles, were prepared by using different microcapsule concentrations: 5, 10, 15 and 20 wt%. For comparison, a coating without microcapsules was also prepared. The coated samples with 5% microcapsule concentration, due to the low amount of released linseed oil, could not properly repair the artificial scratch. In contrast, when the microcapsule concentration was equal to or higher than 10% the volume of the released linseed oil was enough to seal the scratch. However, the coating sample containing 15% nanoparticle-loaded microcapsules was the optimum self-healing coating because it showed comparable SSE values to those of samples containing 20% microcapsule concentration in spite of its lower microcapsule concentration. The EN method was employed as a complementary quantitative technique to study the self-healing behavior of coatings. The calculation of the amount of noise charges using the standard deviation of partial signal (SDPS) plots arising from wavelet analysis made it possible to obtain the SSE values of the coatings. The good agreement between EIS and EN results indicates that the EN technique, as well as the EIS method, can be used successfully for the self-healing evaluation.