The degradation processes of two self-polishing antifouling coatings containing copper-based agents (CuSCN and Cu2O) in 3.5% NaCl solution and the protection effect of the coating systems were studied by electrochemical impedance spectroscopy (EIS), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM/EDS) methods. The results demonstrate that after immersion for 1525 d at room temperature, the two coating systems still have very good protection property for the 5083 Al alloy substrate, manifesting by the high value of the low-frequency impedance. Alternate high and low temperature immersion test (45 °C 12 h + 25 °C 12 h) leads to serious damage to the antifouling topcoat, and the failure is mainly manifested by many micro-pores and micro-cracks. Because the CuSCN antifouling agent particle has bigger diameter and slightly bigger solubility than that of Cu2O agent, the micro-pores established after the agents dissolved and released during the hydrolysis process of the antifouling coating are relatively bigger, which results in more decrease in the impedance and a worse protective property of the coating system for the substrate.

The electrochemical impedance spectroscopy and weight gain tests were performed on carbon fiber/vinyl ester and carbon fiber/bismaleimide composites in 3.5% NaCl solution to study the electrochemical and water absorption behaviors. The microstructure morphology and the flexural property of the composites in the long-term exposure process were analyzed with the scanning electron microscope and four-point bending tests. The results revealed that after long-time immersion (>200 d), the water absorption of the two composites is less than 0.5%. This has little effect on the microstructural integrity, only with slight damage on the fiber/resin interfaces, but results in a significant decrease (about 84%) in the composite flexural property. The variation of the water absorption percentage shows good consistency with that of the resin capacitance (Qc) and is negatively related to the variation of the resin resistance (Rpo) and the low-frequency impedance (|Z|0.01Hz) of the composites. A good linear relationship exists between the variations of phase angles in the middle-frequency range (0.1−10 Hz) and the |Z|0.01Hz. The phase angle at 10 Hz (θ10Hz) may be suggested as a suitable parameter to rapidly evaluate the performance of carbon fiber-reinforced polymer composites, just like for evaluating the protective performance of polymer-coated metals in the literature.

The degradation process and the electrochemical behavior of two anti-corrosion and anti-fouling coating systems (FW-1 and FW-2) in a simulated diurnal cycling immersion environment (3.5% NaCl, 35 °C 12 h + 25 °C 12 h) were investigated by electrochemical impedance spectroscopy (EIS) technology. Combined with the coating gloss, color difference, adhesion strength and scanning electron microscopy (SEM) tests, the micro morphologies and the variations of the performance parameters were comparatively analyzed. The results showed that in the 160 days of immersion, with the hydrolysis of the FW-1 topcoat resin and some pigments dissolved and released, the surface micro-morphology of the coating changes from rough to smooth, thereby increasing the gloss. While, for the FW-2 topcoat, the occurrence of micro pores and tiny cracks results in an increase in the roughness and a decrease in the gloss. The release of the copper ion particles in the antifouling topcoat has an influence on the color, manifesting as obvious rise in the color difference of the coating. The low-frequency impedance (|Z|0.01 Hz) values of both coating samples decreases slowly and provides very good shielding to the carbon steel substrate. The self-polishing of the topcoat has no big effect on the electrochemical performance of the whole anti-corrosion and anti-fouling coating system; the protective performance of the coating system mainly depends on the integrity of the primer and the intermediate paint.