3.2. Microstructure analysis
The surface modified alumina nanoparticles were used in this study to obtain a uniform distribution in the coating matrix. However, this does not guarantee that nanoparticles are compatible with the coating. Therefore, FE-SEM was utilized in order to investigate the nanoparticles distribution in the coating matrix (Fig. 2).It is clear from Fig. 2 that nanoparticles were dispersed in the bulk of the epoxy coating uniformly. The average size of the particles is less than 100 nm indicating that particles were compatible with the coating. As a result, a few agglomerations were seen even using the highest loading of the nanoparticles.
3.3. Anti-corrosion properties investigation
The anticorrosion properties of the epoxy nanocomposites were studied by salt spray test and electrochemical impedance spectroscopy. Both experiments were conducted in sodium chloridesolutions at pH range of 6–7. In fact, the application of the epoxy coating studied in this research is mostly in the atmospheric environments. Therefore, the corrosion tests were done in sodiumchloride solution. To compare the results of EIS and salt spray test measurements, both experiments were conducted in the neutral pH of sodium chloride.
3.3.1. Salt spray test results
Fig. 3 shows the visual performances of the samples exposed to salt spray test up to 1000 h.From Fig. 3 it can be seen that blisters created near scribes for the blank sample. Moreover, corrosion products formed near scribes and beneath the coating for the sample without nanoparticles after 1000 h. It can be seen that addition of 1 wt% nanoparticles caused a decrease in the number of blisters close to scribes and also lower corrosion products creation beneath the coating. Results show that addition of higher nano-alumina particles (2.5 wt% and 3.5 wt%) caused further enhancement of the anticorrosion properties of the coating. Visually, no blister and corrosion products formed near scribes and at the coating/metal interface of the coatings loaded with 2.5 and 3.5 wt% nano-alumina. This indicates that nano-alumina particles could enhance the epoxy coating anticorrosion performance significantly.
3.3.2. EIS measurements
Corrosion resistance of the epoxy coatings loaded with nanoparticles was also studied by EIS. The measurements were carried out after 10, 60 and 135 days immersion in 3.5 wt% NaCl solution. As Mansfeld et al. [17] have reported, the most efficient way to analyze the impedance data, when examining coatings, is in Bode plot format. Fig. 4 represents the Bode plots of different samples. It should be noted that the Nyquist plots of the samples are not represented in this work. As it can be seen from the Bode plots, all the samples show only one time constant indicating high resistance of the systems against electrolyte diffusion. This means that fitting the Nyquist plots through electrochemical equivalent model could not give extra information i.e. coating resistance and coating capacitance values. On can be calculated is only the total resistance including coating resistance (Rc) and charge transfer resistance(Rct). Therefore, the coating capacitance could not be also obtainedin this way.