Impedance at low frequency and breakpoint frequency were obtained from the Bode plots in order to describe the protective performance of different coating systems without modelling the experimental data through equivalent circuit [18–24]. Variations of impedance (at 10 mHz) and break point frequency (fb) as a function of exposure time are shown in Figs. 5 and 6.Fig. 5 shows that corrosion resistance of the coating was significantly improved in the presence of nanoparticles. It is clear from this figure that impedance values of the coatings loaded with 2.5 and 3.5 wt% nanoparticles are much greater than blank sample. A descending trend of impedance can be seen by increasing immersion time. However, the impedance of the coatings containing 2.5 and 3.5 wt% nanoparticles did not significantly change even after 135 days immersion. The break point frequency (fb) is another useful parameter to estimate the degree of coating delamination and the extent of corrosion growth beneath the coating[25]. The breakpoint frequency is the frequency at which the bodeplot slope begin to change. At frequencies higher than fb, the coating shows capacitive and at lower frequencies shows resistive behaviour. The relative increase in delaminated area can be determined from the increase of fb[26]. The lower fb value (lower frequency) shows more capacitive behaviour of the coating. In fact, this means that the coating resistance is high enough to resist against electrolyte diffusion into the coating matrix. Fig. 6 shows that the fb increased by increasing immersion time for the blank sample. However, the fb did not change significantly for the coatings loaded with 2.5 wt% and 3.5 wt% nano-aluminaparticles. These observations clearly show that barrier properties of the blank sample reduced significantly after 135 days immersion. As a result, the electrolyte diffused into the coating matrix and reduced its corrosion protection properties. On the other hand, coatings loaded with nanoparticles showed capacitive behaviour even after long immersion time. This shows lower electrolyte diffusion into the coating/metal interface of these samples than blank one [27,28]. Generally, the impedance data shows that the coating containing 3.5 wt% nanoparticles has the greatest corrosion protective performance than other sample. These observations are in accordance with the results obtained from salt spray test.The mechanism in which the nano-alumina particles could enhance the corrosion resistance of the epoxy coating is approximately clear. Nano-alumina particles could increase the coating resistance against hydrolytic degradation and corrosive electrolyte diffusion even at long immersion times. The improvement in the corrosion resistance of the nano-modified coatings can be attributed to the small size and large surface area of the nanoparticles. These are responsible for the decrease in the number of transport paths for corrosive electrolyte diffusion through the coating. In fact, nanoparticles could significantly enhance the barrier properties of the coating. Moreover, the coating degradation can be reduced in the presence of the nanoparticles