Therefore, the high thickness of our epoxy coating (75–80 m) is also responsible by the detection of only one relaxation process in a broad frequency range. Thus, the wide observed relaxation process is associated mainly to organic coating pore resistance, which represents the extent of ionic conduction through the polymer layer in the immersion solution and capacitance of intact film [42,43]. The coating resistance achieved was about 109.5 cm−2 for all alloy substrate coated with sol–gel films and epoxy primer and remain constant up to 300 days. For aluminium alloy coated with epoxy coating, without sol–gel film, the impedance profile is similar that panels protected with the bilayer system (sol–gel/epoxy coating). However the overall impedance value decreases after 160 days of immersion in NaCl solution achieving 108.9 c −2.Increasing the immersion time it should be noted that the total impedance modulus decreases from 108.5 cm−2 to 107.9 cm−2 after 220 and 300 days, respectively. In the longer assessed time at least three relaxation processes are detected. In this immersion time of aggressive agents, water, oxygen and chloride ions, penetrate through epoxy coating and reach the aluminium alloy surface resulting in the fall of impedance modulus, which doesn’t occur with sample
pretreated with sol–gel nanocoatings. Regarding the differences between the two pretreatments, i.e. with 3Vand 3VE5, it was more difficult to ascertain due to the excellent barrier properties of the epoxy paint, as evidenced from EIS
analysis. However, the obtained results prove the beneficial action of VTMS/TEOS and VTMS/TEOS/EDTPO films as adhesion promoters and as anticorrosive system compared to the samples without sol–gel-films.