## 6 Results and Discussion
Figure 6 shows EIS spectra as a function of time for the bare 8079 alloy when immersed in the NaCl 3.5 wt.% aerated aqueous solution. The impedance responses are similar at all times of sample exposition to the test solution. This demonstrates that the sample quickly reaches a steady-state regime, and its behaviour does not change during the test duration. The impedance-modulus plot, Figure 6a, in which the modulus of impedance as a function of frequency is reported, shows a quasi-resistive behaviour at low frequencies (1 \(\times\) 10\({}^{-2}\)-3 \(\times\) 10\({}^{-1}\) Hz), a capacitive behaviour in the range 3 \(\times\) 10\({}^{-1}\)-1 \(\times\) 10\({}^{3}\) Hz, due to the presence of the double layer and the electrochemical reactions developed at the interface, determining the corrosion of the sample, and a resistive behaviour in the range 1 \(\times\) 10\({}^{3}\)-1 \(\times\) 10\({}^{5}\) Hz, due to the solution resistance. The impedance modulus at 1 \(\times\) 10\({}^{-2}\) Hz is approximately equal to 3.2 \(\times\) 10\({}^{5}\)\(\Omega\)\(\cdot\)cm\({}^{2}\), due to the active behaviour of the interface which, as demonstrated by the potentiodynamic tests, is affected by pitting corrosion. The phase-angle data, reported in Figure 6b, confirm this interpretation by exhibiting a resistive behaviour at low frequencies (phase-angle tending towards 0) and a capacitive range (phase-angle tending towards 80\({}^{\circ}\)) between 3 \(\times\) 10\({}^{-1}\) and 1 \(\times\) 10\({}^{3}\) Hz; after that, the phase angle decreases, reaching 0\({}^{\circ}\) at high frequencies. Figure 7 shows results obtained from carrying out the EIS measurements, as a function of time, for the CCrR sample when immersed in the 3.5 wt.% NaCl aerated aqueous solution. The results clearly demonstrate the very high quality of the products used. The impedance modulus (Figure 7a), which is one of the most important parameters and among others is used to identify the quality of a coating system, reaches, at the lowest frequency and the beginning of the test campaign, (t = 0), the value of approximately 7 \(\times\) 10\({}^{10}\)\(\Omega\)\(\cdot\)cm\({}^{2}\). This value is generally considered very high, demonstrating the remarkable corrosion resistance of the entire protective-layer [18]. Correspondingly, the phase angle (Figure 7b) remains equal to a value of approximately 90\({}^{\circ}\) throughout the frequencies range. After 20 min of the sample exposition to the test solution, the shape of the curves and the sample behaviour did not change significantly, but after 2 days a sharp decrease in phase angle at low frequencies (approaching 0\({}^{\circ}\)) can be seen, as well as the modulus of impedance showing a resistive behaviour between 10 and 10\({}^{-2}\) Hz, reaching a value of approximately 4 \(\times\) 10\({}^{9}\)\(\Omega\)\(\cdot\)cm\({}^{2}\). This means that the electrolyte has reached the interface, diffusing through the organic coating, and has begun to corrode the substrate [18]. The self-healing properties of the CrIII contained in the conversion coating show their effect after 7 days of sample exposure to the aggressive environment. In fact, the impedance modulus reaches approximately the same values as those obtained after 20 min of the test, and the phase-angle increases [19]. At the same time, the data show that both the impedance modulus and the phase-angle curves are shifted downwards in the entire frequency range, demonstrating that the electrolyte continues to penetrate the organic coating. These results confirm the high effectiveness of the CrIII-based conversion layer in protecting the aluminium substrate from degradation. Figure 8 shows the electrochemical behaviour as a function of the time of the CNR sample when immersed in NaCl 3.5 wt.% aerated aqueous solution. The data highlights the stability of the sample throughout the observation period.