## 6 Results and Discussion  
The potentiodynamic-polarisation curves recorded on the bare, CrR and NR samples are shown in Figure 5.  
All specimens show the expected behaviour of Al alloys when in contact with an aqueous solution containing chlorides. The picture shows that the NR sample exhibited a quite different corrosion potential from the other two samples, while all samples were affected by pitting corrosion. In more detail, the curves analysis highlights the fact that the sample NR stands apart, with a corrosion potential equal to \(-0.975\) V vs. SCE, significantly different from that shown by the bare and CrR samples for which, in both cases, the corrosion potential (E\({}_{\text{corr}}\)) is equal to \(-0.793\) V vs. SCE, as reported in Table 4.  
Generally speaking, when the corrosion mechanism is of the activated type, the sample' anodic and cathodic branches of the potentiodynamic curves allow for an estimating of the samples' corrosion current-density using the Tafel approximation. In the case under study, the corrosion current-densities were evaluated in different modes in relationship to the recorded potentiodynamic-curves shape. Analysing the potentiodynamic curves, it is clear that the bare sample is affected by pitting corrosion, due to the presence of the aluminium oxide that forms spontaneously when the material is exposed to air. The sample CrR, on the other hand, has a layer of chromium oxide that protects it, and it shows a metastable pitting between the corrosion potential and approximately \(-0.710\) V vs. SCE, due to the self-healing action of the CrIII-based conversion layer. The NR sample is coated with the conversion layer containing titanium- and zirconium-fluoride salts; it exhibits an activated corrosion-mechanism between the corrosion potential and \(-0.793\) V \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **E\({}_{\text{corr}}\) (V vs. SCE)** \\ \hline Bare & \(-0.793\) \\ \hline CrR & \(-0.793\) \\ \hline NR & \(-0.975\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: E\({}_{\text{corr}}\) value recorded before the potentiodynamic-polarisation test. Figure 5: Potentiodynamic-polarisation curves of the bare, CrR and NR samples, recorded in an aerated aqueous 3.5 wt.% NaCl solution. vs. SCE, and then it shows a metastable-pitting formation between \(-\)0.850 and \(-\)0.800 V vs. SCE; after that, it shows the setting-off of pitting between \(-\)0.800 and \(-\)0.740 V vs. SCE, which is counteracted by the action of the conversion layer deposited on the specimen, which tends to prevent its development. The pitting definitively stabilises when the potential of \(-\)0.740 V vs. SCE is reached. It should anyway be noted that samples showed a cathodic-reduction activated-type mechanism. Starting from these observations, the corrosion current-density was estimated using the following criteria. In the case of the NR sample, both anodic and cathodic branches of the potentiodynamic curves were used to determine the Tafel lines and then the current density of the sample. For the other two coupons, only the cathodic branch was used. In this way, the corrosion current-density of the NR sample was evaluated as approximately equal to 2 \(\times\) 10\({}^{-8}\) A/cm\({}^{2}\). It is approximately one order of magnitude lower than that exhibited by the sample CrR (equal to 2 \(\times\) 10\({}^{-7}\) A/cm\({}^{2}\)) and two orders of magnitude lower than that exhibited by the bare sample (equal to 3 \(\times\) 10\({}^{-6}\) A/cm\({}^{2}\)). These results highlight the effectiveness of the Ti-Zr-based conversion coating, even when compared to the CrIII-based phospho-chromic treatment. The performances of the NR sample can be attributed to the effect of the Ti- and Zr-containing chemicals deposited on the sample surface and to the presence of the polymeric base, which induces relative stability into the conversion layer, while the CrIII-based treatment is more soluble in water. This aspect will be clearly demonstrated by discussing the electrochemical-impedance-spectroscopy results.