## 3 Results and Discussion
### Protective Behavior of PPM-Based Coatings against Corrosion of AA2024  
The coatings of the copolymers are an electrical insulating layer deposited on the silane treated surface of aluminum AA2024. Thus, the protective action of the coating against corrosion of the metallic substrate is merely due to a physical barrier effect, aimed to prevent the direct contact between the aggressive environment (i.e., naturally aerated near-neutral 3.5 wt.% NaCl aqueous solution) and the underlying aluminum surface. Polarization scans are a useful electrochemical method to compare the barrier properties of the here prepared PPM-based coatings containing 6.1% mol/mol and 13.4% mol/mol repeat units with long alkoxy side chains that, extending from the main polymer backbone, act as an internal plasticizer. Figure 5: PPM derivatives coatings on polybenzylsiloxane-modified AA2024. (**a**) Surface coated with CO-PPM containing 13.4% of 4-octyloxy side-chains. (**b**) Surface coated with CO-PPM containing 6.1% of 4-octyloxy side-chains. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Advancing** & **Receding** \\ \hline
6.1\% octyloxy units & 104\({}^{\circ}\) & 100\({}^{\circ}\) \\
13.4\% octyloxy units & 102\({}^{\circ}\) & 101\({}^{\circ}\) \\ AA 2024 [1] & 55\({}^{\circ}\) & 41\({}^{\circ}\) \\ \hline \hline \end{tabular} \({}^{1}\) literature data [1].  
\end{table} 
Table 3: Contact angles. Contrary to single-cycle polarization technique (commonly called pitting scan), in which an anodic scan is immediately followed by a backward one, which has been recognized as a valuable tool for the detailed corrosion morphology analysis of aluminum [21; 22; 23], anodic polarization provides a faster but more qualitative method to study the corrosion phenomena. In this work, the recorded current density was exploited as a diagnostic parameter to identify the occurrence of localized corrosion. In the case of the copolymer with 6.1% octyloxy units, excessive porosity (Figure 5b) and/or presence of cracks in the polymer layer resulted into a worst physical barrier effect, in turns detected by a current increase during the polarization test (Figure 6a) due to localized (i.e., pitting) corrosion, thus resulting in aluminum dissolution. This well-known corrosion phenomenon to which Al is susceptible in the presence of chlorides occurs already at OCP for bare aluminum (Figure S3), resulting in a quick increase of the current density to values of many Ma cm\({}^{-2}\) already at -0.6 V vs. SCE [21; 22; 23]. The resulting shift toward more positive potentials for the abrupt increase of the current density for the coated metal (Figure 6a) with respect to the bare one (Figure S3) is due to the barrier-like protective property of the PPM coatings. Therefore, current densities 10\({}^{6}\) times lower (i.e., few Na cm\({}^{-2}\)) are maintained at potentials even more positive than the OCP and the critical pitting potential of bare AA2024, resulting into a significantly increased resistance towards the aggressive environment. Best anodic polarization curves recorded for silane treated AA2024 coated with a layer of 6.1% and 13.4% octyloxy units are presented in Figure 6a, where the scans started from OCP (between -0.78 and -0.61 V vs. SCE) as detected after 600 seconds of equilibration between the samples and the solution. For sake of reproducibility, different samples of each copolymer were tested. All data are reported in the Electronic Supporting Information (Figures S4 and S5). While at the more negative potentials both types of copolymers assure a comparable good protection of the underlying AA2024 (current densities around 10 nA cm\({}^{-2}\)), the coating prepared with a lower amount of the comonomer exhibited an abrupt increase of the current density starting at ca. Figure 6: **(a)** Anodic polarization curves for AA2024 coated with copolymer with 6.1% (blue lines) and 13.4% octyloxy units (black lines) in naturally aerated near-neutral 0.6 M NaCl solution.