## 3 Results and Discussion
### Protective Behavior of PPM-Based Coatings against Corrosion of AA2024
Right: Optical microscope pictures (under 395 nm light irradiation) of samples coated with copolymer containing 6.1% (**b**) and 13.4% octyloxy units (**c**) after polarizations tests. -0.50 V vs. SCE (Figure 6a and Figure S4). This sudden and almost monotonic increase of current (from nA cm\({}^{-2}\) to mA cm\({}^{-2}\) within 1 V) is a proof of the limited protection ability of the copolymer with 6.1% octyloxy units, that invariably showed defects after the polarization tests, responsible for the direct metal/solution contact (Figure 6b, and Figures S6 and S7). On the other hand, coatings obtained with the copolymer containing octyloxy units (13.4%) exhibited almost stable current density of 13-30 nA cm\({}^{-2}\) up to at least 2.5 V vs. SCE (Figure 6a and Figure S5). These results are compatible with very good isolation of the Al surface from the solution by this coating, as a result of an almost complete absence of cracks and holes (Figure 5a, Figure 6c, and Figures S8, and S9) and a low porosity level. To give a clearer quantitative comparison, AA2024 in de-aerated near-neutral 3.5 wt.% NaCl (a less aggressive analog with respect to the aerated solution employed in this study) exhibits passivation current densities around \(\mu\)A cm\({}^{-2}\)[23], i.e., two orders of magnitude higher than those recorded with the copolymer with 13.4% octyloxy units even applying extremely more oxidizing potentials. By comparing the performance of the two types of coatings (Figure 6a), it is possible to tentatively attribute the better barrier effect of the copolymer with 13.4% octyloxy units to the higher content of long alkyl side chains that act as a more effective plasticizer by increasing compactness, cohesion, and adhesion ability of the cured films. As confirmed by DSC (see the previous section), the addition of a comonomer introducing long side chains resulted in a modified PPM with a glass transition temperature (\(T_{\mathrm{g}}\)) of 31 \({}^{\circ}\)C, lower than pristine PPM of the same molar mass (around 55 \({}^{\circ}\)C [1]). Considering that \(T_{\mathrm{g}}\) of the copolymers is comparable to the temperatures that an AA2024 manufacture can encounter in real application, additional anodic polarization curves were recorded on aluminum alloy samples coated with the best performing copolymer (13.4% octyloxy groups), by setting the temperature of the 3.5 wt.% NaCl solution at 35 \({}^{\circ}\)C, just above the glass transition temperature of the copolymer (Figure S10). The coating, when operating at \(T>T_{\mathrm{g}}\), showed, on average, worse corrosion-protective behavior with respect to that at temperatures lower than \(T_{\mathrm{g}}\). The worsening can be attributed to the higher mobility of the polymer matrix that loses, at least partially, its barrier effect by favoring, for example, the permeation of the solution toward the underneath aluminum alloy surface. Stability of the better performing corrosion-protective coating copolymer (13.4% octyloxy units) was further investigated by applying a constant polarization at 0 V vs. SCE to the aluminum-coated sample for 24 h in naturally aerated near-neutral 0.6 M NaCl solution (Figure 7). Current density, starting from some nA cm\({}^{-2}\) constantly grew reaching, in three hours, a value around 6 \(\mu\)A cm\({}^{-2}\) that remained almost stationary for a relatively long period of time (up to 12 h). After some hours characterized by high instability, current density started again increasing progressively up to ca. 100 \(\mu\)A cm\({}^{-2}\). The reported trend in the current is coherent with a progressive loss of the barrier effect by the polymeric coating, attributable to both increasing permeation of solution through pores and the formation of defects that grow up in time affecting the integrity of the barrier itself. However, it is apparent that the occurrence of defects is confined to few zones mainly localized close to the borders where some contributions of crevice corrosion can take place (Figure S11).