*3.4. Coatings*

Coatings of the copolymers were manufactured by pressing polymer powders onto a silane-pretreated [1] AA2024 specimen, using polyetheretherketone (PEEK) foil to separate the PPM-based polymers from the pressing instrument. Pressing was performed for both copolymers (6.1% mol/mol and 13.4% mol/mol octyloxy units) at a temperature of 120 ◦C for 30 s. The thickness of the resulting films was about 30 μm (Figures S1 and S2), and the coatings appeared very uniform and homogeneous although no rheological additive was added (Figure 5). Notably, at the processing temperature, the low viscosity of the molten polymer with 13.4% mol/mol octyloxy units enhances self-diffusion within the polymeric matrix, and therefore, during film formation, the cohesion between the polymer chains, and thus, coalescence are promoted [20]. Moreover, polymers which contain side chains that do not strikingly hinder self-diffusion may have a greater cohesive strength than non-branched polymers, based on a firmer anchoring of such macromolecules in the polymeric matrix [20].

**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.

Remarkably, it is common knowledge that corrosion inhibition may also be favored by the hydrophobic behavior of polymer coatings and the homogeneity of their surface. Advancing and receding contact angles of water on copolymer coatings confirmed the low wettability of the protective films (Table 3). Moreover, the remarkably low contact angle hysteresis (4◦ and 1◦, for 6.1% mol/mol and 13.4% mol/mol octyloxy units) indicates a very smooth and uniform surface (notably, the contact angle hysteresis was even lower than that on AA2024 itself) [1].



#### *3.5. 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.

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 [21–23], 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<sup>−</sup><sup>2</sup> already at –0.6 V vs. SCE [21–23]. 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<sup>6</sup> times lower (i.e., few Na cm<sup>−</sup>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.

**Figure 6.** (**a**) Anodic polarization curves for AA2024 coated with copolymer with 6.1% (blue lines) and 13.4% ocytyloxy units (black lines) in naturally aerated near-neutral 0.6 M NaCl solution. Right: Optical microscope pictures (under 395 nm light irradiation) of samples coated with copolmyer containing 6.1% (**b**) and 13.4% octyloxy units (**c**) after polarizations tests.

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<sup>−</sup>2), the coating prepared with a lower amount of the comonomer exhibited an abrupt increase of the current density starting at ca. –0.50 V vs. SCE (Figure 6a and Figure S4). This sudden and almost monotonic increase of current (from nA cm<sup>−</sup><sup>2</sup> to mA cm<sup>−</sup><sup>2</sup> 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<sup>−</sup><sup>2</sup> 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 μA cm<sup>−</sup><sup>2</sup> [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 e ffect of the copolymer with 13.4% octyloxy units to the higher content of long alkyl side chains that act as a more e ffective 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*g) of 31 ◦C, lower than pristine PPM of the same molar mass (around 55 ◦C [1]). Considering that *T*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 ◦C, just above the glass transition temperature of the copolymer (Figure S10). The coating, when operating at *T* > *<sup>T</sup>*g, showed, on average, worse corrosion-protective behavior with respect to that at temperatures lower than *<sup>T</sup>*g. The worsening can be attributed to the higher mobility of the polymer matrix that loses, at least partially, its barrier e ffect 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<sup>−</sup><sup>2</sup> constantly grew reaching, in three hours, a value around 6 μA cm<sup>−</sup><sup>2</sup> 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 μA cm<sup>−</sup>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 a ffecting 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). The characteristic "up and down" current spikes are potentially attributable to the initiation of small metastable pits.

The corrosion-protective ability of the best formulated "self-plasticized" polymer coating (i.e., 13.4% octyloxy groups) is in line with the performance already reported for coatings of PPM with addition of siloxanes as an external plasticizer [1], especially in terms of current densities recorded during anodic polarization tests (few nA cm<sup>−</sup>2). This comparison seems to qualitatively confirm that the design of the self-plasticized PPM approach is a good alternative to the more classical one which is based on the addition of external plasticizers.

**Figure 7.** Current density as a function of time for AA2024 coated with a copolymer with 13.4% octyloxy units, polarized at 0 V vs. SCE in naturally aerated near-neutral 0.6 M NaCl solution, duration: 24 h. Inset: Magnification of the first 12 h.
